Local vibrational modes of a vacancy with two hydrogen atoms (VH 2 ) and of a vacancy with one oxygen and two-hydrogen atoms (VOH 2 ) in silicon have been investigated using isotopically enriched 28 Si, 29 Si, and 30 Si single crystals. Infrared absorption spectroscopy revealed shifts in the Si-H stretch frequencies of the two defects when the mass of the silicon host atoms was changed. The observed stretch frequencies can for each defect be accounted for with a simple vibrational model based on two coupled Morse oscillators. The anharmonic contribution to the local vibrational mode frequencies of these two defects is evaluated.

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