Electron energy‐loss spectroscopy (EELS) and reflection high‐energy electron diffraction (RHEED) have been used to investigate thermal and photothermal reactions in dihydride (1×1)::2H surfaces obtained by dosing clean Si(100)2×1 surfaces to a saturation exposure (≊2×1017 cm−2) of Si2H6 at 300 K. With 15‐s anneals at progressively higher temperatures, the (1×1)::2H was transformed to a (2×1):H monohydride phase at temperatures between 655 and 725 K and most of the remaining H evolved by 765 K. Complete recovery of the initial clean‐surface EELS spectrum and 2×1 RHEED pattern, signaling deposition of one layer of epitaxial Si(100)2×1, was obtained between 845 and 955 K. One pulse of 120 mJ cm−2 ArF laser‐beam irradiation resulted in significant hydrogen evolution from Si2H6‐saturated dihydride surfaces at 300 K and complete hydrogen desorption, with the reestablishment of the initial clean‐surface EELS spectrum, at 525 K. The mechanism was found to be photothermal rather than photolytic and the maximum temperature during the ≊20‐ns pulse (at 525‐K starting temperature) was calculated to be 800 K. RHEED patterns from dihydride surfaces irradiated at 525 K were 2×1, but with slightly weakened half‐order diffraction spot intensities. In initial film growth experiments using continuous Si2H6 dosing and pulsed ArF laser irradiation, epitaxial Si was obtained at a steady‐state temperature of 525 K.