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Synthesis and photochemistry of hydrogen telluride

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Synthesis and photochemistry of hydrogen telluride

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Content SYNTHESIS AND PHOTOCHEMISTRY OF
HYDROGEN TELLURIDE
Copyright 2004
by
Sun Young Lee
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CHEMISTRY)
August 2004
Sun Young Lee
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ii
Table of Contents
List of Tables iv
List of Figures v
Abstract vi
Chapter 1 Synthesis of Hydrogen Telluride
1.1 Anomaly in Atomic Weight of Tellurium 1
1.2 Different Synthetic Routes of Hydrogen Telluride 3
1.3 Characteristics of Hydrogen Telluride 11
1.4 Experimental Procedure for the Synthesis of Hydrogen Telluride 14
1.5 Suggestions for Improving the Synthesis of Hydrogen Telluride 19
1.6 References 24
Chapter 2 The Absorption Spectrum of Hydrogen Telluride
2.1 Experimental Details 26
2.2 The Absorption Spectra of Group 6 A Dihydrides 27
2.3 References 31
Chapter 3 The Photochemistry of Hydrogen Telluride
3.1 High-n Rydberg Time-of-Flight Spectroscopy 32
3.2 Laser System for HRTOF Experiment 34
3.3 Detection 36
3.4 Experimental Details 39
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iii
3.5 Photodissociation of Hydrogen Telluride 42
3.6 References 47
Bibliography 48
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iv
List of Tables
Table 1.1 Cathodic Reduction of Tellurium to Hydrogen Telluride 7
Table 1.2 The Formation of Hydrogen Telluride and Its Yield 9
Table 3.1 Molecular Properties of Group 6 A Dihydrides 42
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List of Figures
Fig 1.1 A Vacuum System for the Synthesis of H2 Te 19
Fig 1.2 Modified Vacuum System for the Synthesis of H2 Te 23
Fig 2.1(a) The Absorption Spectra of Group 6 A Dihydrides 27
Fig 2.1(b) The Absorption Spectrum of H2 0 29
Fig 2.2(a) Absorption Spectrum of Hydrogen Telluride 30
Fig 2.2(b) A Higher Concentration Sample Emphasizes the Long Wavelength
Structure 30
Fig 3.1 Schematic Representation o f HRTOF Apparatus and Generation of
Radiations 36
Fig 3.2 Solid Angle for Detection 38
Fig 3.3 Absorption Spectrum and Product Channel Energies 41
Fig 3.4 Center-of-mass Translational Energy Distribution for
the Photodissociation of H2 Te at 266 nm 44
Fig 3.5 Center-of-mass Translational Energy Distribution for
the Photodissociation of H2 Te at 355 nm 45
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ABSTRACT
Unlike light Group 6 A hydrides, the electrons in tUTe are relativistic and the
molecule is highly unstable: hydrogen telluride gas decomposes upon slightest
provocation and tellurium deposit is an obvious product. A detailed account of
different synthesis methods and a thorough presentation of the experimental details
of synthesizing hydrogen telluride gas using zinc telluride as a precursor are
discussed. Problems encountered applying the current method are addressed and
plausible solutions are presented.
The ultraviolet absorption spectrum of lUTe revealed a fascinating long
wavelength tail that extends ~ 400 nm. Employing the High-n Rydberg Time-of-
Flight technique, photodissociation of hydrogen telluride at 266 nm and 355 nm
wavelengths yielded rovibrationally state- resolved translational energy of H-atom
product channels, both the primary and secondary photolysis of fUTe. The spectra
showed a clear anisotropy of the two spin states of the primary photolysis. The
calculated Do(H-TeH) was 22,700 ± 100 cm"1 .
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Chapter One
Synthesis of Hydrogen Telluride
1
1.1 Anomaly in Atomic Weight of Tellurium
Dating from the late 1870’s through the early 1900’s, the interest in making
hydrogen telluride in its gaseous state arose mostly due to experimental anomalies in
the atomic weight determination of various tellurium samples. These anomalies
most likely occurred because of the five stable isotopes of native ores o f tellurium. It
was the decomposition process of hydrogen telluride into its elemental forms
(hydrogen gas and zero-valent tellurium metal) that assisted in the characterization of
the precise atomic weight.
Tellurium, from a consideration of its physical properties, falls into Group 6 ,
Series 7, of the periodic table1 and its reported accurate atomic weight is 127.6.
However, the atomic weight of tellurium is higher than that of iodine, which has an
atomic weight of 126.9. This outcome was somewhat atypical considering the
principles upon which the periodic table was established. During this period, no
element has had its atomic weight more rigidly scrutinized than tellurium2.
M endeleef in 1869 announced the Periodic Law and in it, he pointed out that the
atomic weight of an element could sometimes be corrected as soon as its properties
are known. He suggested that the atomic weight of tellurium must not be 128 but
rather 123 to 126 in order for tellurium to be positioned before iodine.
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This puzzling statement triggered a thorough, long, and arduous investigation
of determining the true atomic weight of tellurium and many scientists postulated
that tellurium must be composed of several elements. In 1889 Brauner reported
tellurium as a complex substance mixed with two elements1 , which cannot be
separated by ordinary chemical means. In the same year, while taking ultraviolet
spectra of tellurium, copper and antimony, Griinwald discovered an impurity
belonging to a tellurium group weighing 212. In the late 1880’s, Mandeleef1 termed
this unknown element dvitellurium. Many chemists ’ from independent methods of
purifying tellurium and measuring its atomic weight, concluded that the atomic
weight of tellurium should be 127.7 amu. Many years later after using larger
amounts of tellurium and better methods of purification, Brauner concluded that it is
highly improbable that the abnormally high atomic weight of tellurium is due to an
admixture of dvitellurium. W. R. Flint4 in 1912 anchored this idea more precisely.
Following a meticulous purification process of tellurium, he reached a conclusion
that tellurium cannot be of a complex body, thus rejecting the idea presented by
Mendeleef.
The speculation about the existence of dvitellurium and non-homogeneity of
tellurium were no longer a pertinent scientific issue worth pursuing. Like the
anomaly exhibited by tellurium, the same kind of periodicity appears in the
placements of Ar (39.9 amu) and K (39.15 amu); Co (59 amu) and Ni (58.7 amu);
Nd (143.6) and Pr (140.5 amu) in the current periodic table. A. van den Broek5
argued that it is the nuclear charge not the atomic weight of an element which should
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establish and determine the position of the element in the periodic table. W.
Muthmann5 concentrated on the general character of tellurium, especially the
isomorphism of the trigonal forms of tellurium and selenium rather than the atomic
weight. Tellurium is fitted for a place in the periodic table along with sulphur and
selenium. It clearly shows a definite relationship with selenium and sulphur and this
is demonstrated by its bivalency as indicated in by the corresponding compounds5,
TeCl2, ThTe, Ag2Te, (T^Hs^Te, et cetera.
The debate over the issue of the anomalies in the atomic weight of tellurium
ended, however it was the theory of isotopes that really changed the point of view of
atomic weights5. There are eight naturally occuring isotopes of tellurium (five stable
and three radioactive) and they are listed in the order of increasing atomic weight;
1 2 0 Te (0.089%), 1 2 2 Te (2.46%), 1 2 3 Te (0.87%), 1 2 4 Te (4.61%), 1 2 5 Te (6.99%), 1 2 6 Te
(18.71%), 1 2 8 Te (31.79%), and 1 3 0 Te (34.48%). Because o f the presence of isotopes,
scientists in the past had difficulties determining the exact atomic weight of
tellurium.
1.2 Different Synthetic Routes of Hydrogen Telluride
The thermodynamic stability6 of ITTe does not promote its formation by a
direct combination of its constituent elements, i.e. H2 + Te -> H2Te. H2 Te is the
only group VIA hydride that cannot be conveniently prepared from the elements
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since it decomposes readily above 273 K6 ’1 4 . Indeed, pure tellurium can be obtained
by the thermal decomposition of H2Te at 573 K.
