Gas phase Raman intensities of C2H2, C2HD and C2D2

Calculation of absolute Raman scattering cross-sections using vibrational self-consistent field/vibrational configuration interaction wave functions

In the present study, the differential scattering cross-sections, depolarization ratios and Raman shifts of small molecular systems are obtained from configuration iteration wave functions of vibrational self-consistent field (VSCF) states. The transition polarizabilities were modeled using the Placzek approximation, neglecting those contributions not arising from the electric dipole mechanism. This theoretical approach is considered a good approximation for samples that absorb in the UV range if the excitation radiation falls in the visible region, as is the case of the molecules selected for the present study, namely: water, methane, and acetylene. Potential energy and electronic polarizability surfaces are calculated by the CCSD(T) and CC3 methods with aug-cc-p(C)V(T,Q,5)Z basis sets. The vibrational Hamiltonian includes the vibrational angular momentum contribution of the Watson kinetic energy operator. As expected, due to the variational nature of the VSCF and vibrational configuration interaction (VCI) methods, the Raman transition wavenumbers are substantially improved over the harmonic predictions. Surprisingly, the scattering cross-sections obtained using the harmonic approximation or the VSCF method better agrees with the experimental values than those cross-sections predicted using VCI wave functions. The more significant deviations of the VCI results from the experimental reference may be related to the significant uncertainties of the measured cross-sections. Still, it may also indicate that the VCI Raman transition moments may require a more accurate description of the electronic polarizability surface. Finally, the depolarization ratios calculated for H₂ O and C₂ D₂ using harmonic and VCI wave functions have similar accuracy, whereas, for C₂ H₂ and C₂ HD, the VCI results are more accurate.

The Infrared Diode Laser Spectroscopy of the nu2 + nu5 - nu2 Hot Band of Acetylene

The infrared diode laser spectrum of the nu2 + nu5 - nu2 hot band of acetylene has been observed within the discharge plasma of acetylene diluted in He. The concentration of acetylene in the nu2 state was modulated by applying a few kHz of ac current, and the spectrum of the hot band was recorded selectively by the discharge modulation technique. In total, 46 P, R, and Q-branch lines have been assigned to the nu2 + nu5 - nu2 hot band and molecular constants including the band origin 727.59318(27) cm-1 were derived. The lifetime of acetylene in the nu2 state was measured to be about 1 ms, proving that the nu2 state is metastable. The collisional relaxation rate constant of acetylene in the nu2 state by helium was measured to be k = 8.01 ± 0.19 x 10(-14) cm3 molec-1 s-1. Copyright 1998 Academic Press. Copyright 1998Academic Press