Vibrational dephasing and the Raman non-coincidence effect of CHBr₃ in isotopic dilution

The ν₁ (C-H) stretching mode of CHBr₃ was investigated in the solvent CDBr₃ at room temperature using three-colour femtosecond CARS and scanning multi-channel Raman difference spectroscopy (SMT-RDS). Small but measurable blue shifts (≤1 cm^-1) of the isotropic part of the Raman scattering relative to the anisotropic component and upon isotopic dilution are the clear signatures of the Raman non-coincidence effect and isotopic lineshift, respectively. The experimental results are discussed in terms of theoretical models providing insight in the intermolecular coupling mechanisms. The measured shift Δν(ID) = 0.73 ± 0.02 cm^-1 upon isotopic dilution is found to be dominated by the attractive interaction and is to be compared with the non-coincidence effect of Δν(NCE) = 1.02 ± 0.05 cm^-1 for the neat liquid. A 5% decrease in the dephasing rate 1/T₂ and a similar change in the linewidth upon inifinite dilution are observed and semi-quantitatively explained via repulsive intermolecular interaction in accordance with an estimate of the corresponding (small) wavenamber shift. Copyright (C) 2000 John Wiley And Sons, Ltd.