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.