Resonance Raman spectra of theS3←Stransition of SO2: Electronic, vibrational, and rotational dynamics in solution

Abstract

Resonance Raman spectra, including absolute scattering cross sections, of sulfur dioxide in the vapor phase and in hexane solution have been obtained at several excitation wavelengths between 235 and 208 nm, on resonance with the strong S3←S0, 1B2←1A1 electronic transition. Due to the narrow rovibronic linewidths in the isolated molecule, the vapor phase spectra are best described as single vibronic level resonance fluorescence and exhibit intensity patterns that depend strongly on excitation wavelength, while the solution phase spectra are only weakly dependent on excitation frequency due to the increased linewidths of the vibronic transitions. The spectra in hexane are dominated by long overtone progressions in the symmetric stretch and its combination bands with the bend. The relative and absolute cross sections are reproduced fairly well through time-dependent wave packet propagation calculations employing an anharmonic S3 state potential previously developed to fit vibronic energy levels of the isolated molecule. The average vibronic homogeneous linewidth in solution is found to be about 560 cm−1 FWHM, implying an electronic dephasing time of approximately 20 fs, and the absence of significant broad fluorescence indicates that the dephasing is dominated by electronic population relaxation. In contrast, the ground state isotropic Raman linewidths are less than 5 cm−1, even for overtones involving up to six stretching quanta, indicating ground state vibrational dephasing times greater than 2 ps. Analysis of the anisotropic band shapes suggests that the rotational motion is not greatly hindered by solvation.