Angle resolved scattering and electron ejection by helium metastable atoms incident on a tungsten (110) surface

A55 Surface Science 103 (1981) 431-437 Q North-Holland Publishing Company HARTREE-FOCK-SLATER-LCAO ACETYLENE-TRANSITION

STUDIES OF THE

METAL INTERACTION

IV-. Dissociation fragments on Ni surfaces; cluster models

Petro GEURTS, Walter RAVENEK and Ad VAN DER AVOIRD Institute of Theoretical Chemistry, The Netherlands

Received 15 July 1980;accepted

University of Nijmegen,

Toernooiveld,

Nijmegen,

for publication 17 September 1980

Using the Hartree-Fock-Slater-LCAO method we have calculated the ionization energies for the acetylene fragments CH, CH2 and CaH adsorbed on small Ni clusters and we have compared these with the UPS spectrum measured for dissociatively adsorbed CaHs on the Ni(ll1) surface. For none of these fragments the calculated spectrum is in one-to-one correspondence with the experimental one. Although one should perform further, more extensive, calculations in order to be conclusive, we suggest as a possible explanation of this discrepancy that other (low intensity or strongly broadened) peaks might be hidden in the experimental spectrum. If such peaks would be found, our results can be used to identify the adsorbed fragments since the spectra calculated for the different species are rather different. On the other hand, we conclude that these spectra do not depend sensitively on the adsorption site or on the position of the adsorbed fragments.

Surface Science 103 (1981) 438-455 @North-Holland Publishing Company ANGLE RESOLVED SCATTERING METASTABLE

AND ELECTRON EJECTION BY HELIUM

ATOMS INCIDENT ON A TUNGSTEN (110) SURFACE

D.J. TITLEY and T.A. DELCHAR Department

of Physics, University of Warwick, Coventry CV4 7AL, England

Received 14 May 1980; accepted for publication 18 September 1980 Angle resolved measurements have been made of the metastable scattering and electron ejection consequent upon the interaction of a thermal energy, helium metastable beam (angular divergence 24’) with a clean and oxygen covered tungsten (110) surface at 300 K. The azimuthal angle used was 6’ off the (111) direction. Reflection of metastable atoms from a clean W(110) surface occurs at angles of incidence greater than 35’ with the fraction reflected increasing, up to 8S0 incidence. The reflection is apparently non-specular and is accompanied by a broadening of the scattered beam to -25” at half height. For the oxygen covered surface reflection is found to occur at angles of incidence less than 35’ and again it is apparently non-specular. The reflection process can be modelled crudely by allowing the incident atoms to experience a repulsive Born-Mayer potential. The apparently non-specular effects are attributed to rainbow scattering from the periodic potential at the surface. Polar plots of the electron ejection process show marked angular effects for both the clean and oxygen covered surface, which vary with angle of incidence. Electron yield measurements show that the yield of ejected electrons is greater for the oxygen covered surface and both the clean and oxygen covered surface give a yield which increases with increasing angle of incidence. The ejected electron spectrum for both the clean and oxygen covered surface is unlike that which can be expected from a two-electron resonance ionization/Auger neutralization process. The oxygen covered surface shows a peak in the energy spectrum which corresponds with UPS measurements. It is concluded that, at normal incidence, for an oxygen covered surface, the electron ejection mechanism may be Auger de-excitation.