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(Δk = 0), the sin term goes to unity and intensity is maximized. For clusters, refraction of
light is reduced from that in the bulk medium such that the index of refraction in air is
suitable. Equation 4.1 can be simplified according to equation 4.2 assuming Δk = 0
( ) ( ) ( ) 4.2 2 2
2
1 1
2
3 3 I ω ∝ N ⋅ I ω ⋅ I ω
where N is the molecular number density. The quadratic dependence of the CARS signal
on pump intensity demonstrates the need for high power pulsed lasers while the quadratic
dependence on the number density limits the concentrations of samples available for
study. For reference, ro-vibrational CARS spectra have been previously measured for
H2.
3
4.3. CARS Optical Setup
The experimental layout of the helium droplet apparatus used for CARS
experiments has already been described in Chapter 3. However, only Chamber (1) is
employed in CARS studies, see Fig. 3.1. A schematic of the optical setup for CARS is
shown in Fig. 4.2. Two lasers are required for CARS experiments in order to produce the
pump and Stokes beam as discussed in section 4.2. The pump beam is provided by a
pulsed Nd:YAG laser (Continuum Inc. Powerlite Precision II 8020). The Nd:YAG laser
generates an approximately 8 ns pulse with a maximum output of 1000 mJ/pulse, 425
mJ/pulse, and 215 mJ/pulse for the fundamental (1064 nm), second (SHG – 532 nm), and
76
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 100 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | (Δk = 0), the sin term goes to unity and intensity is maximized. For clusters, refraction of light is reduced from that in the bulk medium such that the index of refraction in air is suitable. Equation 4.1 can be simplified according to equation 4.2 assuming Δk = 0 ( ) ( ) ( ) 4.2 2 2 2 1 1 2 3 3 I ω ∝ N ⋅ I ω ⋅ I ω where N is the molecular number density. The quadratic dependence of the CARS signal on pump intensity demonstrates the need for high power pulsed lasers while the quadratic dependence on the number density limits the concentrations of samples available for study. For reference, ro-vibrational CARS spectra have been previously measured for H2. 3 4.3. CARS Optical Setup The experimental layout of the helium droplet apparatus used for CARS experiments has already been described in Chapter 3. However, only Chamber (1) is employed in CARS studies, see Fig. 3.1. A schematic of the optical setup for CARS is shown in Fig. 4.2. Two lasers are required for CARS experiments in order to produce the pump and Stokes beam as discussed in section 4.2. The pump beam is provided by a pulsed Nd:YAG laser (Continuum Inc. Powerlite Precision II 8020). The Nd:YAG laser generates an approximately 8 ns pulse with a maximum output of 1000 mJ/pulse, 425 mJ/pulse, and 215 mJ/pulse for the fundamental (1064 nm), second (SHG – 532 nm), and 76 |
