1. Contribution from the Surface Science Research Centre and Chemistry Department, Manchester University, Manchester M13 9PL, U.K., Physics Department, UMIST, P.O. Box 88, Manchester M60 1QD, U.K., Dipartimento di Fisica, Via Valerio 2, 34127 Trieste and Laboratorio TASC-INFM, S.S. 14 Km 163.5 Basovizza, 34012 Trieste, Italy, and Sincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, 34012 Trieste, Italy