Photoconductivity and the external photoelectric effect in PbS

Abstract We have measured the yields for photoconductivity and external photoernission in the vacuum ultraviolet spectral region, 584–3000 A, using polycrystalline layers and natural crystals. The photoconductive yield in PbS layers rises approximately linearly with incident photon energy hv up to 7.5 eV; further extension of the measurement was thwarted by the magnitude of the external photocurrent. The rise in yield, as previously reported by us, is attributed to collision multiplication beginning at hvt = 2 eV. The observed photoelectric threshold for galena crystals was ∽ 5 eV, and for layers ∽ 6 eV. Both kinds of specimen exhibit very large external photoyields at high hv . 10–20 per cent at hv = 14.5 eV. The energy distribution of emitted electrons was examined using retarding-potential methods. The high photoelectric thresholds imply that any electron with sufficient energy to escape would be subject to scattering due to collision multiplication, an effect expected normally to limit the attainable photoelectric yield. From the high ultimate yields that were observed, and from the changes in the energy distribution of the emerging electrons with increasing hv , we infer that scattering by collision multiplication, considered as a function of hv , is not a monotonically rising function, but has a maximum. The data suggest strongly that this occurs for incident photon energies of slightly more than 9 eV.