KINETIC-ENERGY DISTRIBUTIONS OF O- PRODUCED BY DISSOCIATIVE ELECTRON-ATTACHMENT TO O-2 PHYSISORBED ON A KR SUBSTRATE

We report measurements of the kinetic-energy (E(k)) distributions of O- produced by low-energy electron impact (6-18 eV) on thin disordered films consisting of 0.15 ML of O-2 physisorbed on 5 ML of Kr, all condensed on polycrystalline Pt. The present measurements confirm that, for incident electron energies E(e)less than or equal to 12 eV, dissociative electron attachment (DEA) to physisorbed O-2 involves the (2) Pi(u), (2) Sigma(u)(+) (I), and (2) Sigma(g)(+) (I) resonances of O-2(-)*; of these, only the latter is found to dissociate to the second limit O-(P-2)+O(D-1), via a nonadiabatic curve crossing. Above E(e)=12 eV, DEA to O-2 is found to proceed mainly via the (2) Sigma(x)(+) (II) [x = g and/or u] state(s), leading to O-+O(D-1) fragments. No evidence for O-2(-)* resonance states which dissociate to the third limit O-+O(S-1) is found below E(e)=18 eV. The results also support the general notion that postdissociation interactions which lead to ion energy loss consist mainly of binary large-angle elastic collisions of the desorbing O- with adsorbate particles in the van der Waals solid. Contributions of energy-loss electrons to the DEA O- E(k) distributions are found to most likely originate in small O-2 clusters at, or near, the surface of the Kr substrate.