Hydrogen telluride can however, be synthesized via many routes. Heating a
mixture of tellurium with nonstoichiometric titanium hydride at 630-650 K will
generate H2Te6. In this case the hydride dissociates from the titanium to give free
hydrogen which then reacts with tellurium in a 2 : 1 ratio to give the desired product.
Another route is the reaction of H2 gas with tellurium electrodes in a discharge tube
which is catalyzed by trace amounts of any volatile organic compounds, such as
dilute acid, to give H 2Te7.
o
A lesser known procedure, put forward by A. C. Voumasos is the formation
of hydrogen telluride based on the cleavage o f a carbon-hydrogen bond by tellurium.
The product is formed when elemental tellurium is heated with anhydrous sodium
formate, Na(H C02 ) at 720-800 K together with some gaseous impurities. The
product was barely enough to run any subsequent reactions. The yield however, is
generally low because at the temperature at which the reaction takes place much of
the products initially formed dissociate back to elemental tellurium and hydrogen
gas. In a similar procedure, he discovered that the reaction of calcium formate with
O C
Te at highly elevated temperature also forms H2Te . In yet another procedure , the
reduction of tellurium complexes such as tellurite or tellurate, T e02, with titanous
chloride in buffered solution with acetic acid and its corresponding ammonium salts
give rise to the desired product H2 Te at 60-70 °C. Two more practical techniques of
obtaining hydrogen telluride are the action of acids on metal tellurides and
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electrolysis using tellurium cathodes. H. Davy5 first discovered the gaseous form of
hydrogen telluride via the electrolysis route as well as the acid on metallic tellurides
route. Numerous chemists employed either or both of these methods to obtain usable
quantities of hydrogen telluride. There has not been any report of a new mode of
synthesis for f^T e in the literature in so far, with most chemists preparing hydrogen
telluride in situ by employing one of the above-mentioned methods or the following
methods.
In the electrolysis method, electricity is passed through an electrolytic
solution between the two electrodes, commonly with a tellurium cathode and
platinum anode. By this means, H. Davy5 observed that hydrogen telluride is
evolved when a solution of potassium hydroxide reacts with the tellurium cathode.
When J. W. Ritter5 performed a similar cathodic reduction of tellurium he discovered
that the electrolytic solution became colored. Therefore by induction, he concluded
that hydrogen telluride must be responsible for the color change since no other
reaction intermediate would cause such an effect.
W. Hempel and M. Weber9 obtained 44.2 % yield of hydrogen telluride from
an electrolysis process with an electrolyte made with 30 % sulphuric acid/70 % water
with a current of 4.5 amperes when applied to a platinum anode and a tellurium
cathode under a potential of 75 to 120 volts. J.C. Poggendorff5, in a similar
experiment, reported practically the same result in generating hydrogen telluride by
electrolysis. Brauner10 also utilized cathodic reduction of tellurium into hydrogen
telluride with a yield of 45 % and 55 % of hydrogen gas. The description of the
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electrolysis set-up is as follows. Inside of a wooden box, an insulating barrier was
placed. Atop the barrier layer, a thin film of zinc was layed. A glass apparatus was
then placed securely. From the bottom up, a thin walled glass tube that contained
tellurium was inserted. Tellurium was melted inside the glass tube and copper wire
was attached before tellurium solidified. Cutting the tip of the glass tube exposed
tellurium. Platinum wire acted as anode. The apparatus was filled with 50 %
sulfuric acid/water solution. Under the potential o f 75-110 volts, a current of 4.5
amperes was applied. The outside cooling temperature of the whole apparatus was
kept at -78 0 and the temperature of electrolyte was kept at 0 °.
E. Emyei11 obtained hydrogen telluride mixed with 5 - 6 % hydrogen gas in
an electrolysis procedure when the device is subjected to a potential of 220 V in a
cooled (-15 0 to -20 °) electrolytic solution of 50 % sulphuric acid/water. Dennis
and Anderson3 employed a meticulously oxygen free environment by removing
oxygen from the electrolyte to obtain a better yield upwards of 82 % in their
investigation. However, the most problematic contaminant to several different
techniques was most likely hydrogen gas.
For the reaction of Vi Te2 + 2 e' +2 H+ -> H 2Te, the standard potential12 is
to -0.50 V at 293 K. The electrolysis of a solution of H 2SO4 or H3PO4 using a
tellurium cathode is the most efficient means for preparation of hydrogen telluride in
terms of yield. Using less volatile electrolytes7 may eliminate the formation of a
harmful analogue of tellurium and volatile sulfur compounds that are difficult to
separate from hydrogen telluride.
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Dennis and Anderson3 reported using 50 % orthoposphoric acid in lieu of
H 3PO4 as a less volatile electrolyte in cathodic reduction of tellurium into hydrogen
telluride. By using orthophosphoric acid instead of H3PO4 as the electolyte, the
volatile phosphorous compounds where eliminated giving rise to smaller amounts of
impurities such as of H 2S and E^Se. The electrolyte was boiled to free dissolved
oxygen, thus keeping the reaction condition as oxygen free as possible, and then was
kept at 273 K.
Cathodic reduction of tellurium using 0.1 M HC1 solution containing KC1 as
a co-electrolyte also produces hydrogen telluride7. Table 1.1 summarizes the results
of the electrolysis method of synthesizing hydrogen telluride with substantial product
yield.
Table 1.1: Cathodic Reduction of Tellurium to Hydrogen Telluride
Electrolyte Voltage (V) Electrolyte Temp. (K) Yield (%) Ref.
50 % H2S04 2 2 0 255-258 94 1 0
30 % H2SO4 75-120 273 44.2 9
50 % H3PO4 1 1 0 273 12-43 3
50 % H3PO4 1 1 0 273 22-82 3
Even though the electrolysis method has its merit in producing hydrogen
telluride with a significant yield, a more experimentally friendly method can also be
used to prepare the gas. That alternate route is the action of acids on metal tellurides
Acid hydrolysis of metal tellurides is a much simpler method for the preparation of
hydrogen telluride gas than the electrolysis route.
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By heating potassium and tellurium together and treating the resulting
complex with dilute hydrochloric acid, H. Davy4 discovered a violent effervescent
union of tellurium and hydrogen whose smell resembled the putrid stench hydrogen
sulfide. The gas produced, H 2Te, decomposed to elemental tellurium and hydrogen
when it came in contact with air. Dilute aqueous mineral acids with metal tellurides
typically give poor yields of hydrogen telluride, but this method is used to prepare
T 0
the gas in small quantities ’ .
Dennis and Anderson3 in 1914 published an extensive report on synthesizing
hydrogen telluride via electrolysis and the action of acids on metal tellurides. In
particular, they explored one particular metallic telluride that could produce larger
quantities of hydrogen telluride. Five different metals were introduced to make the
metallic tellurides; zinc, antimony, iron, manganese, and aluminum. Individually
powdered metal species were mixed in proportion called for by the balanced
emperical formula with a powdered tellurium sample. These mixtures were then
placed in a flask and heated to a red glow or a dull redness until a metallic telluride
was formed. While the combinations of most species with tellurium took place
quietly, the union of aluminum and tellurium created an almost violent explosion.
During a larger scale experiment, the reaction between aluminum and tellurium
created such a violent reaction that fine particles o f the telluride erupted from the
mouth o f the flask into the air. Each metallic telluride was different in its
appearance. ZnTe had a brownish red color while Sb2Te3 was tin-white in color.
FeTe was black, but MgTe was pure white in color. Similar to ZnTe, Al2Te3 was of
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a brownish color. There was an evolution of a mixture of H 2 and f^T e after each
metallic telluride was subjected to decomposition by water or cold, dilute
hydrochloric acid. However, antimony and iron telluride remained unaffected by
hydrolysis with water or acid.
ZnTe exhibited slower reactivity towards water or dilute acid, while MgTe
and Al2Te3 were very responsive. Notably, hydrolysis on Al2Te3 had a robust,
exothermic reaction accompanied by the expected release of a mixture of gases (H2
and H2Te). Overall, it was ascertained that aluminum telluride was the best
candidate for generating hydrogen telluride.
