Kinetic-energy release in the dissociative double photoionization of OCS

Abstract

Kinetic-energy release distributions (KERDs) of fragment ions produced in the dissociative double photoionization of OCS have been studied in the photon energy region of 37–100 eV by use of synchrotron radiation and the photoion–photoion coincidence (PIPICO) method. The most probable KERDs are determined for the three major dissociation channels—namely, OC++S+, O++S++C, and C++S++O—of OCS2+ at selected photon energies. This was accomplished by analyzing the spectral profile of the PIPICO peaks measured at the pseudomagic angle (∼55°), which minimizes any effect of the anisotropic angular distributions of the fragment ions. For the OC++S+ channel at hν=37 eV, the KERD ranges from 3.0–6.3 eV with an average kinetic-energy release (AKER) of 4.5 eV, whereas the O++S++C channel at hν=100 eV exhibits a wide KERD (5.0–23.7 eV) with an AKER of 12.8 eV. The nearly continuous KERDs derived for the direct double photoionization from valence orbitals are discussed in relation to the electronic states of OCS2+. From the measurements of the AKERs as a function of the excitation photon energy, it is concluded that the fragment ions are produced in their excited states at the respective dissociation thresholds for most of the dissociation channels examined. This result is in sharp contrast to previous observations.