The distribution of Si oxidation states in 1.7-, 2.0-, and 3.0-nm-thick downstream plasma enhanced chemical vapor deposited SiO2 film upon Si has been studied by soft x-ray photoemission, using synchrotron radiation. The spectra from the as-deposited 3.0-nm-thick oxide show two prominent Si oxidation states, the Si4+ and the Si3+, with the Si3+ oxidation state preferentially located at the oxide vacuum interface. The Si3+ state is considerably larger than that observed in thermal SiO2 and is radiation and thermally sensitive, partially converting to the Si4+ state during annealing and extended irradiation. The analysis of the photoinduced conversion from the Si3+ to the Si 4+ oxidation state suggests that the paramagnetic component of the Si3+ structure matrix can be correlated with the structure of the E' defect center, as measured by electron spin resonance. A model is proposed for the SiO2 deposition mechanism whereby a temperature activated surface oxidation of the Si3+ structure is taking place.