Even though aluminum telluride requires a 2:3 molar ratio of Al:Te, using an
excess of aluminum (30 to 64 %) will produce the desired gas (H2Te) in higher
overall yield. The hydrolysis of Al2Te3 with water yields H2 Te in lower yield than
the action of dilute HC1 on Al2Te3. A study by Gunn6 reinforces the above
statement. The reaction of HC1 with Al2Te3 provides the most efficient means of
producing hydrogen telluride. Table 1.2 lists the reagents used and the subsequent
yields based on this process.
Table 1.2: The Formation of Hydrogen Telluride and Its Yield__________
Metal Telluride Acid Yield (%) Ref.
Al2Te3 Dil. HC1 1 0 3
MgTe Dil. HC1 < 1 0 3
ZnTe Dil. HC1 unknown 3
Al2Te3 Dil. HC1 7.6 1 2
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Robinson and Scott13 also prepared A^Tea by a direct combination of
aluminum and tellurium following the procedure described by Dennis and
Anderson3. Al2Te3 was added in a dropwise fashion to a dilute HC1 solution. The
evolved gases were dried by passing over CaC^ and P2O5 before being condensed in
a vessel cooled by liquid air. Eighty grams of Al2Te3 yielded only about 2 cubic
centimeters of liquid hydrogen telluride.
Brauner10 also studied the acid decomposition of Al2Te3 into H^Te. Small
portions of aluminum telluride were added to 100 ml of a 4N HC1 solution under a
stream o f inert nitrogen gas. When the last portion of Al2Te3 was added, the solvent
was heated for a short time to convert any non-reacted metal tellurides to hydrogen
telluride. Meanwhile, while studying ligands containing the elements of Group 6 A,
Pettit et alu prepared hydrogen telluride in a similar fashion. They began by heating
a mixture of 80 g Te and 30 g A1 to a red glow, to produce aluminum telluride. They
then added this metal telluride in small quantities to deoxygenated 2M HC1 at 273 K,
which generated the desired gas (H2Te). Stewart et al. 15 were also able to prepare
H2Te in the same way by adding an acid to aluminum telluride which subsequently
produced hydrogen telluride. A dropwise addition of 4N HC1 was added to 99.9 %
pure Al2Te3 and the gas was purified by trap-to-trap distillation in vacuo.
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1.3 Characteristics of Hydrogen Telluride
With the exception of H2O, the dihydrides of the group VIA elements are
toxic and must be handled with extreme care1 6 . In general, 16 the thermal stability
and bond strength decreases from H2S to H2P 0 . H2Se as a pure gas is thermally
stable up to 280 °C, but decomposes at temperatures beyond. The dihydrides, such
as H2S, T^Se and E^Te are extremely poisonous gases with revolting odors1 6 . The
toxicity16 of H2S even exceeds that of HCN, which was once used as the gas of
choice for executions (the gas chamber). The toxicity7 of E^Se exceeds that of H2S
and the toxicity of E^Te is comparable to E^Se.
Hydrogen telluride is a colorless, very toxic9,10,17 gas that is unstable to
17 •
moisture and undergoes thermal and photo decomposition . It also decomposes m
the presence of trace amounts of air as well as corks and rubber stoppers1 0 . Great
care must be taken in terms of handling and storage to ensure the gas remains in a
dry, inert, dark, all-glass environment. H2Te bum s11 in air giving off pale blue
flames; the combustion products being H2O and TeC> 2 . Pure liquid H2Te is colorless,
but with a trace amount of oxygen the liquid is deep red, slowly turning to an opaque
crimson liquid3 as the material decomposes into Te0 2 and water. A pure solid
sample of hydrogen telluride is a white crystalline powder, which melts to a colorless
13 18
liquid . Liquid hydrogen telluride, however, decomposes when exposed to light
and becomes faintly yellow colored. A thoroughly dried gas sample of hydrogen
telluride is stable in light, but decomposes13 in contact with dry oxygen gas.
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In 1907 when Baker and Bennett2 were working with H2Te, the severity or
the toxicity effect upon exposure to this compound was not known. In their report,
they revealed, “It is not generally known that tellurium hydride is poisonous. All the
time we were working with this gas, we both suffered from severe headaches,
although as little as possible of the gas was allowed to escape into the air. On one
occasion, one o f us inhaled, by accident, a bubble of the gas. It caused a very severe
attack o f vertigo, which lasted for three days, and was accompanied by a low
temperature, 95.2 -95.8 °F.” This account of the dangerous effects of hydrogen
telluride gas on humans is astonishing. It is now a well-known fact that hydrogen
telluride gas is very toxic and upon inhalation or exposure to a small amount, acute
lung inflammation10 along with throat and nasal inflammation is a common health
hazard. A development of garlic breath from inhalation has been mentioned via
private conversations with other chemists.
The sensitivity to light has been a major hurdle in the synthesis hydrogen
telluride. Legions of chemists5 "6 , 9-10 have proclaimed that the action of light
accelerated the decomposition of hydrogen telluride. While preparing the gas, Gunn6
discovered that the fluorescent room light aided in the decomposition process, and
therefore the handling of the gases was largely done in the dark. The rather rapid
decomposition process of ELTe hindered accurate measurements since tellurium
deposits were left. So, in preparation o f the gas, chemists protected the apparatus
from the light, often performing their experiments in the dark. In some cases, the
collection vessels were painted black15 to exclude any extraneous light.
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• j
To probe the effects of light on hydrogen telluride, Dermis and Anderson
performed an experiment to determine its sensitivity to light and its behavior at
ordinary temperatures. They prepared pure, dry hydrogen telluride gas and filled a
glass bulb, which was completely sealed from the outside environment. By the same
method, four bulbs containing hydrogen telluride were prepared. These four bulbs
were placed in an upright, slightly inclined, position to facilitate descending
movement of hydrogen telluride. Two of the bulbs were positioned under the dark
and the other two exposed to a diffused room light. After some time relapse, the four
bulbs were examined in red light. The inspection revealed that all four bulbs were
identical in appearance: the walls of the glass bulbs were covered in a ‘mirror-like’
finish, alluding to a decomposition process of hydrogen telluride into an elemental
form of tellurium and hydrogen gas. Emyei11 also found that a similar
decomposition process occurs in sealed tubes, even when stored for several days at
sub zero temperatures. It was determined that even at 0 °C, hydrogen telluride
decomposes without any external stimuli, into Te° and H2. This astonishing result
shows that even with exclusion of light, hydrogen telluride gases slowly dissociate.
In order to retard the decomposition process, any sample of hydrogen telluride must
be stored cold or used in situ.
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1.4 Experimental Procedure for the Synthesis of Hydrogen Telluride
The preparation of hydrogen telluride is difficult to carry out at room
- 7
temperature, where it is unstable in all environments . As with any preparative
method of generating hydrogen telluride, the yields are critical depending on the
measures taken to prevent any spontaneous decomposition process generating
elemental tellurium as a silvery metal deposit and hydrogen gas. Stringent exclusion
of oxygen , water, and light need to be exercised. The chief contaminant in this
process is hydrogen gas, as mentioned by Emyei11 and it can be removed by
condensing H2 Te at 77 K 1 3 . All materials coming in direct contact with the gas,
including joints and reaction vessels and containers need to be made o f glass.
Rubber seals should be avoided, as the hydrogen telluride gas would disintegrate any
connection made with this material1 0 .
The work by Stewart et al. 15 provides a means for a starting point in terms of
visualizing the reaction set-up. The application of their apparatus was easy to
conceive, for none o f the reports thus far on the synthesis of H2 Te disclosed the
intimate details of the amounts of reagents used.
The following insights were taken into careful consideration and applied as
needed. First, their two-trap system was employed: the first trap was maintained at
-22.8 °C by a CCLt/dry ice slush to remove residual H2 0 and the second trap at
liquid N 2 temperature to collect the product. Secondly, due to the instability o f H2 Te
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in the presence of light, the entire synthesis was carried out in the dark. Lastly, the
product was stored in a 1 liter Pyrex bulb painted black and stored at 78 K.
