The interaction of thermal atomic hydrogen (H0) with very thin amorphous hydrogenated silicon (a-Si:H) films is investigated using real time in situ infrared spectroscopy. Hydrogen bonded in isolated and in clustered network configurations is identified from the analysis of the IR line positions and the kinetics of the hydrogen uptake in hydrogen-depleted, hydrogen-deficient and deuterated samples. The use of very thin films is important for this mode identification, since the penetration of atomic hydrogen is not diffusion limited and is therefore very uniform. The analysis yields an IR mode for the isolated SiH groups centered at ∼1985 cm-1, and a newly identified IR mode for platelet-like SiH groups at ∼2033 cm-1. On the basis of this mode identification, the relative reaction probabilities for H saturation of Si dangling bonds, the insertion into strained Si-Si bonds and the H abstraction reaction are determined to 1:0.44:0.26. The kinetic evolution of the two SiH bulk modes successfully describes the measured structural changes and etching of a-Si:H during exposure to H0. © 1998 American Institute of Physics.