Photofragmentation of molecular adsorbats: Theoretical study of photoionization and photodesorption

Conference · Tue Dec 31 04:00:00 EST 1996

OSTI ID:447595

Buechner, M; Atabek, O; Bejan, D; Raseev, G ^[1]

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1. Universite de Paris-Sud, Orsay (France)

This work studies photofragmentation of molecules absorbed on solid surfaces namely the photoionisation, photodesorption and the influence of adsorbate coverage. In the first part, the effects of the hindered rotation motion of molecular adsorbate are studied. This motion changes the orientation of the molecular axis and consequently influences strongly the ejection direction of photoelectrons. In the models of the literature these effects have been neglected up to now and our approach includes the influence of the surface in a realistic way. We calculated the angular distributions of electrons excited from the 4 sigma orbital of physisorbed (CO/Ar) and chemisorbed CO (CO/Ni(100)) systems including the hindered rotation. For CO/Ar, the results are strongly influenced by this rotation. For CO/Ni(100) the hindered rotation plays a minor role appearing as a broadening in the structures of the angular distribution of photoelectrons. We present a recipe that enables the estimation of a mean tilt angle of the adsorbate about the surface normal taking into account the hindered rotation. The second part includes photodesorption in the non-thermal regime. For a photon excitation energy between 2 and 7 eV, the adsorbate is first electronically excited. Then, the deexcitation transfers a part of energy to the nuclear modes of the adsorbate, especially to the one associated to desorption. With the excited electron, another part of energy departs into the solid. A multidimensional model for photodesorption, that couples the nuclear motions, was derived and applied to CO adsorbed on top of Cu(111). The hindered rotation is strongly coupled to the desorption mode but the internal vibration and hindered translation are decoupled from these two modes. Experimentally, a bimodal fragment velocity distribution was found and attributed to a thermal and nonthermal desorption mechanisms. The results of the present model show similar bimodal distributions only for a nonthermal regime.

• OSTI ID: 447595
• Report Number(s): CONF-960343--
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
40 CHEMISTRY
66 PHYSICS
ANGULAR DISTRIBUTION
CARBON MONOXIDE
DESORPTION
FRAGMENTATION
MOLECULES
NICKEL
PHOTOIONIZATION