The hydrogen telluride gas was synthesized following a refined and modified
version, similar to Stewart et al.u and Dennis and Anderson3. Both research groups
used powdered Al2Te3 and dilute hydrochloric acid to prepare hydrogen telluride;
however, we chose powdered ZnTe and concentrated hydrochloric acid. The
balanced reaction is as follows:
ZnTe + 2HC1 -» ZnCl2 + H2 Te
Zinc telluride was chosen as a precursor mostly, because of unfamiliarity
with Al2Te3 and the dangers associated with making it from its elemental forms.
Aluminum telluride is not commercially available whereas, its counterpart, zinc
telluride is and thus lends another reason for this choice. ZnTe can be purchased
from most chemical suppliers and in very high purity (99.999 %). This compound
was acquired from Alfa Aesar and used as-is without any further purification. The
compound is a brilliant reddish brown color and has a lumpy appearance.
Prior to synthesis, the reaction vessel and collection vessel were dried
overnight at 1500 in an oven and assembled while still hot the following day. Into a
N2 -atmosphere glove box were placed, the reaction flask with Teflon stopcock,
rubber septum, magnetic stirring bar, and a bottle of zinc telluride. Twelve grams of
zinc telluride were crushed to a fine powder with a mortar and pestle and the
pulverized ZnTe was added to the 150 ml round bottom flask equipped with a
sidearm, and the mouth of the flask was sealed with a rubber septum.
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The flask was then taken out of the glove box and attached to a series of two
U shaped tubes and a collection vessel connected by greased ground-glass ball-and-
socket joints with Teflon® stopcocks on either side of each joint. The system was
connected to the glass vacuum line via Tygon® tubing and evacuated. The entire
set-up was evacuated for approximately one hour prior to synthesis to ensure the
complete removal of any air that might be trapped in the system. During the one-
hour evacuation process, the entire apparatus (reactor, traps, and collection vessel)
was flame-dried to remove any residual water from the surfaces of the glass. The
collection vessel was then spray painted black so as to maintain a dark environment
for the temporary storage of the gas (vide supra).
Meanwhile two different types of slush baths were made. A slush bath17 can
be defined as a coolant consisting of a low melting liquid that has been partially
frozen by mixing with liquid nitrogen or dry ice. For this experiment, the first trap
utilized an ethylene glycol/dry ice bath, which was prepared by slowly adding pieces
of dry ice into a Dewar flask containing ethylene glycol while continuously stirring
the mixture until the desired consistency and the temperature were reached. In a
similar manner, an «-pentane/LN2 bath was prepared and used as the coolant for the
F^Te gas collection trap. During the experiment, occasional blending in of more
liquid nitrogen or dry ice was needed to maintain a constant temperature.
Once all traps were sufficiently cooled, a pressure-equalizing addition funnel
was attached to the reaction flask containing the ZnTe under N 2 pressure and the
entire set-up was re-evacuated for approximately 2 0 minutes prior to addition of acid
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into the funnel. About twenty milliliters of concentrated, 12.1 molar, HC1 was
bubbled with dry N 2 for approximately 30 minutes to ensure the removal of any
dissolved gases. The acid then was added to the addition funnel via a small-diameter
(1/16" I.D., 1/8" O.D.) Teflon® tubing cut at a sharp 45° angle under N 2 pressure to
avoid contaminating the acid with air or moisture. A metal cannula was avoided
because o f its reaction with concentrated hydrochloric acid. Once all the acid was
transferred via Teflon® cannula, rubber stopper was removed and replaced with a
glass stopper and the joint was clamped securely.
The reaction was performed under a static nitrogen atmosphere vented to a
mercury bubbler in a well-ventilated hood without any light source apart from a dark
red light used in photography. In a darkened room, portions of 4 molar equivalents
of concentrated HC1 were added to the ZnTe with vigorous stirring. Reaction
progress was monitored by the rate of bubbling of the HCl/ZnTe solution, and the
progression of the reaction color from red to dark grey. As the reaction proceeded,
the Teflon® stopcocks became discolored by metallic tellurium. Although
discoloration served as a visual aid in a darkend room, the decomposition process
was evident.
The H^Te gas produced passed through the first trap (-15 °C dry ice/ethylene
glycol slush cooled U-tube) to remove excess water and to the second trap (-131 °C
n-pentane/LN2 slush cooled U-tube) for the final collection of the gaseous product.
The melting point10 of hydrogen telluride is -49 °.
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18
After the reaction was complete, the reaction flask was sealed off and the U-
tubes and the collection vessel were evacuated. The collection vessel was cooled in
a LN2 bath and opened to the U-tubes as the «-pentane/LN2 bath was removed. This
process allowed the H2 Te to diffuse into the collection vessel free from water and N2.
The yield was improved by back-transferring hydrogen telluride trapped in the
vacuum line LN2 trap, which is placed prior to a vacuum pump, to the collection
vessel and cooling the vacuum line trap was essential. Once collection was
complete, the vessel was sealed and removed from the apparatus. It then was placed
in LN2 Dewar inside of a black acrylic box and transported to spectrophotometer
within five minutes of collection.
The product yields were extremely variable from batch to batch depending on small
variations in the synthetic process, such as how well zinc telluride mixed with
hydrochloric acid and sample collection time, which usually lasted for about an hour.
The entire synthesis took about six hours to complete. The following scheme, Figure
1 .1 , accompanies the vacuum system used for the synthesis of hydrogen telluride.
All joints were ground ball-and-socket joints apart from the o-ring connection
between the collection vessel and the glass vacuum line. The second U-tube has
larger diameter than that of the first U-tube. The capacity of the collection vessel is
260 ml in volume.
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19
Figure 1.1: A Vacuum System for the Synthesis of H2 Te
First
F-Tube
Second
U-Tube
Collection
Vessel
W
1.5 Suggestions for Improving the Synthesis of Hydrogen Telluride
The sample lifetime can be significantly enhanced with an improved
purification process. Although a rather sophisticated vacuum trap-to-trap distillation
apparatus was employed, the purity of the gas was the biggest concern. Still the
production of hydrogen gases predominated and unfortunately there was incomplete
removal of residual water, which aided in degradation process. The decomposition
process was visually evident from the mirror-like finish on the glassware; elemental
tellurium deposits on all of the surfaces of the apparatus.
In order to adequately remove water, the need for drying agents seemed to be
of paramount importance. Drying the product prior to collection should be utilized,
and this can be accomplished by using calcium chloride, CaCl2, in conjunction with
phosphorous oxide, P2O5. Several literature sources6,10,14 report having a drying
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20
agent tube prior to trap-to-trap distillation for the sole purpose of removing trace
amounts of water prior to collection and storage of the product. CaCl2 serves to free
the gases from some of the moisture that they contained while P 2O5 serves to
complete the drying of the gases. Using drying agents such as above will aid in
purity of the gases and increase the lifetime.
This drying process can be implemented by the following suggestion. The
width of the current experimental set-up is approximately thirty inches and the set-up
must be enclosed within the working space of a fume hood. Because of this limited
space issue, the best place to add on a purification section to the reaction would be in
between the side arm of the round bottom flask and the first U-tube (see Figure 1.2
for clarification). Brauner10 conceived the idea of having two units of drying agent
traps to ensure complete water removal form the desired product to minimalize
decomposition. With a shortened Kontes joint from the flask, the ball-and-socket
joint will be replaced by a series two repeating units of glass tubes equipped by 0 -
ring joints on each end: each unit will contain an arrangement of glasswool-CaC^-
glasswool-P2 0 5 -glasswool. O-ring joints would be better suited for this connection
because it would not have any spatial hindrance (its width is narrower than that of
groun ball-and-socket joints) and it would ensue easier packing of the drying agents
into the tube and also easier assembly. This new set-up will improve the purity of
the gases. However, in Dennis and Anderson’s report3, tubes containing drying
agents and inner walls of the glass apparatus that contained H2 Te became blackened
due to a spontaneous decomposition of the gas into an elemental tellurium. This
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21
process unfortunately decreased the percent yield of the purified product, but does
increase the purity of the gas that is collected.
