Coherent anti-stokes raman spectroscopy of transient species Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/th83m239t

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  • The CARS apparatus at OSU was modified to permit studies in the 0-500 cm⁻¹ spectral region. The new low frequency capabilities were then used to study molecular clusters formed in free expansion jets of carbon dioxide. A strong cluster band was observed at 73 cm⁻¹, with a weaker shoulder at about 90 cm⁻¹. Spectra recorded in the v₁ symmetric stretching region under the same expansion conditions suggested the low frequency cluster bands were due to large aggregates. Comparison with a spontaneous Raman spectrum of crystalline CO₂ confirmed this, as the cluster bands were found to coincide with E[subscript g] and T[subscript g] librational modes in the solid. The 266 nm UV photodissociation of methyl iodide was studied in the region of the v₁ symmetric stretch of the methyl radical. At initial 20 - 200 ns photolysis/probe delays, near thermal rotational distributions were seen at pressures of a few Torr, indicating that the translational energy imparted to the CH₃ upon dissociation was rapidly transferred to other degrees of freedom. Zero delay studies in the symmetric stretching region gave "near nascent" conditions in which nearly equal amounts of vibrational excitation for the v₂ = 0,1,2 states was observed. Analysis of high resolution spectra of the methyl radical in the symmetric stretching region yielded accurate rotational constants for two states: (1000), B = 9.49662 cm⁻¹, C = 4.69505 cm⁻¹ (1100), B = 9.17738 cm⁻¹, C = 4.76442 cm⁻¹ The C-H bond length in the (1000) state was calculated to be 1.0836 Ȧ̇̇ while the equilibrium bond length was determined to be 1.0706 Ȧ. The v₁ fundamental was determined to be 3004.346 cm⁻¹ and, from the observation of the difference band v₁ + v₂ - v₂, the anharmonicity constant x₁₂ was found to be -5.9 cm⁻¹. The possible reaction of CH₃ with 0₂ was monitored via the pure rotational CARS signal for oxygen. No evidence for the formation of the peroxy intermediate CH₃-0₂ was seen within 5 ns after dissociation. This result is consistent with thermodynamic predictions at the elevated temperatures (1000K) of the reaction mixture deduced from the observed pure rotational distributions of oxygen.
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