Surface induced dissociation in slow collisions of H-2(+) and O-2(+): information from the ion impact electron spectra

Measured and modeled ion impact electron spectra for slow collisions (50 eV ) of H-2(+) and O-2(+) ions with W(1 1 0) surfaces under grazing incidence are presented. The work function of the surface was varied by its exposure to alkali atoms. Tn order to model the energy spectra the following procedu re was pursued (Refs. [1-3]: P.A. Zeijlmans van Emmichoven, P.A.A.F. Wouter s, A. Niehaus, Surf. Sci. 195 (1988) 115; P. Eeken, J.M. Fluit, A. Niehaus, I. Urazgil'din, Surf. Sci. 273 (1992) 160; H. Muller, V. Kempter, Surf. Sc i. 366 (1996) 343): a set of coupled rate equations, representing the proje ctile in its various charge and excitation states, is solved as a function of the projectile's distance with respect to the surface; The energy spectr um is constructed from the contributions of the various Auger-type processe s occuring along the projectile's trajectory. The following molecular aspec ts are taken into account: For H-2(+), Auger processes can end at either on e of the two final states H-2((1)Sigma(g)(+)) and H-2((3)Sigma(u)(+)) (whic h after its population will dissociate spontaneously), and the influence of the vibrational excitation of the projectile ion with its consequences for the electron spectra is considered. For O-2(+), Auger de-excitation (AD), connected with electron emission, can take place during the spontaneous dissociation of the repulsive states O-2 *(1,3)Pi(u) populated at low work functions. Under these conditions the modeled electron energy spectra, both for H-2(+) and O-2(+) collisions, agree satisfactorily with the experimental results. This allows for the identification of the sequence of electron transfer pr ocesses taking place during the collision, and gives information on the mec hanisms for surface-induced dissociation in slow collisions of molecular io ns. (C) 1999 Elsevier Science B.V. All rights reserved.