Thus far hydrochloric acid was added to a powdered zinc telluride, as the
basic procedure was adapted from the work by Stewart et a/.1 5 ; However, others9 ' 1 1 ,
13-14 had it otherwise: small portions of metallic telluride were added to a cooled
solution of dilute hydrochloric acid. Laing and Pettit14 have also suggested adding
metallic telluride to a cooled acid solution. Implementing the above idea to make
hydrogen telluride would imply changing the procedure and assessing the effect of
the temperature of the solvent in yielding the desired product. This proposition
would avoid having incomplete mixture of zinc telluride and hydrochloric acid, even
though vigorous stirring was applied. Given the thermal volatility of hydrogen
telluride produced and the reaction between zinc telluride and hydrochloric acid does
not produce heat, chilling a solution of acid might not be a bad idea. In order to cool
the flask containing the acid, having an ice bath underneath the flask would suffice
the task.
The quantity of the acid used in the current experiment was a double the
amount for which the reaction stoichiometry called, therefore adding fractions of
zinc telluride into the solution while vigorously stirring the acid solution would
ensure that the reaction mixture was a homogenous solution. Stirring the mixture
was specifically introduced to avoid having a cakey texture, but sometimes did not
produce a homogenous solution in the past.
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2 2
In order to achieve an oxygen free environment, Brauner1 0 boiled
hydrochloric acid to remove dissolved gases. However, the process of boiling
concentrated hydrochloric acid is tedious and dangerous. A more practical method
of releasing dissolved gases is to bubble nitrogen gas into the acid solution.
In the past, establishing the product yield presented a problem after the
synthesis was complete, for product produced fluctuated from batch to batch. In
order to address this predicament, adding a thermocouple gauge in between the first
and the second U tubes will help to determine the pressure of the gases collecting
into the vessel. Therefore it will indicate the relative yield, and will aid in judging
whether the product will be enough to run any subsequent photochemical
experiments.
Any modification made to the current synthesis set-up will inevitably
accompany procedural change and the following scheme, Figure 1.2, illustrates the
envisioned changes in the vacuum system that will be used in the future synthesis of
hydrogen telluride gases. As prescribed above in the text, the pressure equalized
addition funnel now contains finely powdered zinc telluride and hydrochloric acid is
in a flask, which is being cooled by an ice-bath. Drying agent tubes are inserted
between the flask and the first U-tube and thermocouple gauge (Duniway TCG-531)
in between the two U-tubes.
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Figure 1.2: Modified Vacuum System for the Synthesis of H^Te
*2.
n-Pcntatie/LNi LN : E thylene G lycol/D ry Icc
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24
1.6 References
1. W. L. Dudley and E. V. Jones, J. Am. Chem. Soc. 34, 995 (1912).
2. H. B. Baker and A. H. Bennett, Chem. Soc. Trans. 91, 1849 (1907).
3. L. M. Dennis and R. P. Anderson, J. Am. Chem. Soc. 36, 882 (1914).
4. W. R. Flint, J. Am. Chem. Soc. 34, 1325 (1912).
5. J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical
Chemistry, Volume XI, Te, Cr, Mo, W, (Longmans, Green and Co., London,
1922).
6. S. R. Gunn, J. Phys. Chem. 68, 949 (1964).
7. Inorganic Reactions and Methods, Volume I, Vol., edited by J. J. Zuckerman
(VCH Publishers, Deerfield Beach, 1986).
8. A. C. Voumasos, Chem. Ber. 43, 2272 (1911).
9. W. Hempel and M. G. Weber, Z. Anorg. Chem. 77, 48 (1912).
10. G. Brauner, Handbuch der Praparativen Anorganischen Chemie (Ferdinand
Enke Verlag, Stuttgart, 1975).
11. E. Emyei, Z. Anorg. Chem. 25, 313 (1900).
12. S. A. Awad, J. Phys. Chem. 66, 890 (1962).
13. P. L. Robinson and W. E. Scott, J. Chem. Soc. 972 (1932).
14. D. K. Laing and L. D. Pettit, J. Chem. Soc., D. 2297 (1975).
15. P. A. Montano, H. M. Nagarathna, D. Newlin, and G. W. Stewart, J. Chem.
Phys. 74, 5558 (1981).
16. F. A. Cotton, G. Wilkinson, C. A. Murillo, and M. Bochmann, Advanced
Inorganic Chemistry, 6th ed., (Wiley-Interscience, New York, 1999).
17. Encyclopedia o f Inorganic Chemistry, Volume 8, Vol., edited by R. B. King
(Wiley, New York, 1994).
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25
18. Dictionary o f Inorganic Compounds, Volume 3, Vol., edited by J. E. Macintyre,
F. M. Daniel, and V. M. Stirling (Chapman and Hall, New York, 1992).
19. R. E. Rondeau,/. Chem. And Eng. Data. 11, 124 (1966).
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26
Chapter Two
The Absorption Spectrum of Hydrogen Telluride
2.1 Experimental Details
The UV-VIS absorption cell is made of Pyrex glass measuring 8 cm in length
and 2.54 cm in diameter. In order to fit into the rest of the glass line leading up to
the molecular beam, a Teflon® stopcock and an o-ring joint were attached to the cell.
The high grade 5 mm thick MgF2 windows (CVI Laser Corporation) with diameter
25.4 mm and focal length 50mm were fixed to the cell with high-density glue. Prior
to taking a UV-VIS absorption spectrum of hydrogen telluride, varying pressures of
argon gas were filled into an absorption cell. The range of argon gas pressures was
from 600 mTorr to 1200 mTorr with 200 mTorr increments. The data was saved to a
computer and used as the background samples.
The collection vessel containing freshly prepared hydrogen telluride gas was
attached to the same glass line as the absorption cell. The liquid nitrogen trap was
removed and the solidified hydrogen telluride was allowed to melt under its own
pressure. The system was evacuated shortly after to remove adsorbed hydrogen
gases. Hydrogen telluride gas was then allowed to transfer from the collection vessel
to the UV-VIS cell and the pressure of the gas was monitored with a thermocouple
gauge (Duniway TCG-531). By controlling the stopcock of the cell and pumping off
the excess amount of gases, only the desired pressure of gas was filled in the cell
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27
according to pre-determined argon measurements. The cell containing a specific
pressure of gas was placed in a black acrylic box and transported to a Varian Cary
Series 50 spectrophotometer, whose resolution is 1.5 nm, without further delay to
obtain absorption spectra. All spectra were background substracted. The deposition
of tellurium in mirror-like form on the cell windows was observed at the end of the
run and provided further evidence for the photochemical decomposition of hydrogen
telluride.
2.2 The Absorption Spectra of Group 6A Dihydrides
The absorption onset for the Group 6A dihydrides shows a bathochromic
shift going from the lightest to the heaviest. Figure 2.1 (a) depicts the absorption
spectum of H2S, F^Se, and F^Te.
Figure 2.1 (a): The Absorption Spectrum of Group 6A Dihydrides1
— s
-6
20.000 40000
W * v w number. o n ,* 1
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28
The shift to the longer wavelength absorption suggests that as with heavier
molecule, the electrons are not as tightly bound to the nucleus.
Often the overall shape and possible structures2 of the absorption spectrum
can reveal the dissociation dynamics. When nuclei are subjected to Coulombic
force, the electronic charges redistribute and that is refleted in electronic transitions.
Once an electronic transition is complete, the atoms can vibrate or rotate. The
vibrational/rotational energies have discrete levels, which can be stacked on top of
each electronic level. The absorption spectra show structures characteristic of
vibrational energy levels of the molecule.
In that respect, water is a well-understood system. The first absorption band
of water has been ascribed to electronic transition X 'Ai A 'B i: promotion of non
bonding p orbital to a* orbital. The diffuse structure approximately has a range from
188 nm to 140 nm and has an absorption maxima at ~ 165nm. This particularly
structureless feature reflects the eigenenergies of the symmetric stretch mode in the
upper electronic state2 and indicates a direct dissociation mechanism.
