Relaxation processes following excitation and ionization of SiF4in the vicinity of the silicon 2pthreshold. II. Dissociation of the molecular ions

摘要

Partial ion yields from photoionization of SiF4have been measured with timehyphen;ofhyphen;flight mass spectrometry, photoionndash;photoion coincidence and triple photoion coincidence techniques using synchrotron radiation from 100 to 136 eV, in the region of the silicon 2pedge where strong resonances are found. From the photon energy dependence of positive ion pairs and of doubly charged fragment intensities, with a suitable normalization procedure, we have estimated the total double photoionization cross section. Below the Si2pedge, the double to single dissociative ionization branching ratio follows the resonance behavior, and is consistent with the photoelectron results of de Souzaetal. (Paper I) and discussed in terms of multibond breaking dissociation pathways of residual excited ions (singly and doubly charged) produced by resonant Auger decay processes. Above the Si2pedge, the fragmentation is found to originate from dissociative double ionization occurring after normal Auger processes. The small additional contribution of triple dissociative ionization is consistent with a second order Auger process. In contrast, the underlying valence ionization continuum is responsible for all parent intensity and most of the SiF+3fragment expected from the normal dissociative ionization channels. The measurement of appearance energies of fragment ion pairs by the photoionndash;photoion coincidence method, offers a direct measurement of the first direct double ionization onset in SiF4and new values for other dissociative SiF2+4states which complement those found by Auger spectroscopy.