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In situ and spectroscopic ellipsometry measurements have been applied to probe damage mechanisms induced by low‐energy hydrogen ion bombardment of single‐crystal silicon. Ion beam voltages from 20 to 1500 V have been employed, covering the range found to passivate grain boundaries in polycrystalline Si and near‐surface defects in dry etched single‐crystal Si. The spectroscopic data for room‐temperature samples after bombardment provide information needed to model the in situ data, obtained using a fixed photon energy of 3.4 eV at a nominal sample temperature of 200 °C. Models for the in situ ellipsometry data for ion energies ≪500 V allow determination of the evolution of damaged surface layers as a function of exposure time. Two layers dominate the time evolution of the optical data: (1) a polycrystalline Si (p‐Si)‐like layer, modeled as an effective medium mixture of c‐Si, a‐Si, and void, which develops on the scale of 1 min (dose: 6×1017 ions/cm2), and (2) a more slowly evolving transparent layer, interpreted as an oxide. The latter layer, also observed in earlier studies, may be attributed to oxidation caused by vacuum residual oxygen ions in the hydrogen ion beam. The oxidation consumes the top‐most p‐Si‐like region of heaviest damage.