Temporal Development of Absorption Spectra in Alkali Halide Crystals Subsequent to Band-Gap Excitation

Abstract

The electronic states of a semiconducting or of an insulating crystal often are treated as though the lattice were rigid, quite indifferent to the presence of carriers. However, departures from this picture are well known. In polar AB crystals, conduction-band electrons have enhanced mass, existing as polarons; and in many halide salts the hole is actually immobilized. Fields associated with an electron in the vicinity of such a hole, a self-trapped exciton (STE), can give rise to substantial displacements of nuclei and even to photochemistry. Transient lattice defects [1], with near-unity yields [2], can be recognized in optical absorption or in emission. Such absorption may cover much of the spectrum, from the near-infrared to the near-ultraviolet. For the alkali halides, there are minor yields of stable or long-lived defects; these populations have been studied for many years by conventional spectroscopic techniques. At low temperatures the ground-state ³Σ_u ⁺ STE is stable except for a spin-forbidden radiative transition to the electronic ground state of the crystal. However, certain higher states of the STE lead to generation of F-H defect pairs; this has been demonstrated in double-excitation experiments [3]. The stability of the ground-state STE against such defect creation is associated with only a small barrier — about 110 meV in NaCl [4]. We have employed short-pulse white-light spectroscopy to record absorption of STE, F, and F-like species early after band-gap excitation by two 266 nm photons. We interpret the temporal evolution of the spectra in terms of thermal migration of Cl atoms over the above barrier [5,6]. In this system, photochemical defect production follows promotion to a potential sheet on which the barriers to halogen diffusion are small.