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Exciton-phonon interaction in 3d transition metal compounds

Abstract Phonon structures observed in crystal-field bands of V-, Cr-, Mn- and Ni-halides have been investigated by absorption spectroscopy around the liauid helium temperature. Most of the structures are interpreted in terms of many-phonon processes induced by first-order linear electron-phonon interaction. However, evidence of two-phonon vibronic progressions is reported for some intra-configurational crystal-field transitions. It is considered that the main mechanism for the occurrence of two-phonon progressions involves the direct coupling of the incident photon to a two-phonon state via the second-order dipole-moment operator.