ULTRAFAST SPECTROSCOPY OF HOLE AND EXCITON SELF-TRAPPING IN HALIDE CRYSTALS

Subpicosecond and picosecond optical absorption spectra following band -gap excitation of alkali halide crystals have been measured and analy zed. It is found that a simple rule relates the low-energy edge of the ''N band'' previously attributed to the one-center (Landau-type) hole polaron, the photon energy of the (two-photon) excitation pulse, and the band gap. The rule explains the quantitative photon-energy depende nce of the N-band position and its presence or absence in all of the d ata compiled from the present work and other published works, within t he framework of two-photon cross-correlated absorption of the excitati on and probe pulses as well as pulse dispersion considered separately in the optics and in the sample. It is concluded that the one-center s elf-trapped hole in alkali halides has not yet been identified by abso rption spectroscopy in the visible-uv spectral range. The self-trapped hole and/or associated self-trapped exciton is observed in the two-ce nter (V-k) cofiguration. (C) 1998 Elsevier Science B.V. All rights res erved.