SUBPICOSECOND SPECTROSCOPY OF HOLE AND EXCITON SELF-TRAPPING IN ALKALI-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. This rule describes important features of all subpicosec ond absorption spectra of excited alkali halides compiled from the pre sent work and other published works, within the framework of two-photo n cross-correlated absorption of the excitation and probe pulses as we ll as pulse dispersion in the optics and in the sample. It is conclude d that the one-center self-trapped hole in alkali halides has not yet been identified by absorption spectroscopy in the visible-uv spectral range. The self-trapped hole and/or associated self-trapped exciton is observed in the two-center configuration. We also report studies usin g the fourth harmonic of an amplified Ti:sapphire laser to excite insu lators with band gaps as large as 11.6 eV by two-photon absorption and observe the subsequent development of defect spectra.