| Summary: | Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2002. === Vita. === Includes bibliographical references. === Modifications of Cavity Ringdown Spectroscopy (CRD) for use in the UV region are discussed. These modifications partially overcome the low sensitivity of CRD in the UV (due to lower mirror reflectivity, R - 0.99) by removal of non-exponential contributions to the ringdown decay. We have also developed a novel fitting algorithm to accurately extract the ringdown decay time from the ringdown signal. The resulting sensitivity is comparable to that achieved in the visible region (10-8 cm-1), where mirror reflectivities are higher (R - 0.9999). Application of this new UV-CRD apparatus and software for acetylene absolute absorption cross section measurements is also discussed. The observation and assignment of high-resolution spectra of the A' A" - X1 A' transition of HCBr and DCBr, recorded using Frequency Modulation Spectroscopy, is described in the second section. The 2 band of HCBr (vo = 11957 cm'l) and the 2' band of DCBr (vo = 12349 cm-1) have been observed for both naturally occurring isotopes of bromine. The presence of several K, subbands indicates that the molecule exhibits bent molecule rotational structure in these vibrational levels. Analysis of the rotational structure that is reported here has required a reassessment of rotational assignments of previous work published for HCBr. The lower singlet state is isolated, i.e. there is no evidence of triplet state perturbations in the vibrationless level of the ground state. Rotational constants derived for the ground state of all naturally occurring bromine isotopomers were used to estimate structural parameters. === (cont.) The barrier to linearity for the A 'A state is also estimated to be - 13600 cm'. Finally, the application of sensitive spectroscopic techniques to measurements of combustion-relevant radical reaction rates is discussed. Unsuccessful attempts to record the spectra of two target radicals, cyclohexadienyl (C6H7) and 2-phenyl ethyl (C6HsC2H5), in the gas phase are described. The radicals are generated via flash photolysis and absorption spectra are recorded in both liquid and gas phase. A specially designed multipass cell (Herriott cell) used for gas phase absorption and kinetic measurements is described. Investigation of the oxidation rate of cyclohexadienyl (C6H7) in liquid phase is also presented. === by Leah M. Ruslen. === Ph.D.
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