In this study, we examine the adsorption and thermal reactions of digermane (${\mathrm{Ge}}_{2}$${\mathrm{H}}_{6}$) on the Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surface with high-resolution core-level photoemission spectroscopy using synchrotron radiation. At 325 K, the digermane dissociatively chemisorbed to produce ${\mathrm{GeH}}_{3}$, ${\mathrm{GeH}}_{2}$, GeH, and SiH species. The sticking coefficient at zero coverage deduced from the photoemission intensity is \ensuremath{\sim}0.5. Successive annealing of the digermane-saturated surface to higher temperatures causes further decomposition of ${\mathrm{GeH}}_{3}$ and ${\mathrm{GeH}}_{2}$ and the desorption H from GeH and SiH, leaving atomic Ge on the surface. In light of the sufficiently large chemical and surface shifts in their core-level binding energies for different surface species, those processes were identified by examining the evolution of Ge 3d and Si 2p line shapes. Experimental results indicate that the reaction for H release from GeH not only occurred in a large temperature range but also depended heavily on the ${\mathrm{Ge}}_{2}$${\mathrm{H}}_{6}$ adsorption coverages. Two reaction routes for H release from Ge sites were used to describe the large reaction temperature spreads accurately. GeH decomposition by transferring the H atom to a surface Si dangling bond took place for low coverages at \ensuremath{\le}590 K, and ${\mathrm{H}}_{2}$ thermal desorption occurred for higher coverages in the range of 590--770 K. The former process of atom transfer of H from Ge to Si sites was directly observed in the Ge 3d and Si 2p photoemission spectra. \textcopyright{} 1996 The American Physical Society.