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Study of Raman spectra of KBr doped with F-centres by excitation in the F and K bands

Raman spectra of KBr(F) found by excitation of the low-energy side of the F band can be well interpreted by the off-resonance approximation when the '1s-2p'-like transition of the F electron is taken into consideration. A 58% softening of the force constant between the F-centre and its nearest neighbours affects the positions of the two Gamma 1+ resonant modes at 110 and 121 cm-1. The relative values of the electron-phonon coupling parameters have been determined in a phenomenological manner by considering the vibrations of the first five shells of ions neighbouring the F-centre. When the laser line is tuned from the F to the K band, an enhancement of the LO modes is observed in the Raman spectra relative to the vibrations of Gamma 1+ symmetry. The F-centre Raman scattering theory developed by Robbins and Page (1977, 1978) accounts for the change in lineshapes of the first-order Raman spectra when the interference effect between the F and K band contributions to the Raman scattering is taken into consideration, but it cannot justify the relative values of the electron-phonon coupling parameters.