The formation and restoration of defects by F-2 laser irradiation with high laser fluence were investigated fur modified silica glasses, and the results were compared with those by ArF excimer laser irradiation. F-2 laser irradiation induced oxygen deficient centers (ODCs) and E' centers via one-photon-absorption processes, while ODC and E' defects are generated by two-photon-absorption processes by an ArF excimer laser. As-doped SiOHs and photoinduced SiOHs enhanced the formation of defects markedly in the case of F2 laser irradiation. F-2 laser light transformed isolated SiOH bonds into hydrogen-bonded SiOHs, while such a process did not occur under ArF excimer laser light. These results suggest that silica glass networks were dissociated by two types of processes. The dominant process is the formation of pairs of E' centers and NBOHCs, followed by conversion to the SiHs and SiOHs as a result of chemical reactions with hydrogen molecules in silica glass at room temperature, The other is the generation of ODC defects accompanied by interstitial oxygen molecules, which are also decomposed partly into E' centers with the aid of F-2 laser light.