No abstract

High resolution core-level electron spectroscopy on free molecules and atoms AIP Conf.Absolute photoabsorption crosssection measurements of simple molecules in the corelevel region Rev. Sci. Instrum. 66, 1531 (1995);We have performed the first experiments to examine the neutral fragmentation paths following direct core-level excitation in a molecule. Using monochromatized synchrotron radiation in the range 100-140 eV, we have monitored the dispersed UV /optical fluorescence resulting from excitation of a Si 2p electron in SiF 4 , The main features in the fluorescence spectrum have been identified as emission from the SiF 4+ D state and from excited SiF, Si, F, and Si + • Features in the fluorescence excitation spectra are assigned to excitation of a Si 2p electron to unoccupied valence orbitals, Rydberg orbitals, and shape resonances. There is a large enhancement in the yield of excited-state fragments following core-to-Rydberg excitation, which is due to the greater probability of the core-excited Rydberg state decaying, via a resonant Auger process, to highly excited, unbound states of SiF / .

No abstract

No abstract

We present an analysis of the photolysis of an adsorbed molecule by monochromatic synchrotron radiation. The system studied was CF 3 Cl adsorbed on Si͑111͒͑7ϫ7͒ at 30 K, and the techniques employed were photoemission spectroscopy ͑PES͒ and photon-stimulated desorption ͑PSD͒. The valence-level photoemission spectra, under various photon exposures, show that the photolysis effect induced by the incident monochromatic synchrotron radiation photons ͑hϭ98 or 110 eV͒ occurs on this adsorbed gas-solid system. Variations of the F Ϫ and F ϩ PSD ion yields were measured as a function of monochromatic ͑110 eV͒ photon exposure at three adsorbate coverages ͑the lowest doseϭ0.3ϫ10 15 , the medium doseϭ0.8ϫ10 15 , and the highest doseϭ2.2 ϫ10 15 mol/cm 2 ͒. For the lowest CF 3 Cl-dosed surface, the photon-exposure dependencies of the F Ϫ and F ϩ yields show the characteristics: ͑a͒ at early stages of photolysis, the desorption of F Ϫ yields is mainly due to dissociative attachment ͑DA͒ and dipolar dissociation ͑DD͒ of the adsorbed CF 3 Cl molecules induced by the photoelectrons emitting from the silicon substrate, while at high photon exposure the F Ϫ formation by electron capture of the F ϩ ion is the dominant mechanism; ͑b͒ the F ϩ ion desorption is associated with the bond breaking of the surface SiF which is formed by reaction of the surface Si atom with the neutral fluorine atom or F Ϫ ion produced by scission of the C-F bond of CF 3 Cl, CF 2 Cl, or CFCl species. A kinetic model was proposed for the explanation of the photolysis of the lowest CF 3 Cl-dosed surface. Based on this model and the variation rates of the F Ϫ /F ϩ signals during 110 eV photon bombardment, a photolysis cross section of ϳ1.9 ϫ10 Ϫ17 cm 2 was determined. The photolysis cross section as a function of incident photon energy in the range 98-110 eV, near Si(2p), was also obtained, which shows a threshold at around 100 eV, corresponding to the same rise as the total electron yield ͑TEY͒ spectrum of the clean silicon substrate. This edge-jump indicates an enhancement of the photolysis efficiency by indirect excitation of the substrate core-level ͓Si(2 p)͔ electrons, and confirms the substrate-mediated excitation as the photolysis mechanism for the lowest-dosed surface. However, for the medium-and the highest-dosed surfaces both the direct photodissociation and the substrate-mediated dissociation of the adsorbed CF 3 Cl occur at early stages of photolysis, while only the substrate-mediated dissociation was observed at higher photon exposure. For photolysis of all three surfaces the SiF is the only fluorosilyl product observed on the surface.