Effect of environmental O2 on dynamical process of the Si(111)"1~1"¨7~7 structural phase transition has been investigated. In a macroscopic view, growth of the 7~7 DAS structure on the quenched surface after pulsed laser irradiation was monitored in-situ with RHEED under different O2 pressures. In an atomic scale view, the n~n DAS domain growth was observed with high temperature STM. It has been found that the oxides formed in the "1~1" matrix or phase boundary between n~n DAS-1~1 and the n~n domain boundary is responsible for the suppression of the 7~7 growth. Under the higher O2 pressure, not only suppression of the 7~7 growth but also erosion of the 7~7 took place. The threshold O2 pressure which suppresses the 7~7 domain growth but does not attack the 7~7 domains themselves was higher for the specimen kept at 600 than that at 465 because of the much less phase boundaries in the former.
The normalized (3/7,4/7) reflection intensity changes after laser irradiation on samples at 465 (a) and at 600 (b). The more the O2 pressure, the more the intensity increase was suppressed. Exposure to O2 is less effective at 600 than 465 .