Off-axis cavity ring down spectroscopy with exponential and phase shift detection : absorption spectra of weak transitions at low temperatures.

Abstract

In this dissertation the ability of the off-axis laser injection in cavity ring down experiments is shown. Modulated continuous wave and pulsed lasers are used to obtain the absorption spectra of weak transitions at low temperatures. The absorption spectra of the red atmospheric bands of O₂ (A-band and γ-band), the overtone (Δυ =4) band of HD and the C-H (Δυ = 4, 5 and 6) overtone spectra of polyatomic molecules (C₂H₆, CH₂D₂, CHD₃, C₃H₄, C₆H₁₀ and C₅SiH₁₀) are presented. The integrated absorption of each band was calculated as a function of the density of the gas samples and used to obtain the band strength of these weak transitions. In addition, the Fourier-transform (IR and NIR) spectra between 2500 and 13000 cm⁻­­­¹ are presented for CH₃D, CHD₃, C₆H₁₀ and C₅SiH₁₀. The local mode harmonic frequency and anharmonicity were obtained. The harmonically coupled anharmonic oscillator (HCAO) model was used to calculate energy levels and assign the absorption bands to particular transitions of CH₃D, C₆H₁₀ and C₅SiH₁₀. Also, overtone intensities of CHD₃ were calculated using ab initio quantum mechanical methods and compared with the experimental data. Spectral simulations were performed in order to confirm the transitions and the temperature of the molecules in the cell. Lastly, photolysis of ethyl acetate via high vibrational overtone absorption is proposed.