Stretching mode interaction of equal and unequal pairs of SeH−and SeD−defects in alkali halides

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Abstract
The stretching-mode (SM) absorption of isolated SeH− or SeD− molecular defects in alkalihalides show a resolved spectral structure due to the different reduced masses of the six natural stable Se isotopes. In a FTIR study of these defects in KCl and KBr, we have observed new SM absorptions of defect pairs which scale quadratically with the defect concentration. For “equal pairs” SeH−-SeH− and SeD−-SeD−, the fundamental SM absorption shows (in contrast to “mixed” SeH−-SeD− pairs) a distinctly different Seisotope splitting, while the overtone splitting of all pairs remains similar to that of the isolated defects. Besides a simple classical coupled-oscillator treatment, we present a quantum-mechanical calculation, explaining the observed spectral behavior by resonance interaction of the molecules in pairs of equal (or “nearly equal”) frequencies.

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