Page 154 |
Save page Remove page | Previous | 154 of 238 | Next |
|
small (250x250 max)
medium (500x500 max)
Large (1000x1000 max)
Extra Large
large ( > 500x500)
Full Resolution
All (PDF)
|
This page
All
|
enthalpy and entropy of the liquid upon mixing. The P, T, and x relationships of liquid
hydrogen isotopic mixtures have previously been studied in the temperature range of 18 –
28 K.33 Lambert34 measured the excess enthalpy, HE, for a number of compositions of p-
H2/o-D2 mixtures at 20.4 K. The excess enthalpy was expressed as:
HE = α⋅x(H2)⋅x(D2) 5.6
where α = 11.8 cal/mol. A value of HE was also reported for an equimolar mixture of p-
H2/o-H2 at 20.4 K of 0.4 cal/mol, which is almost 8 times smaller.34 Similar results for p-
H2/o-D2 mixtures were also observed by Knaap et al.35 However, less agreement is
observed between studies of the excess volume of H2/D2 mixtures, in which the deviation
from ideality was observed to be in the range of -0.6 to -1.2% at temperatures from 15 to
20.4 K34,35 . This is in contradiction to the positive values of the deviation predicted by
Prigogine.13,28 In addition, as observed by Knapp35, a non-zero excess entropy implies
that mixtures of H2/D2 isotopes do not form “regular solutions” where both excess
entropy and molar volume are equal to zero.36
Clusters of about 104 molecules (X = 1 %) as studied in the present work differ
from the bulk phase in that they have a large fraction of surface molecules of about
20%.37 Molecular dynamics simulation of classical binary Lenard-Jones clusters show a
broad phase diagram depending on the size of the particles involved and the relative
depth of the potential wells.38 For example, classical Lenard-Jones clusters with εAA <
εAB < εBB energy parameters and σAA > σAB > σBB size parameters for species A and B are
130
Object Description
| Title | Infrared and Raman spectrosopy of molecules and molecular aggregates in helium droplets |
| Author | Sliter, Russell Thomas |
| Author email | sliter@usc.edu; sliterr@gmail.com |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Chemistry |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2011-04-21 |
| Date submitted | 2011 |
| Restricted until | Unrestricted |
| Date published | 2011-04-26 |
| Advisor (committee chair) | Vilesov, Andrey F. |
| Advisor (committee member) |
Reisler, Hannah Kresin, Vitaly V. |
| Abstract | This dissertation covers several different aspects of spectroscopy of molecules and molecular clusters embedded in low-temperature matrices, such as helium droplets. First, details on the formation and optimization of He droplets will be discussed. A new method of measuring droplet sizes for cw nozzle expansions using mass spectrometry was developed. The results of the measurements of the sizes of the droplets in pulsed expansion as a function of temperature will be described. Details on the electron-impact ionization of He droplets will also be discussed as well as a simple method of modeling the ionization and excitation of He atoms in the droplet. In addition, preliminary measurements on the size distribution of He droplets produced at very low temperature of 5 – 7 K in continuous expansion will be addressed.; Using matrix isolation in He droplets, vibrational spectra of clusters consisting of para-H₂ or para-H₂/D₂ have been obtained using coherent anti-Stokes Raman spectroscopy (CARS). The vibrational frequency of para-H₂ molecules obtained upon expansion of neat para-H₂/D₂ gas or liquid was found to be very similar to that in bulk solid samples having equal composition. On the other hand, spectra in clusters obtained upon expansion of 1% para-H₂/D₂ clusters seeded in He are liquid and have a considerable frequency shift, which indicate phase separation of the two isotopes in clusters at low temperature. The onset of phase separation in para-H₂/D₂ mixtures is predicted at approximately 3 K providing further evidence of super-cooled liquid hydrogen clusters.; To address the Raman spectra observed in liquid H2 clusters, vibrational and rotational spectra of bulk liquid para-H2 at temperature of T = 14 – 26 K and of solid at T = 6 – 13 K have been obtained using coherent anti-Stokes Raman scattering technique. The vibrational frequency in the liquid increases with temperature by about 2 cm⁻¹, and the shift scales with the square of the sample’s density. An extrapolation of the vibrational frequency in the metastable para-H₂ liquid below the freezing point is discussed. The results indicate that the vibron hopping between the molecules is active in the liquid, similar to that previously found in the solid.; Matrix isolation has also been performed in argon solid matrices based on a custom-made cryogenic optical cell. Single water molecules have been isolated in solid Ar matrices at 4 K and studied by ro-vibrational spectroscopy using FTIR in the regions of the v₁, v₂, and v₃ modes. Upon nuclear spin conversion at 4 K, essentially pure para-H₂O was prepared followed by subsequent fast annealing generating ice particles. FTIR studies of the vapor above the condensed water upon annealing to T ≥ 250 K indicate fast re-conversion of nuclear spin to equilibrium conditions. Our results indicate that nuclear spin conversion is fast in water dimers and larger clusters, which preclude preparation of concentrated samples of para-H₂O, such as in ice or vapor. |
| Keyword | Helium droplets; laser spectroscopy; matrix isolation; superfluidity; clusters |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m3778 |
| Contributing entity | University of Southern California |
| Rights | Sliter, Russell Thomas |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-Sliter-4404 |
| Archival file | uscthesesreloadpub_Volume23/etd-Sliter-4404.pdf |
Description
| Title | Page 154 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | enthalpy and entropy of the liquid upon mixing. The P, T, and x relationships of liquid hydrogen isotopic mixtures have previously been studied in the temperature range of 18 – 28 K.33 Lambert34 measured the excess enthalpy, HE, for a number of compositions of p- H2/o-D2 mixtures at 20.4 K. The excess enthalpy was expressed as: HE = α⋅x(H2)⋅x(D2) 5.6 where α = 11.8 cal/mol. A value of HE was also reported for an equimolar mixture of p- H2/o-H2 at 20.4 K of 0.4 cal/mol, which is almost 8 times smaller.34 Similar results for p- H2/o-D2 mixtures were also observed by Knaap et al.35 However, less agreement is observed between studies of the excess volume of H2/D2 mixtures, in which the deviation from ideality was observed to be in the range of -0.6 to -1.2% at temperatures from 15 to 20.4 K34,35 . This is in contradiction to the positive values of the deviation predicted by Prigogine.13,28 In addition, as observed by Knapp35, a non-zero excess entropy implies that mixtures of H2/D2 isotopes do not form “regular solutions” where both excess entropy and molar volume are equal to zero.36 Clusters of about 104 molecules (X = 1 %) as studied in the present work differ from the bulk phase in that they have a large fraction of surface molecules of about 20%.37 Molecular dynamics simulation of classical binary Lenard-Jones clusters show a broad phase diagram depending on the size of the particles involved and the relative depth of the potential wells.38 For example, classical Lenard-Jones clusters with εAA < εAB < εBB energy parameters and σAA > σAB > σBB size parameters for species A and B are 130 |
