Optical emission characteristics of vacuum ultraviolet light which is effective in decomposing source gases used for deposition of hydrogenated amorphous silicon were investigated. The emission line from H atoms at 121.5 nm corresponds to the absorption peak of source gases of silicon hydrides. This emission line intensity increases markedly by mixing hydrogen with helium from 5 to 10% in concentration. The deposition rate of hydrogenated amorphous silicon films by windowless photochemical vapor deposition using this strong emission line at 121.5 nm is proportional to the emission intensity, which reaches as high as 270 angstrom/min.