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 analyzed. 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 subpicosecond absorption spectra of excited alkali halides compiled from the present work and other published works, within the framework of two-photon cross-correlated absorption of the excitation and probe pulses as well as pulse dispersion in the optics and in the sample. It is concluded 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 using the fourth harmonic of an amplified Ti:sapphire laser to excite insulators with band gaps as large as 11.6 eV by two-photon absorption and observe the subsequent development of defect spectra.