Coupling between molecular and charge transfer excitations in mixed alkali halide-cyanide crystals

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Abstract
The optical absorption spectra of mixed alkali halide cyanide crystals [M(CN)x Y 1−X where M = Na, K, Rb and Y =F, CI, Br, I] were measured at temperatures ranging from 300 K down to 4.2 K and CN- concentrations from 102% up to pure cyanide crystals. The absorption cross sections of the parity forbidden X 1σ+→a'3σ+ vibronic transitions of the CN molecular ions were found to present a strong, non-linear dependence on the x-composition of the mixed crystal as well as on the halide ion. This behavior is explained by the coupling of the triplet molecular excitations to singlet charge transfer excitations in their neighborhood. The experimental data were fit with a coupling energy of a few meV, depending mainly on the type of excitations involved (localized or collective) and not on the particular halide ion. The coupling mechanism is discussed and it is found to be due to the overlap between the wavefunctions of the two excitations.

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