TEMPERATURE AND COVERAGE CONTROL OF O-2 PHOTOFRAGMENTATION ON SI(111)7X7

The photofragmentation study of O-2 adsorbed on Si(111)7 x 7 shows tha t two markedly different regimes can be selected by varying the sample temperature and the O-2 coverage. At room temperature the photofragme ntation is essentially dominated by direct processes whereas at low te mperature (30 K) the photofragmentation evolves from indirect to direc t processes when increasing the O-2 coverage from 0.04 to 0.2 L. These effects are tentatively assigned to the temperature and coverage depe ndence of the adsorption sites of O-2 on Si(111)7 x 7.