Mechanistic studies of the photodissociation of physisorbed O₂/graphite

Abstract

We have investigated the vacuum ultra-violet photodissociation of molecular O2 physisorbed on graphite using synchrotron radiation with photon energies in the range 13-35 eV. The yield of desorbed O+ ions shows a threshold at approximately 19.5 eV and resonances at 24.5 eV and 28.5 eV, contrasting strongly with the gas phase photodissociation cross section. The principal mechanism of O+ production appears to be dipolar dissociation driven by photoelectrons generated in the substrate. An enhancement in the detection efficiency of low-energy ions, compared with previous measurements, has identified an additional dissociation mechanism leading to the desorption of O- ions. Specifically, the results support a channel for low-energy O- desorption, attributed to the dissociative attachment of photoelectrons, again generated in the graphite substrate, to the physisorbed O2 molecules.

Photon-stimulated desorption of F− ions from CF3Cl adsorbed on Si(111)-7×7

“…However, relatively little work has been undertaken to study photon-stimulated desorption of negative ions. [1][2][3][4][5] Several photodissociation/photodesorption mechanisms have been proposed. [6][7][8][9][10][11][12][13] For the desorption of negative ions, the incident photon may excite the adsorbed molecule AB to a neutral Rydberg state.…”

“…In the study of gas-phase CF 3 Cl, Jochims et al 26 have measured the photoabsorption spectrum, and observed various absorption maxima in the photon energy range 9-25 eV. In an electron energy loss spectroscopy ͑EELS͒ experiment of gas-phase CF 3 Cl, King et al 27 have classified the Rydberg states involved in the transitions for the features in the EELS spectrum.…”

“…In an electron energy loss spectroscopy ͑EELS͒ experiment of gas-phase CF 3 Cl, King et al 27 have classified the Rydberg states involved in the transitions for the features in the EELS spectrum. Schenk et al 28 have also performed DD experiments of F Ϫ from gas-phase CF 3 Cl using synchrotron radiation in the photon energy range 10-30 eV, and observed various bands in the F Ϫ ion yield curve. They assigned a variety of ion-pair processes to the production of F Ϫ .…”

“…Photon-stimulated desorption of F À ions from CF 3 We report the photon-stimulated desorption of negative ions induced by direct dipolar dissociation and dissociative electron attachment. The photon-stimulated desorption of F Ϫ ions from CF 3 Cl physisorbed on a Si͑111͒-7ϫ7 surface at 30 K in the photon energy range 12-35 eV was studied.…”

“…The F Ϫ ion yield exhibits four resonances, at 12.8, 16.2, 19.5, and 22.3 eV, quite unlike the gas phase photodissociation cross section. The intensities of these resonances depend strongly on the CF 3 Cl coverage in a manner which varies from peak to peak. The resonances at 19.5 and 22.3 eV, which have a significant enhancement in the monolayer regime, are due to electron mediated dipolar dissociation of adsorbed CF 3 Cl molecules.…”