The thermal behavior of an ammonia-covered Si(100) surface is investigated by infrared spectroscopy and density functional methods. Upon adsorption at room temperature, (Si) N H2 and Si-H species are formed on the surface. Comparison of the vibrational studies with density functional calculations suggests that the (Si) N H2 structures are preferentially located on the same side along the silicon dimer row on a (2×1) reconstructed Si(100) surface, although a mixture of different long-range configurations is likely formed. Decomposition of these (Si) N H2 species is observed to start at temperatures as low as 500 K. Theoretical predictions of the vibrational modes indicate that at this point, the spectrum is composed of a combination of (Si)2 NH and (Si)3 N vibrational signatures, which result from insertion of N into Si-Si bonds. Our computational study of the formation of (Si)2 NH structures indicates that subsurface insertion is more feasible if the strain imposed during the insertion in a Si dimer is attenuated by a (Si)2 NH structure already inserted in the neighboring dimer along the same silicon dimer row. This cooperative reaction lowers the energetic requirements for subsurface insertion, providing a theoretical explanation for the mechanism of thermal decomposition of N H3 on Si(100) and for other systems where subsurface migration is observed experimentally. © 2007 The American Physical Society.