Visible light photoemission and negative electron affinity of single-crystalline CsCl thin films

CsCl has been known to show extraordinary photoemission by visible (VIS) light only after ultraviolet (UV) light irradiation. To elucidate this phenomenon, photo yield and photoemission spectrum of a single-crystalline CsCl thin film were measured in ultrahigh vacuum. Photo yield measurements confirmed that photoelectrons were emitted from F-centers induced in the CsCl thin film by preceding UV irradiation. Absence of the similar phenomenon in the NaCl film indicated that VIS photoemission in alkali halides was caused not only by defect formation but also by negative electron affinity.