Hydrogen‐bombardment induced damage in single crystal silicon as a function of the substrate temperature, ion energy, and ion dose was studied usinginsituspectroscopic ellipsometry over the photon energy range 2.0–5.5 eV under high vacuum conditions. The incident hydrogen ion energies were 300 and 1000 eV, and the doses were 1015–1018ions/cm2.Insituspectroscopic ellipsometry results showed that the damage layer thicknesses for the samples bombarded at elevated temperatures are smaller than for samples bombarded at room temperature and subsequently annealed at the same elevated temperature. The diffusion coefficient for hydrogen in silicon of 6 × 10−15cm−2/s was obtained from theinsituspectroscopic ellipsometry data.