Vibrations of hydrogen-chemisorbed Si(100) surfaces with 3 X 1 and 1 X 1 structures were studied by buried metal layer-infrared reflection absorption spectroscopy. The SiH2 bend scissors mode was found to split into two distinct peaks at 902 and 913 cm(-1); they were assigned to isolated dihydride (ID) and adjacent dihydride (AD), respectively. The observed differences in the dependence on annealing temperature showed that the small peak near 2090 and the 2107 cm(-1) peak were assignable to the symmetric stretching mode of ID and less stable AD, respectively.