For the absorption spectrum of H2S, unlike its lighter counterpart water, it has
structures with absorption maxima around 198 nm4. The absorption ranges from 260
nm to 160 nm. The structure is attributed to coupling of the bound 'Bi and
dissociative 1 A2 states; the molecules are excited first into a bound state but
dissociate via 1 A2 state. The structure also suggests an indirect dissociation
mechanism2. Figure 2.1 (b) illustrates the absorption spectrum of H20 showing three
lowest absorption bands, A, B, and C.
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29
Figure 2.1 (b): The Absorption Spectrum3 of H2O
HUO
t o
5
o
s 8 1 0 s 7 9 11
^ photon ^ ^
As with hydrogen telluride, there are more outer electrons including d
electrons. The molecule itself is much heavier and has more diffuse electrons: it has
more electronic states contributing to its photochemistry. The absorption for
hydrogen telluride spans from 400 nm to ~ 180 nm. The absorption spectra for
hydrogen telluride are shown in Figure 2.2. Unlike the lighter counterparts of Group
6A dihydrides (H2O, H2S, and lUSe), the absorption spectrum of hydrogen telluride
revealed a long tail extending to ~ 400nm. The spectrum provides an insight into
light sensitivity of hydrogen telluride gas as it extends into the visible region. The
structured tail suggests a spin-forbidden transition (singlet -> triplet) and also around
the 200 to 300 nm region, there can be a lot more complicated curve crossings in the
upper electronic states. It was also noted that the progression at the long wavelength
tail region appears at energies that are close to that of the TeH (2rii/2) excited spin-
orbit channel5.
Figure 2.2 (a) illustrates the main absorption feature, which includes a
pronounced structure peaked around 250 nm and the long wavelength tail extending
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30
to ~ 400 nm5. Using a higher sample concentration, an emphasized tail region is
shown in Figure 2.2(b).
Figure 2.2 (a): Absorption Spectrum of Hydrogen Telluride5 (b) A Higher
Concentration Sample Emphasizes the Long Wavelength Structure5
C
3
€
c d
d >
o
c
410 420 370 380 390 400
V )
c
3
€
CD
CD
O
c
■ i
o
( / )
.Q
CD
200 250 300 350 400 450
wa\elength (nm) wavelength (nm)
If the PES (potential energy surface) were known for the system of H 2Te, it
might be easier to complete the picture o f understanding the complicated structures
that were observed in the absorption spectrum. However, at this point of the current
experiment, the shortest wavelength member of this progression was interpreted as a
clue that the 2n i /2 level might play an important role in the photochemistry5.
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31
2.3 References
1. C. F. Goodeve andN. O. Stein, Trans. Farad. Soc. 27, 393 (1931).
2. R. Schinke, Photodissociation Dynamics, (Cambridge University Press,
Cambridge, 1995).
3. P. Gtirtler, V. Saile, and E. E. Koch, Chem. Phys. Lett. 51, 386 (1977).
4. C. Y. Roberts and F. Z. Chen, J. Quant. Spectrosc. Radiat. Transfer 60, 17
(1998).
5. J. Underwood, D. Chastaing, S. Lee, P. Boothe, T. C. Flood, and C. Wittig,
Chem. Phys. Lett. 362, 483 (2002).
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32
Chapter Three
The Photochemistry of Hydrogen Telluride
3.1 High-rc Rydberg Time-of-Flight Spectroscopy
Atoms in states of high principal quantum number, Rydberg atoms, possess
unique properties such as long life-times and susceptibility to field ionization. The
HRTOF (High-« Rydberg H-atom Time-of-Flight) method has been widely
employed in studies of photodissociation and bimoleular reaction dynamics yielding
H atom products. In the HRTOF technique, H atoms are photoexcited to high-n
levels immediately after being produced, and allowed to fly as neutrals to a detector
where they are field ionized and counted.
The primary advantages of this method involve several factors. Most
importantly, the products evolved are spatially neutral H atoms, thus resolution-
limiting space-charge effects are avoided. Secondly, the mass analysis is not
necessary as the products are selected spectroscopically. The third benefit is that the
relative uncertainty in flight length can be made much smaller than is possible with
conventional photofragment spectroscopy. The last advantage of using HRTOF
technique is that high efficiencies may be achieved during the Rydberg tagging
process because photoexcitation is doubly resonant.
Because H atom fragments are much lighter than their counterffagments,
measured recoil velocities are far larger than the initial velocity of the parent
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33
molecule, leading to very small uncertainties in measured kinetic energy release
distribution. As a result, vibrational resolution is normally achieved and also in
some cases, rotational structure can be observed.
The principal of the HRTOF (High-n Rydberg Time-of-Flight) technique
involves measuring the velocity (time-of-flight) of the H-atom products, and using
the law of conservation o f energy and momentum to convert to the kinetic energy
release of the photofragments. The observed spread of the kinetic energies reflects
the internal energy of the unobserved moiety2. The translational energy of a mass-
selected photofragment is measured from its time of flight between the region of
photodissociation and the detector.
The internal energy o f a photofragment TeH resulting from photodissociation
of the hydride HTeH in the process (HTeH + hv -> H + TeH) can be obtained by
measuring the translational energy of the H atom. Since this has no internal energy,
that of the molecular fragment, E jnt(TeH), is given by the following formula:
E int(TeH) = hv - Do(HTe-H) -E trans, where the terms include hv; the energy of
photon, Do; dissociation energy of HTe-H bond, E transi the total translational energy
of two fragments. The advantage of the H atom as a photofragment is that it carries
most of the translational energy2. For instance, in the photodissociation of H2Te to
give H + TeH, the conservation of translational energy requires that E trans, to tal =
(1 2 9 / 1 2 8 ) Etrans(H).
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34
3.2 Laser System for HRTOF Experiment
Three different laser radiations were employed in the current HRTOF
experiment.
Photolysis Laser
An Nd:YAG laser (Continuum Powerlite 9010) served as a photolysis laser
producing its fourth harmonic generation of 266 nm radiation and third harmonic
generation of 355 nm radiation. Along the beam path to the chamber, an iris controls
the flux of the radiation such that only a center o f beam goes through and a half
wave plate (Newport) that polarizes the photolysis radiation either horizontally or
vertically with respect to the lab frame. The radiation is focused into the chamber
and counterpropagates with the 365 nm radiation for the Rydberg transition. Typical
output energy from the photolysis laser was about 80 mJ/pulse; however, several
experiments were performed with varying powers ranging from 20 to 65 mJ/pulse.
Lyman-a Transition
Lyman-a radiation at 121.6 nm promotes H(1 £)-> H(2 P). The frequency
tripling scheme, a nonlinear frequency mixing process, generates the VUV radiation
at 121.6 nm. A dye laser (Continuum ND 6000) operating with LDS 750 dye is
pumped by the second harmonic generation of a Nd:YAG laser (Continuum
Powerlite 8010). The output of the dye laser is focused and doubled by KDP crystals
inside of an Autotracker (Inrad Autotracker III). The frequency o f the fundamental
364.8 nm radiation is tripled to produce 121.6 nm radiation. This VUV radiation is
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35
then focused at the interaction region by a MgF2 lens. The intensity o f the VUV
radiation is estimated by measuring the NO+ ion current from the photoionization of
NO. This gives an intensity of ~108 photon/pulse which corresponds to a frequency
tripling efficiency of 10'6. An enhanced laser intensity has been observed when a
certain amount of Ar is added into the Kr cell. Typical energy output prior to
tripling cell was approximately between 10 and 15 mJ.
Rydberg Transition
The 365nm radiation for the Rydberg excitation H(22 P) -> H(n~50) is
produced from 308 nm radiation by a XeCl excimer laser (Lambda Physik Compex
201) pumped by a dye laser (Lambda Physik FL 3002) operating with DMQ dye.
The output laser beam is focused and introduced into the interaction chamber. The
output power was on average 10 to 13 mJ. It is imperative that the Rydberg state
excitation must temporally overlap with the Lyman-a transition because the lifetime
of H(2 P) is short-lived, 1.6ns. The resulting Rydberg hydrogen atoms have a
lifetime of 100ms.
