Graduate Thesis Or Dissertation
 

Application of laser spectroscopy to structure and dynamics of small molecular systems

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  • A variety of laser spectroscopic methods are employed in this thesis research for studies of CF₃NO, an interesting model photodissociative system, and for investigation of low energy rotations and vibrations of small molecules and low molecular weight clusters in free jet expansions. Nonradiative energy decay for electronically excited CF₃NO is investigated in an argon matrix. By comparing the visible absorption spectrum with pulsed laser photoacoustic and fluorescence excitation spectra it is argued that all nonradiative decay processes are fast, thus eliminating the likelihood of transient separation of CF₃ and NO photofragments in the matrix cage or appreciable decay through a low lying triplet state. No evidence is found for a "channel two" decay mechanism in the matrix in contrast to gas phase work of others. Upper state vibrational relaxation is seen to be exceedingly fast by consideration of fluorescence spectra, and the fluorescence quantum yield is estimated to be 0.015 from measured emission lifetimes. The photochemistry of CF₃NO is investigated in gas phase studies using Coherent Anti-Stokes Raman Spectroscopy (CARS) to probe the nascent rotational populations of both the CF₃ and NO photofragments. The frequency of the v₁ fundamental, which has not been observed previously by any Raman technique, is established with more certainty than in the past. The experimental spectra are modeled adequately by calculations assuming a completely statistical distribution of rotational energies for both fragments. Upper state B and C constants are estimated for the CF₃ species. Studies are also reported on the applicablility of photoacoustic Raman spectoscopy (PARS) and CARS to investigations of low frequency modes of simple molecules. The development of these methods for use in our labs in the low shift regime is discussed in some detail and the two techniques are compared and constrasted. CARS is employed to observe pure rotational transitions of molecules cooled in free jet expansions and to study the low frequency vibrations of small van der Waals clusters of jet cooled CO₂.
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