Electron-ion coincidence studies of the behaviour of molecules upon core excitation

Abstract

We have measured the time of flight mass spectra of several molecules, which include carbon disulfide (CS2) and thiophene (C4H4S), in coincidence with Auger electrons which result from the decay of core excitations in these molecules. These experiments allow us to observe the behaviour of parent ions of the molecules which are selected for their excitation energy and also for the site of the original excitation.We have observed strong state selectivity in the fragmentation of CS2. As a result of this, there is a strong correlation between electronic states of the parent ion and electronic states of the fragments which arise from these states. This correlation can be deduced from the observed kinetic energy releases in fragmentation, which are nearly independent of the electronic excitation energy of the parent ion.We have also observed a certain amount of site selectivity in the fragmentation of a molecule as large as thiophene. This shows up as a difference in the yields of certain fragment ions from states in the same region of excitation energy of the parent ion, depending on the site of the original excitation of the molecule. Clearly, such an observation argues against complete statistical behaviour in the fragmentation of thiophene.