A simplified version of a schematic HRTOF apparatus is shown in Figure
3.1. This illustration indicates the polarized photodissociation radiations (266 and
355 nm) and radiations employed to create high-n Rydberg hydrogen atoms (121.6
nm + 365 nm).
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36
Figure 3.1: Schematic Representation of HRTOF Apparatus1 and Generation of
Radiations
detector
H atom flight path
53.5 cm
Lyman-a, 121.6 nm
^ Rydberg, 365 nm
pulsed nozzle
molecular beam
horizontal polarization
photolysis laser
266 nm, 355 nm
vertical polarization
— 266 nm
355 nm
365 nm
FHG
Dye
Dye
THG
Nd:YAG
Excimer
Nd:YAG
3.3 Detection
In the view of the lab frame, the detection set-up is featured in Figure 3.1.
Molecular beam and the photolysis beam lie perpendicular with respect to the
detection axis. The molecular beam provides a collision free environment, i.e.
molecules travel with a well-defined velocity without any collision among
molecules. In order for a greater probability of transition to occur, the transition
dipole moment of a molecule, f i , must align the electric field vector, E . It follows
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37
from p . £ = \ju\s\ cos 6 3> where 0 is the angle between the p and E. Hence when the
angle is zero, i.e. transition dipole moment and the electric field vector are perfectly
aligned, the transition probability is the greatest and the molecule will absorb a
photon. The detection scheme is set up such that the detection is sensitive to
anisotropy. If the photolysis laser light is polarized along the z-axis in the lab frame,
therefore parellel or vertical along with the detection axis, then a molecule with its
dipole pointing in the same z direction will be observed, albeit there are molecules
whose dipole moments are aligned in other directions. Not every molecule is
oriented perfectly in line with the polarized electric field vector, some will have a
cosine angle dependence within the solid angle limit and they will make the
transition and will be detected according to the detection limit.
The solid angle is determined by the length of the time-of-flight (djoF) tube
from the interaction point and the radius of the multichannel plates (T m c p ) - Therefore
any modifications in the time-of-flight tube length and the radius of MCP’s will
change the solid angle. The solid angle obtained from this experiment is 6.8° and
Figure 3.2 depicts the diagram of the solid angle. The change in the direction of E
either vertical or horizontal with respect to the detection axis will give information
about the different transition dipole moments of a molecule, and those molecules
whose p align with E will have the greatest transition probability. Whether the
current set-up is mechanically sensitive to detect one electronic state over another is
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38
up for debate and in order to solve the problem, such changes in the solid angle
should occur.
Figure 3.2: Solid Angle for Detection
fM cp= 6.35 cm
UCP
In order to prolong the lifetime of Rydberg atoms, a weak dc electric field
(about 20 V/cm) is applied in the laser interaction region where molecular beam
intersects with all three radiations. It prevents residual H+ ions, which were
produced from the photodissociation events, from drifting toward the MCP detector.
The weak field helps to break down the selection rule . When hydrogen atoms are
immersed in an external electric field, without any magnetic field present, the
energies of sublevels are shifted and mixed . The Stark effect provides a pass for a
lower / state, which is initially populated via optical excitation, to switch into a
higher Rydberg state with higher I states, which have longer lifetime than that of
lower I states. Thus, Rydberg H atoms in this electric field achieve extra stability
and do not decay prior to reaching detectors. The high-n Rydberg atoms thus
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39
produced are radiatively metastable and stay highly excited for at least 100 ms, while
they drift with their nascent velocities to the Chevron MCP detector. Upon arrival at
the detector, the excited atoms are efficiently field-ionized as they pass a wire mesh
in front of the detector; they are then detected as ions.
3.4 Experimental Details
The HRTOF technique has been applied to the H atom product to obtain the
center-of-mass (c.m.) translational energy distribution. The molecule under
consideration, H2Te in the current experiment, is seeded to a small concentration in
argon as a carrier and is emitted through a 0.75 mm diameter pulsed nozzle assembly
(General Valve, Series-9) into a high vacuum source chamber. A glass vacuum line
was installed in order to accommodate hydrogen telluride gas made freshly from the
synthesis. The collection vessel containing hydrogen telluride gas was attached to
the glass line and the gas was allowed to thaw under its own pressure. The
molecular beam was produced by mixing hydrogen telluride and argon gas in small
concentration. The molecular beam was differentially pumped. The expanding gas
pulse is then skimmed by means o f a small aperture (~ 1 mm diameter) located
directly 2 cm downstream in front of the nozzle assembly4. The expansion allows
isentropic flow of the parent molecules into the main interaction chamber. The
resulting highly collimated gas beam enters the main chamber and has a low internal
energy and a well-defined velocity distribution.
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40
The polarization of the photolysis laser beam was achieved by rotating a half
wave plate and the linear polarization was nearly pure. Hydrogen atoms produced
from photodissociation were excited by 121.6 nm VUV radiation to the 2 2 P state.
This radiation was generated by tripling the 364.7 nm output from a Nd:YAG
pumped dye laser in krypton, and was focused in the interaction region by a MgF2
lens. An excimer pumped dye laser further excited the H atoms from 2 P to high-n
Rydberg level (n ~ 40-90) and the high-w Rydberg states are radiatively metastable
and remain highly excited for many tens of microseconds. A small percentage of
these excited atoms, the neutral Rydberg atoms, drifts with their nascent velocities to
a multichannel plate (MCP) detector (Burle Electro-Optics), where they are detected
as ions after being efficiently field ionized in front of the MCP. The flight path is
53.3 cm. Time-of-flight spectra were recorded and averaged. Then they were stored
in a computer and then they were later converted to center-of-mass (c.m.)
translational energy spectra.
The previously taken absorption spectrum is converted in wavenumber scale
from wavelength in order to show the product channel energies. The following
figure, Figure 3.3 depicts the locations in the absorption spectra that are accessed by
using 266 and 355 nm photolysis radiations. The two arrows indicate the
photodissociation wavelengths used in this study. The primary photolysis
photoproduct channels were accessed via photolysis at 266 and 355 nm radiations:
H2Te + hv -> H(2S) +HTe(2 TIi/2) 2 lT3/2). The H-TeH bond dissociation energy, Do,
obtained from this present study is 22,740 cm'1 and it is indicated by the TeH (2 f l 3/2)
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41
+ H product channel. The spin-orbit excited product channel, TeH ( rii/2) + H, is
5 2 2
shown using the value of theoretical spin-orbit splitting ( II3/2- IT1/2).
Figure 3,3: Absorption Spectrum and Product Channel Energies1
Absorption
50 -
40 -
266 nm
a
o
0
J 30 -
G O
u
( D
X
1
a
< D
£ 2 0 -
£
355 nm
10 -
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42
3.5 Photodissociation of Hydrogen Telluride
Hydrogen telluride gas has gained its notoriety of thermal instability, foul
smell, and light sensitivity since mid 1800’s. The interest in the anomaly of the
atomic weight of tellurium led to a legion of hydrogen telluride syntheses. The chief
motivation behind pursuing the photodissociation of hydrogen telluride arose owing
partly to its unique ultraviolet absorption features that had not been observed in other
Group 6A dihydrides and the lack of photochemical experiments in the past. The
electrons in H2Te are relativistic in origin and therefore the molecule itself embodies
the heavy atom effect and highly correlated spin-orbit coupling. Qualitative effects
of relativity can be summarized in the following statement: due to the relativistic
mass increase, the effective Bohr radius will decrease for inner electrons with large
average speeds6. In general, relativistic energy contributions increase6 by Z2.
The large TeH spin-orbit splitting5 of 3815 cm'1 compared with lighter Group
6A monohydrides (OH, SH, and SeH) is indicative of relativistic effect. There is a
marked difference in the spin-orbit (S/O) splitting energies within the group and
below is a summarized table for the Group 6A dihydrides.
