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Chapter VI. Temperature dependence of the Raman spectra of
liquid para-hydrogen
6.1. Introduction
Superfluidity of hydrogen, predicted three decades ago,1 continues to elude
experimental observation. However, hydrogen may constitute a new class of molecular
superfluids that are characterized by anisotropic interaction and rotational angular
momentum of the particles. Normal hydrogen consists of two modifications: para-H2
(pH2) and ortho-H2 (oH2). pH2 molecules are spinless Bosons (I = 0) and at low
temperature reside in the ground rotational state J = 0, which makes pH2 the prime
candidate for observation of superfluidity. pH2 has a calculated superfluid transition
temperature at ≈ 1 K in the bulk2 and ≈ 2 K in small clusters.3 Freezing of hydrogen at
13.8 K is the primary experimental obstacle. Although the rate of freezing is predicted to
be very low at T ≤ 3 K,4 numerous attempts at supercooling liquid hydrogen in
macroscopic droplets,5 clusters,6,7 porous media,8 or in the bulk9 have proved
unsuccessful. Recently, we have assembled clusters having about 104 pH2 molecules at
low temperatures by free jet expansion of pH2 gas seeded in He.10 Using rotational
spectra measured via coherent anti-Stokes Raman scattering (CARS), we have shown that
such clusters remain liquid at estimated T = 1 – 2 K, i.e., close to the predicted superfluid
transition temperature. On the other hand, we observed that the vibrational CARS
frequency of the liquid pH2 clusters is higher than that in solid, but lower than that in the
bulk liquid at 18 K.11 We have speculated that this change indicates a strong dependence
of the vibrational frequency of the pH2 liquid upon temperature, which may in turn be
138
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 162 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | Chapter VI. Temperature dependence of the Raman spectra of liquid para-hydrogen 6.1. Introduction Superfluidity of hydrogen, predicted three decades ago,1 continues to elude experimental observation. However, hydrogen may constitute a new class of molecular superfluids that are characterized by anisotropic interaction and rotational angular momentum of the particles. Normal hydrogen consists of two modifications: para-H2 (pH2) and ortho-H2 (oH2). pH2 molecules are spinless Bosons (I = 0) and at low temperature reside in the ground rotational state J = 0, which makes pH2 the prime candidate for observation of superfluidity. pH2 has a calculated superfluid transition temperature at ≈ 1 K in the bulk2 and ≈ 2 K in small clusters.3 Freezing of hydrogen at 13.8 K is the primary experimental obstacle. Although the rate of freezing is predicted to be very low at T ≤ 3 K,4 numerous attempts at supercooling liquid hydrogen in macroscopic droplets,5 clusters,6,7 porous media,8 or in the bulk9 have proved unsuccessful. Recently, we have assembled clusters having about 104 pH2 molecules at low temperatures by free jet expansion of pH2 gas seeded in He.10 Using rotational spectra measured via coherent anti-Stokes Raman scattering (CARS), we have shown that such clusters remain liquid at estimated T = 1 – 2 K, i.e., close to the predicted superfluid transition temperature. On the other hand, we observed that the vibrational CARS frequency of the liquid pH2 clusters is higher than that in solid, but lower than that in the bulk liquid at 18 K.11 We have speculated that this change indicates a strong dependence of the vibrational frequency of the pH2 liquid upon temperature, which may in turn be 138 |
