Selective photoionization and photodissociation of molecules following inner shell excitation

Present knowledge on the dynamics of elementary decay processes of discrete resonances especially the transitions in small molecules photoexcited by synchrotron radiation is reviewed. It is shown that autoionization and direct dissociation must be considered on the same time scale. This is of primary importance for understanding the problem of site selective fragmentation in polyatomic molecules. Firstly, it is shown that direct dissociation of σ* core excited states precedes autoionization, using the HBr molecule photoexcited near the 3d bromine edge, as a model system. The extension of such phenomenon to different polyatomic molecules either totally symmetric like the silane molecule (SiH4) photoexcited near the silicon 2p edge, or various species of lower symmetry, are discussed. Secondly, it is shown that among the various electronic decay channels of the core excited state, there are some selective pathways, which govern the formation of singly and doubly charged ions. The discussion is restricted to the excitation of outermost core levels. Consequently, the core excited state can only be coupled by autoionization to the valence ionization continua. Lastly, the site selectivity of fragmentation, and its limitations are analysed. This part is illustrated by using TOF mass spectrometry, photoion-photoion coincidence and triple photoion coincidence measurements in SiH4 and CH3Br. 43 refs.; 16 figs