Table 3.1: Molecular Properties of the Group 6A Dihydrides________
Molecule Re [A]
© e[°]
D0 [cm'1 ] S/O [cm'1 ]
H2 0 1.808 7 104.52 7 41279.43 8 126 1 0
h 2s 2.524 7 92.12 7 31536.14 8 378 1 0
H2Se 2.759 7 90.57 7 27630.65 9 1815 1 0
H2 Te 3.135 7 90.26 7 22740 1 3815 5
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43
The center-of-mass (c.m.) translational energy spectrum of hydrogen telluride
was obtained from the photodissociation at 266 nm radiation. The radiation was
linearly polarized in either horizontal or vertical fashion in a lab frame in order to
observe any anisotropic photoproduct distributions. The spectrum revealed a strong
spatial anisotopy for the dissociation: the different spin-orbit states were accessed
and many rovibrational, internal states of TeH were populated. The spin-orbit
ground product channel, TeH ( IT3/2), was derived from the transition dipole moment
perpendicular to the H2 Te molecular plane and the spin-orbit excited state, TeH
(2 T1i/2 ), was from the transition dipole moment lying in the plane.
Using various spectroscopic constants5’1 1 , all of the observed TeH excitations
were fitted. The major peaks from the primary photolysis process were
spectroscopically assigned and the secondary photolysis of H2 Te gave rise to lumpy
features around higher energy region. The center-of-mass translational energy
distribution from the photodissociation of H2 Te at 266 nm is featured in Figure 3.4
and the internal states of TeH have been labelled.
The spin-orbit splitting energy has been approximated to be ~ 3820 cm '1 , and
it was determined by taking the energy difference between TeH (2r i 3/2, v=0) and TeH
( n1 / 2 , v=0). This value is in an excellent agreement with the accurate value of
3814.48 cm '1 obtained spectroscopically5. The distribution shown in Figure 3.4
yielded the bond dissociation (H-TeH) energy, Do = 22,740 ± 30 cm '1 . This
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44
experimental value is within the theoretical prediction suggested by Sumathi and
i
Balasubramanian .
Figure 3.4: Center-of-mass Translational Energy Distribution for the
Photodissociation of H^Te at 266 nm.
266 nm horizontal
266 nm vertical
15000 20000 10000 5000
-1
c.m. translational energy (cm )
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45
Under the same experimental condition, the photodissociation experiment at
355 nm radiation was performed.
Figure 3.5: Center-of-mass Translational Energy Distribution for the
Photodissociation at 355 nm
2n 3/2,v=o
355 nm horizontal
2 .
0 n
355 nm vertical
1000 1200 1400 1600 1800
2000 6000 8000 10000 4000
-1
c.m. translational energy (cm )
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46
Going into a longer photolysis wavelength allowed for the examination of the
photodissociation behavior of hydrogen telluride at its distinctive long absorption tail
region. The longer wavelength absorption region of hydrogen telluride was accessed
and the center-of-mass translational energy distributions revealed a significant
population inversion: there was a tendency toward the selectivity for spin-orbit
excited states of TeH. The spectra at both photolysis wavelengths (266 and 355 nm)
generated a strong spatial anisotropy that is consistent with the rapid dissociation
behavior. Figure 3.5 shows the center-of-mass translational energy distributions for
the 355 nm dissociation. The arrow indicates the distribution of the c.m.
t 'j
translational energies associated with the TeH ( TIi/2, v=0) rotations, i.e. rotational
levels of TeH. After spectroscopic fitting, the average rotational energy is
determined to be ~ 60 cm4 . Again, the photolysis of TeH led to lumpy features in
the spectra. A preliminary estimate of the branching indicated that about two thirds
of TeH is found in their spin-orbit excited states.
The photodissociation dynamics associated with different photolysis
wavelengths for hydrogen telluride present unique aspect of photochemistry: namely
the presence of the long wavelength tail in the absorption spectrum and the
preferential population of spin-orbit excited states of TeH in the 355 nm
photodissociation processes. The interpretation of the 355 nm photodissociation
results lends itself to collaborative high-level theoretical treatment and more
comprehensive analyses that follow the preliminary findings.
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47
3.6 References
1. J. Underwood, D. Chastaing, S. Lee, P. Boothe, T. C. Flood, and C. Wittig,
Chem. Phys. Lett. 362, 483 (2002).
2. J. Michael Hollas, High Resolution Spectroscopy, 2n d Ed., (John Wiley & Sons
Ltd., New York, 1998).
3. P. W. Atkins and R. S. Friedman, Molecular Quantum Mechanics, 3rd Ed.,
(Oxford University Press, Oxford, 1997).
4. J. Zhang, M. Dulligan, J. Segall, Y. Wen, and C. Wittig, J. Phys. Chem. 99,
13680(1995).
5. D. A. Gillett, J. P. Towle, M. Islam, J. M. Brown, J. Mol. Spectrosc. 163, 459
(1994).
6. P. Pyykko, Chem. Rev. 88, 563-594 (1988).
7. L. Pisani and E. Clementi, J. Chem. Phys. 101, 3079 (1994).
8. D. R. Stull and H. Prophet, Project Directors, JANAF Thermochemical Tables,
2nd Ed., Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. (U.S.) 37, (1971).
9. S. T. Gibson, J. P. Greene, and J. Berkowitz, J. Chem. Phys. 85, 4815 (1986).
10. K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, (Van
Nostrand Reinhold Company, NY, 1979).
11. E. H. Fink and K. D. Setzer, J. Mol. Spectrosc. 138, 19 (1989).
12. K. Sumathi and K. Balasubramanian, J. Chem. Phys. 92, 6604 (1990).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
48
Bibliography
P. W. Atkins and R. S. Friedman, Molecular Quantum Mechanics, 3rd Ed.,
(Oxford University Press, Oxford, 1997).
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H. B. Baker and A. H. Bennett, Chem. Soc. Trans. 91, 1849 (1907).
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Enke Verlag, Stuttgart, 1975).
F. A. Cotton, G. Wilkinson, C. A. Murillo, and M. Bochmann, Advanced
Inorganic Chemistry, 6th ed., (Wiley-Interscience, New York, 1999).
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D. A. Gillett, J. P. Towle, M. Islam, J. M. Brown, J. Mol. Spectrosc. 163, 459
(1994).
C. F. Goodeve and N. O. Stein, Trans. Farad. Soc. 27, 393 (1931).
S. R. Gunn, J. Phys. Chem. 68, 949 (1964).
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J. Michael Hollas, High Resolution Spectroscopy, 2nd Ed., (John Wiley & Sons
Ltd., New York, 1998).
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49
K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure, (Van
Nostrand Reinhold Company, NY, 1979).
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(Wiley, New York, 1994).
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F. M. Daniel, and V. M. Stirling (Chapman and Hall, New York, 1992).
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Chemistry, Volume XI, Te, Cr, Mo, W , (Longmans, Green and Co., London,
1922).
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(1998).
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R. E. Rondeau, J. Chem. And Eng. Data. 11, 124 (1966).
R. Schinke, Photodissociation Dynamics, (Cambridge University Press,
Cambridge, 1995).
D. R. Stull and H. Prophet, Project Directors, JANAF Thermochemical Tables,
2nd Ed., Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. (U.S.) 37, (1971).
K. Sumathi and K. Balasubramanian, J. Chem. Phys. 92, 6604 (1990).
J. Underwood, D. Chastaing, S. Lee, P. Boothe, T. C. Flood, and C. Wittig,
Chem. Phys. Lett. 362, 483 (2002).
A. C. Voumasos, Chem. Ber. 43, 2272 (1911).
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50
J. Zhang, M. Dulligan, J. Segall, Y. Wen, and C. Wittig, J. Phys. Chem. 99,
13680(1995).
Inorganic Reactions and Methods, Volume I, Vol., edited by J. J. Zuckerman
(VCH Publishers, Deerfield Beach, 1986).
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Asset Metadata

Creator Lee, Sun Young (author)

Core Title Synthesis and photochemistry of hydrogen telluride

School Graduate School

Degree Master of Science

Degree Program Chemistry

Degree Conferral Date 2004-08

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag chemistry, inorganic,chemistry, physical,OAI-PMH Harvest

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