Marie-Joëlle Ramage

Adsorption of O2 on Si(111)7 × 7 compared at 300 and 30 K

Abstract Adsorption of O2 on Si(111) 7 × 7 at 30 K is studied by UV photoelectron spectroscopy and compared to previous results at 300 K. This demonstrates the existence of two distinct metastable states of O2 adsorbed on Si(111)7 × 7. Furthermore, it is shown that the initial relative population of these metastable states depends on both the temperature and the O2 coverage.

Selective ion photodesorption from NO adsorbed on Si(111)7×7 following core excitation

Selective ion photodesorption from NO adsorbed on Si(111)7×7 has been investigated by core electron excitation. In a first step, the adsorption of nitric oxide on the clean Si(111)7×7 surface has been studied over the temperature range from 15 K to room temperature using ultraviolet photoemission spectroscopy, x-ray photoemission spectroscopy, and photon stimulated desorption of ions. At room temperature, NO was found to react with the surface silicon adatoms to form silicon oxynitride sites (SiNxOy). At low temperature and low coverage, in addition to a permanent dissociative adsorption, a transient molecular adsorption has been observed. At low temperature and high coverage, NO was found to be condensed. In a second step, ion photodesorption induced by core electron excitation of both the adsorbate and the substrate has been studied. Quite interestingly, selective ion photodesorption was observed by core electron excitation of both dissociatively adsorbed NO at room temperature and condensed NO molecule...

Molecular reactions in condensed carbon monoxide studied by ion photodesorption

Ion photodesorption processes from carbon monoxide condensed in multilayers have been studied for photon energies between 12 and 60 eV. In the low energy range (12–25 eV), only the CO+ ion desorbs and the desorption is induced by the formation of excited neutral states. In the high energy range (25–60 eV), a large variety of ions desorb: C+, O+, CO+, C2O+, (CO)2+, C3O2+, and C3O+. The desorption threshold for these ions is around 30 eV and is assigned to the formation of satellite states. The ion desorption is related to fragmentation processes and ion-molecule reactions on the surface. Comparison with gas phase results show that the dissociative ionization of CO can be modified in the condensed phase by charge-transfer processes. Kinetic energy measurements of the desorbed ions support the proposed mechanisms.

Photodesorption from CO ices

The photon excitation of multilayers of condensed CO leads to the desorption of a great number of different ions, C+, O+, CO+, C2O+, C3O+, C4O+, (CO)+2 and C3O2+. The threshold energy for desorption of the most abundant ions (C+, O+) is around 30 eV. Such a high threshold is believed to result from a two electron excitation in the solid. The two main resonances observed at lower energies (15.35 eV and 17 eV) in the CO+ ion yield curve of which the threshold energy is 14.3 eV are assigned to two excitonic levels of solid CO.