Photoelectric Emission from Single-Crystal KI

Photoelectric emission measurements were made on single crystals of KI. Yield curves, obtained after irradiation in the first fundamental absorption band, are similar to those observed on evaporated films and the features are attributed to direct ionization and exciton-induced emission from $F$ centers of density ∼${10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$, only 10 times lower than for films. At 80°K additional structure appears. A small yield for quantum energy below 2.5 ev is attributed to the presence of $F^{\prime }$ centers. At 5.5 ev, the position of $\beta$-band absorption, a small peak is observed. An inversion in the exciton-induced emission yield at 5.8 ev is associated with the sharply peaked surface reflectivity. While the ratio of exciton-induced to direct ionization is slightly higher in crystals than in films, the effective cross section for exciton-induced ionization is still ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$; thus, the photoemission provides direct evidence for energy transfer from excitons but not necessarily for migration.