Exciton-phonon interaction in 3d transition metal compounds

Phonon structures observed in crystal-field bands of V-, Cr-, Mn- and Ni-ha lides have been investigated by absorption spectroscopy around the liquid h elium 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.