Hydrogen and deuterium in bond-centered (BC) and platelet-like configurations were detected in hydrogenated (and deuterated) amorphous silicon thin films deposited from SiH4 and SiD4. plasmas. Infrared absorptions due to these configurations were measured using in situ multiple total internal reflection Fourier transform infrared spectroscopy in a differential mode. where changes in the as-deposited a-Si: H(D) films were observed during D,(H,) plasma exposure. This method coupled with preferential replacement of H(D) by D(H) in BC and platelet-like configurations over the isolated bulk SiH(SiD) configurations enabled detection of these modes without interference from the strong SiH(SiD), absorptions. The Si-H(D) stretching modes for BC hydrogen and BC deuterium were observed at similar to1950 and similar to1420 cm(-1), respectively, while those for platelet-like, hydrogen and deuterium were detected at similar to 2033 and similar to 1480 cm(-1) respectively. (C) 2004 American Vacuum Society.