Cavity ring down and Fourier transform infrared spectroscopy at low temperatures (84-297 K): Fermi resonance and intensities of the C-H fundamental and overtone (Delta(upsilon) = 1-6) transitions of CHD(3).

The C-H stretch fundamental and overtone absorptions of CHD(3) have been obtained using Fourier transform infrared (FTIR), near-infrared, and phase shift and pulsed cavity ring down (CRD) techniques at temperatures between 84 and 297 K. The partially resolved rotational-vibrational spectra of CHD(3) that included the fundamental transition nu(1), the overtone transitions, and combination bands 5nu(1) and 4nu(1) + 2nu(5) and 6nu(1) and 5nu(1) + 2nu(5) were obtained and compared with the simulated spectra at the corresponding temperature. The strength of the Fermi resonance between levels upsilonnu(1) and (upsilon - 1)nu(1) + 2nu(5) with upsilon = 2-6 was calculated using the experimental perturbed energies and relative peak intensities. The integrated absorption was calculated as a function of the density of the gas samples and used to obtain the cross section and the band strength of the Deltaupsilon = 5 and 6 transitions. Ab initio molecular orbital calculations were done to obtain the dipole moment function (DMF) as a function of the C-H internuclear distance. Intensity calculations with the DMF correctly predict the order of magnitude of the experimental band strengths.