Transient trapping/detrapping of electrons at the Si(100)/SiO2 outer surface is studied studied in vacuum or with an O2 ambient (between 10-3 and 30 Torr) following internal electron photoemission from Si. Photoemission-current (produced by a 150 fs, 800 nm laser source) and contact-potential-difference techniques were used to investigate a wide variety of n- and p-doped samples at 300 K with thermally grown, steam grown, and dry oxides with thickness ≤5 nm as well as samples with the oxide layers removed. Characteristics of the steam grown oxide were also studied at 400 and 200 K. For samples in vacuum charging is attributed to direct filling of at least two families of traps, one related to the oxide and the other the Si/SiO2 interface. For samples in O2, details of oxygen-assisted surface charging as reported previously [Phys. Rev. Lett. 77, 920 (1996)] are given. A fast, Coulomb-repulsion driven spillover of surface charge from the irradiated spot to the rest of the surface was detected. Oxygen aids trap filling of the in-vacuum filled and gas-sensitive traps and also detrapping (the efficacy of which increases strongly from 400 to 200 K) when the optical excitation source is removed. Surface transient charging and charge trapping efficacy for the oxidized samples are not very sensitive to sample preparation. A mobility of the trapped charges, probably hopping between traps and also Coulomb-repulsion driven, was measured. © 2000 American Institute of Physics.