First-principles C band absorption spectra of SO 2 and its isotopologues.

The low-energy wing of the C∼B21←X∼1 A1 absorption spectra for SO2 in the ultraviolet region is computed for the 32S,33 S,34 S and 36 S isotopes, using the recently developed ab initio potential energy surfaces (PESs) of the two electronic states and the corresponding transition dipole surface. The state-resolved absorption spectra from various ro-vibrational states of SO2 (X∼1 A1 ) are computed. When contributions of these excited ro-vibrational states are included, the thermally averaged spectra are broadened but maintain their key characters. Excellent agreement with experimental absorption spectra is found, validating the accuracy of the PESs. The isotope shifts of the absorption peaks are found to increase linearly with energy, in good agreement with experiment.