Photodissociation of doubly-charged ions

Abstract

Photodissociation of doubly-charged ion has been induced by radiation from an argonion laser and investigated using a sensitive double-focusing mass spectrometer of reversed geometry. Of the molecular ions studied, only those of benzene and benzonitrile were found to photodissociate. The kinetic energy release distribution f( T) for both ions was found to be independent of photoexcitation energy in the range 2.41–3.47 eV and with the same respective values of f( T) as for unimolecular dissociation. These results confirm that dissociation of doubly-charged ions is a process involving a predissociation mechanism with the major part of T being due to Coulombic repulsion of the charges on the fragment ions. However, the degree of dishing, which depends on several instrumental factors as well as on the translational energy spread of the fragment ions, was more marked in the case of photodissociation than for unimolecular dissociation. This is probably due to the good collimation of the laser beam and the fact that photodissociation occurs only for ions photoexcited in the volume of this beam. Thus, there is an effective pre-collimation of the photoexcited ions.