Kinetic-energy release and intercharge distance of the sulfur dioxide dication (SO22+)

The kinetic-energy release distributions (KERDs) of the fragment ion pairs (O+ + SO+ and O+ + S+) produced in dissociative double photoionization of sulfur dioxide have been determined by analyzing the photoion-photoion coincidence spectra measured in the energy region of 37-130 eV by use of a time-of-flight mass spectrometer and synchrotron radiation. It should be noted that the KERDs obtained for the O+ + S+ channel represent lower limits because the kinetic energy of the neutral O atom is not involved. The kinetic-energy releases (KERs) in the O+ + SO+ dissociation channel do not vary but remain almost constant across this broad range of excitation energies. This means that low-lying electronic states of SO22+ below 37 eV are mainly responsible for this channel. However, the average KERs in the O+ + S+ dissociation channel depend strongly on the excitation energies in the 40-60-eV region and weakly above 60 eV. This indicates that high-lying electronic states of SO22+ as well as low-lying ones, contribute to this channel. From the observed KERDs in the O+ + SO+ and O+ + S+ channels of SO22+, the range of the intercharge distances is estimated by assuming that the KERs are simply given by Coulomb repulsion. The estimated intercharge distances for the O+ + S+ channel are wide and deviate largely from the equilibrium internuclear distance, which is not possible by direct double ionization only and which indicates a significant role of indirect processes. (C) 2001 Elsevier Science B.V.