Photoemission from Silver into Sodium Chloride, Thallium Chloride, and Thallium Bromide

The photoelectric work function for the transfer of electrons from a metal directly into an insulating crystal has been investigated for Ag on samples of NaCl, TlCl, and TlBr. The results obtained do not confirm the existence of this process in the samples studied although the apparatus had a current sensitivity of 1×{}10^-17 ampere. Mott and Gurney have suggested that the work function of the metal-insulator contact would be less than the vacuum work function of the metal by an amount equal to the electron affinity of the insulator. Previous experimental evidence supporting this picture of the contact appears less conclusive in the light of the results of this investigation.During the investigation of the thallium halides, photosignals were observed far beyond the intrinsic edge (0.38{μ} for TlCl and 0.46{μ} for TlBr at -150^∘{}C). Intrinsic irradiation at low temperature appreciably enhanced the long-wavelength photoresponse. The enhanced response could be bleached by heating the sample to room temperature. It was established that the photoresponse was a property of the crystal rather than photoemission from Ag into the crystal.