Valence Hole Localization in Molecular Auger Decay
- PDF / 713,808 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 11 Downloads / 167 Views
VALENCE HOLE LOCALIZATION IN MOLECULAR AUGER DECAY
D.A. Lapiano-Smith, C.I. Ma, K.T. Wu,' and D.M. Hanson Department of Chemistry, State University of New York,Stony Brook, N.Y. 11794-3400 "On leave from Department of Chemistry, State University of New York, College at Old Westbury, Old Westbury, New York 11568 ABSTRACT Studies of the unimolecular decay, following the excitation of core electrons of the carbon and fluorine atoms in carbon tetrafluoride and silicon and fluorine in silicon tetrafluoride by monochromatic, synchrotron radiation, provided evidence for a "valence bond depopulation" fragmentation mechanism. The fragmentation processes were examined using time-of flight mass spectroscopy. The mass spectra show the distribution of ions collected in coincidence with low and high energy electrons. Distinct changes in the mass spectra with atomic site of excitation and photon energy are observed. The observation of F 2รท ions in the time-of-flight mass spectra following excitation of a fluorine is electron in SiF, is significant because it provides direct evidence for the formation of a localized, twohole, final2 valence state that persists on the time scale of fragmentation. In contrast, the lack of F + ions from CF 4 , indicates that the fragmentation occurs through a delocalized two-hole state. I. INTRODUCTION. Evidence for a "valence bond depopulation" mechanism is reported in this paper. The term "valence bond depopulation" refers to an Auger final state in which the two valence holes produced by Auger decay are highly correlated and localized on the atomic site of the core hole. The concept of localization has been proposed to explain the Auger spectra of some metals,1 2 semiconductors," and gas phase molecules",",6 and electron and photon stimulated desorption of ions from solid surfaces.? Two valence holes created by Auger decay of the core hole become confined or localized on the same atomic site because of the large energy that must be dissipated if they separate. The rate of this dissipation may be slow compared to that for other processes. The presence of such localized final states is dependent upon the magnitude of ionicity and polarity in the bonds, bond lengths and the Coulomb repulsion between the holes. An interesting possibility, stemming from the explanation of the spectra, is that the electronic relaxation and subsequent bond dissociation will be localized around the atomic site of excitation. This is possible if the two valence holes created by Auger decay of the core hole, can be localized around a single atom for a time sufficient to allow bond rupture to occur. Carbon and Silicon tetrafluoride were specifically chosen in order to observe whether or not two hole correlation and localization occurs in the valence shell of fluorine. For an Auger transition, involving a particular bonding orbital, the process will occur through that region of the valence electron density immediately around the atomic site of excitation. 6 However, due to the large magnitude of ionicity of the Si-F bonding
Data Loading...
