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2.3. Size Distribution of Very Large He Droplets
2.3.1. Introduction
The average values obtained in Fig. 2.1 were measured by the deflection of a
continuous beam of He droplets upon capture of Xe atoms from a secondary beam6,7 or
by the deflection of negatively charged droplets9 in an electric field for NHe < 104 and 105
< N <107
He , respectively. Unfortunately, these experiments are not universal in which
these results cannot be applied to different systems in other laboratories. For example,
the recent introduction of a pulsed nozzle He expansion cannot rely on continuous nozzle
results. Due to the delicate nature of these experiments, reproduction is difficult and
specialized. In addition, these experiments cannot be applied to systems larger than 107
atoms. Fig. 2.1 shows there is a considerable range of temperatures of 5 – 7 K which has
not been characterized, i.e., of unknown average size and size distribution. Here we will
present the first determination of the size distribution of the droplets in this regime.
Recently, a method of measurements of average droplet sizes has been developed
in our laboratory10 which is based on the extinction of the droplets in collision with room
temperature He gas. The results are in quantitative agreement with previous works in
regime 3 in Fig. 2.1.6,7,9 In addition, this technique has also provided average sizes of the
droplets obtained at T0 = 5 – 7 K. Here, we have developed a method for measuring the
droplet size distribution. Details on the experiments for measuring clusters sizes can be
found elsewhere.10 The remainder of this chapter is devoted to measurements on the size
14
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 38 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 2.3. Size Distribution of Very Large He Droplets 2.3.1. Introduction The average values obtained in Fig. 2.1 were measured by the deflection of a continuous beam of He droplets upon capture of Xe atoms from a secondary beam6,7 or by the deflection of negatively charged droplets9 in an electric field for NHe < 104 and 105 < N <107 He , respectively. Unfortunately, these experiments are not universal in which these results cannot be applied to different systems in other laboratories. For example, the recent introduction of a pulsed nozzle He expansion cannot rely on continuous nozzle results. Due to the delicate nature of these experiments, reproduction is difficult and specialized. In addition, these experiments cannot be applied to systems larger than 107 atoms. Fig. 2.1 shows there is a considerable range of temperatures of 5 – 7 K which has not been characterized, i.e., of unknown average size and size distribution. Here we will present the first determination of the size distribution of the droplets in this regime. Recently, a method of measurements of average droplet sizes has been developed in our laboratory10 which is based on the extinction of the droplets in collision with room temperature He gas. The results are in quantitative agreement with previous works in regime 3 in Fig. 2.1.6,7,9 In addition, this technique has also provided average sizes of the droplets obtained at T0 = 5 – 7 K. Here, we have developed a method for measuring the droplet size distribution. Details on the experiments for measuring clusters sizes can be found elsewhere.10 The remainder of this chapter is devoted to measurements on the size 14 |
