Identification of a Fermi resonance for a defect in silicon: Deuterium-boron pair

We interpret an anomalous isotope shift in the vibrational spectrum of the deuterium-boron pair in silicon in terms of a Fermi resonance between the second harmonic of the transverse boron vibration and the fundamental longitudinal deuterium vibration. A simple quantum-mechanical treatment accounts for the reported observations and further predicts the oscillator strength and polarization of infrared absorption of the perturbed second harmonic of the boron vibration. We observe this second harmonic, now allowed due to its mixing with the deuterium mode, in agreement with the model.