RAMAN RESONANCE DE-ENHANCEMENT IN THE EXCITATION PROFILE OF CS2

The total differential Raman cross section of the symmetric vibrationa l mode of CS2 (652 cm(-1)) in liquid phase has been measured as a func tion of excitation wavelength from the visible to the ultraviolet. The resulting excitation profile shows a strong preresonance enhancement when the excitation wavelength is less than 300 nm. The cross section measured at 240 nm is about three orders-of-magnitude larger than the nu(4) dependence for Raman scattering. The observed preresonant effect appears to be dominated by the B-1(2)((1) Sigma(u)(+)) <-- (1) Sigma( g)(+) transition. A minimum in the excitation profile occurs at a wave length that is associated with the peak of the near-UV absorption band (similar to 320 nm). The observed dip in the profile is ascribable to a quantum interference between the B-1(2)((1) Sigma(u)(+)) and the tw o Renner-Teller components, B-1(2) and (1)A(2)((1) Delta(u)). The tran sition from the ground state to the lower electronic state is electron ically forbidden, but it becomes vibronically allowed due to the Renne r-Teller interaction. This may be the first observation of Raman reson ance de-enhancement due to the interference involving three excited st ates. (C) 1998 American Institute of Physics.