Page 174 |
Save page Remove page | Previous | 174 of 238 | Next |
|
small (250x250 max)
medium (500x500 max)
Large (1000x1000 max)
Extra Large
large ( > 500x500)
Full Resolution
All (PDF)
|
This page
All
|
lieu of the actual data below the freezing point in hydrogen, it is instructive to review the
temperature dependence of the density of liquid 4He.28 The density of 4He liquid
increases upon decrease of the temperature until it reaches superfluid transition
temperature at T ≈ 2.17 K. Below this temperature, the density suddenly decreases from
1.462 g/cm3 to 1.451 g/cm3 and stays to within 0.1% of the latter value down to T = 0 K.
If a similar drop in density is realized in the case of liquid hydrogen upon predicted
transition to the superfluid state, the frequency of the Q1(0) line must have similar
discontinuity, thus providing a convenient monitor of the phase transition.
So far, substantial supercooling of macroscopic samples of liquid hydrogen could
not be demonstrated. Recently, we have shown that supercooled pH2 clusters of about
104 molecules can be prepared in a cryogenic expansion of pH2 seeded in an excess of
He.10 Based on the equation of state of the adiabatic gas expansion, the temperature of
the clusters was estimated to be in the range of T = 1 - 2 K. The frequency of the Q1(0)
line of the liquid clusters was found to be 4150.4 ± 0.1 cm-1. Using the extrapolated
frequency in Fig. 6.6 will place the temperature of the clusters in the range of T < 5 K.
Therefore, the temperatures of the clusters are indeed much lower than the freezing
temperature of the pH2 liquid at 13.8 K, which supports extensive supercooling.
Unfortunately, in clusters, the temperature is not the sole cause of the frequency shift.
Additional shift may also be caused by the finite size effects in the clusters.7 Upon
decrease of the cluster size, the Q1(0) frequency must approach its single molecule value,
i.e., the frequency is expected to rise. Therefore, the frequency of the Q1(0) line in
clusters at a given temperature is expected to have some upward shift with respect to that
150
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 174 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | lieu of the actual data below the freezing point in hydrogen, it is instructive to review the temperature dependence of the density of liquid 4He.28 The density of 4He liquid increases upon decrease of the temperature until it reaches superfluid transition temperature at T ≈ 2.17 K. Below this temperature, the density suddenly decreases from 1.462 g/cm3 to 1.451 g/cm3 and stays to within 0.1% of the latter value down to T = 0 K. If a similar drop in density is realized in the case of liquid hydrogen upon predicted transition to the superfluid state, the frequency of the Q1(0) line must have similar discontinuity, thus providing a convenient monitor of the phase transition. So far, substantial supercooling of macroscopic samples of liquid hydrogen could not be demonstrated. Recently, we have shown that supercooled pH2 clusters of about 104 molecules can be prepared in a cryogenic expansion of pH2 seeded in an excess of He.10 Based on the equation of state of the adiabatic gas expansion, the temperature of the clusters was estimated to be in the range of T = 1 - 2 K. The frequency of the Q1(0) line of the liquid clusters was found to be 4150.4 ± 0.1 cm-1. Using the extrapolated frequency in Fig. 6.6 will place the temperature of the clusters in the range of T < 5 K. Therefore, the temperatures of the clusters are indeed much lower than the freezing temperature of the pH2 liquid at 13.8 K, which supports extensive supercooling. Unfortunately, in clusters, the temperature is not the sole cause of the frequency shift. Additional shift may also be caused by the finite size effects in the clusters.7 Upon decrease of the cluster size, the Q1(0) frequency must approach its single molecule value, i.e., the frequency is expected to rise. Therefore, the frequency of the Q1(0) line in clusters at a given temperature is expected to have some upward shift with respect to that 150 |
