Journal of Chemical Physics, Vol.109, No.4, 1352-1358, 1998
Raman resonance de-enhancement in the excitation profile of CS2
The total differential Raman cross section of the symmetric vibrational mode of CS2 (652 cm(-1)) in liquid phase has been measured as a function 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 wavelength 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 two Renner-Teller components, B-1(2) and (1)A(2)((1) Delta(u)). The transition from the ground state to the lower electronic state is electronically forbidden, but it becomes vibronically allowed due to the Renner-Teller interaction. This may be the first observation of Raman resonance de-enhancement due to the interference involving three excited states.
Keywords:AROMATIC-AMINO-ACIDS;FORBIDDEN ELECTRONIC-TRANSITIONS;CROSS-SECTIONS;N-METHYLACETAMIDE;LASER EXCITATION;BENZENE VAPOR;ULTRAVIOLET;SPECTROSCOPY;SPECTRA;INTENSITY