ELECTRONIC-TRANSITIONS AND DISSOCIATION IN LOW-ENERGY GRAZING COLLISIONS OF O-2(+)X(2)PI(G) AND A (4)PI(U) WITH W(110) PARTIALLY COVERED BYALKALI ATOMS

Electron energy spectra from slow (50 eV) O2+ X2PI(g) and a 4PI(u) mol ecular ions colliding under grazing incidence with W(110) surfaces are reported. The surface work function was varied by the exposure to alk ali (Na, K, Cs) atoms. The electron energy spectra are interpreted in terms of various inter- and intra-atomic Auger processes: For collisio ns with clean W(110) and alkali coverages below 0.3 monolayers (ML) (i n terms of the first completed adlayer at room temperature) Auger capt ure is observed only. At least for O2+ X2PI(g) dissociative neutraliza tion does not occur. Beyond this coverage core excited molecular state s (molecular Rydberg states), formed by the addition of a 3s sigma(g) electron to the respective ion core, become populated. Their decay man ifests itself by Auger deexcitation. In addition we see inter- and int ra-atomic Auger processes involving core excited states of atomic oxyg en, most probably O-(4S)3s2. These states originate from the collisio n induced dissociation of the core excited molecular states. At least for O2+ a 4PI(u) the dissociation via core excited molecular states ap pears to be efficient. From the energy spectra of the O- ions scattere d into 90-degrees with respect to the ion beam direction we conclude t hat under the chosen conditions the O2+ projectile mainly interacts wi th the alkali adatoms.