OBSERVATION OF 2 LONG-LIVED MOLECULAR-STATES OF O-2 ON SI(111)7X7 AT ROOM-TEMPERATURE

Adsorption of O-2 on Si(111)7 x 7 at room temperature is studied by photoemission. Low exposures of oxygen, from 0.1 to 10 langmuir, lead to the formation of two surface states, at 3.8 and 5.1 eV below the Fermi level. The state at 3.8 eV was previously assigned to molecular oxygen chemisorbed in the paul, para and grif configurations. The state at 5.1 eV can be assigned to a different adsorption configuration of molecular oxygen on Si(111)7 x 7. The lifetime of both states is the same within the experimental uncertainty. The lifetime of the 3.8 eV state is found to be of the order of 400 min, i.e. much longer than the previously measured lifetime.