High resolution stimulated raman spectroscopy Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/d504rn98d

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  • The high resolution stimulated Raman spectrometer at OSU was modified to improve its sensitivity and to extend its range of applications to low Raman shifts and to the spectroscopy of solids and liquids. As part of the characterization of the spectrometer, optical Stark effects on rotational and vibrational transitions of N₂ were investigated and the pure rotational spectrum of C₄N₂ was recorded for the first time. Stimulated Raman spectroscopy (SRS) was used for the first time for high resolution Raman spectroscopy of cold molecular liquids and crystals. Accurate measurements of the linewidth and the Raman shift for the very narrow vibrational transition in condensed nitrogen (liquid, solid-β, and solid-α phases) were made between 110 and 15 K. These measurements establish the temperature dependence of the N₂ vibrational frequency and linewidth for the condensed phases and show the effect of the phase transitions on the vibrational Raman spectrum. This data was used in the analysis of SRS spectra of large N₂ clusters formed in free jet expansions which were observed for the first time with the SRS technique. By probing at different points along the axis of the expansion changes in the vibrational Raman spectrum are observed which signify the formation of liquid aggregates, followed by strong supercooling and subsequent freezing to the solid beta phase. High spatial resolution of the SRS probing results in good time resolution for the observation of the extremely fast condensation and freezing phase transitions. Aggregate temperatures are determined by using the temperature dependence of the vibrational frequency measured in the static samples and the assumption that the observed clusters are large enough to be treated as bulk material. That this assumption is valid is shown by several independent size estimates which indicate that the means cluster diameter is in the range of 50 - 500 nm. Monomer temperatures are determined from the rotational Q-branch structure and the extent of aggregation is found to be about 10 % from the relative Raman intensity of monomer and cluster spectra. The results show that SRS offers unique diagnostic capabilities for the study of condensation processes in free jet expansions.
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