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 cond ensed on polycrystalline Pt. The present measurements confirm that, fo r incident electron energies E(e)less than or equal to 12 eV, dissocia tive 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 fo r 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 not ion that postdissociation interactions which lead to ion energy loss c onsist mainly of binary large-angle elastic collisions of the desorbin g O- with adsorbate particles in the van der Waals solid. Contribution s 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 surfac e of the Kr substrate.