Excitonic two-phonon progressions in transition metal compounds

Evidence of two-phonon structures is reported for some intra-configurationa l crystal held transitions in layered V- and Cr-halides. In particular, the crystal field spectrum of CrBr2 (3d(4)) has bean shown for the first time, since the exciton-phonon interaction, despite its unfilled e(g) orbitals, vanishes to first order yielding a two-phonon sequence in the (3)A(1)(G) (3)A(2)(F) band. It is suggested that the main mechanism for the occurrence of two-phonon progressions consists in the direct coupling of the incident photon to a two-phonon state via the second-order dipole moment operator. (C) 2000 Elsevier Science B.V. All rights reserved.