Fragmentation of SO+2 prepared in state selected vibrational levels

Abstract

The threshold photoelectron (TPE) and TPE-coincident photoion (CPI) mass spectra of SO2 have been determined over the energy range 15.844 to 16.674 eV with a resolution of about 0.018 eV FWHM. This is sufficient to prepare most individual vibronic states in populations exceeding 85%. In the 2B2 state, excitation of the stretching mode up to ν1=5 is observed, while in the same state the ν2 bending mode may be excited concurrently with not more than one quantum. In this energy range SO+ formation sets on at 15.930±0.005 eV, suggesting that the onset at 0° K should be 15.965 eV±0.005 eV. S+ is produced over a very narrow energy range only; the onset is at 16.334 eV, coincident with the (310 2B2 state of SO2+; S+ disappears at 16.674 eV. This is interpreted in terms of competition between the assymetric stretch mode and the bending mode for intermodal energy transfer from the symmetric stretch mode, with excitation of the bending mode required to give S+.