We report on the optical properties ofa‐Si : H prepared by cathodic sputtering of a Si target in an 80% argon/20% hydrogen reactive gas at ∼9×10−3Torr. The deposition rate &fgr; (by means of the rf voltage) and the substrate temperature were varied. The total hydrogen content [H] and the concentration of hydrogen involved in SiH bonds [H1] or in SiH2bonds [H2] were measured. We find [H]≳[H1]+[H2], two times higher in some preparation ranges (&ngr;?15 A˚/min or &ngr;=100 A˚/min), which implies the occurrence of hydrogen atoms H′ not bonded in the usual SiH or SiH2forms. The optical absorption coefficient, the optical energy gapE0, and the refractive indexnswere measured.E0decreases asvincreases, is independent ofTsfor &fgr;≳30 A˚/min, and decreases asTsincreases forv≳30 A˚/min.nsincreases toward the crystalline value as &fgr; increases andTsdecreases. The similarity of the properties of the layers obtained by sputtering and glow discharge is emphasized. We suggest equilibrium between hydrogen bonds as SiH and SiH2and the other form(s) and that the optical gap is controlled by the deformation of the silicon matrix by the various hydrogen species.