Photoelectric Effect from Borosilicate Glass

Abstract

Values of the photoelectric quantum efficiency of glass for different wavelengths have been determined and data have been obtained on the status of electrons placed on glass surfaces. One of the borosilicate glasses commonly used in discharge tube manufacture and soda glass were studied in vacuum with 2537 A photons from a quartz Heraeus mercury arc and with light from a glow discharge operating in pure A and Ne from 10⁻¹ to 10 mm pressure. Clean borosilicate glass in vacuum at 22°C with fields of some 1000 v/cm across it yields photoelectric current densities of the order of 10⁻¹² amp/cm² for the full light of a 110 volt arc focussed on 10 cm² area through quartz optics. This current comes from the 3–4% Na impurity atoms on the surface. Soda glass yields currents in proportion to the density of Na atoms. The normal conductivity of the glass comes from Na⁺ ions of some 0.5 volt activation energy normally present. It is enhanced 10‐fold by Na⁺ ions liberated at the surface by light. Na⁺ ions next the cathode cannot leave the glass, but electrons enter the glass from the metal cathode backing of lower work function to neutralize the ion space charge as it builds up near the cathode. Electrons sprayed onto the glass surface have a mobility of ∼5×10⁻⁵ cm²/volt sec in the glass in contrast to the value of ∼1×10⁻⁷ cm²/volt sec for the Na⁺ ions reported by Kraus. Photons above 3300 A do not contribute to the conductivity in any fashion. The photoelectric yield for 2537 A was obtained directly in absolute magnitude by use of a vacuum bolometer. For the light from glow discharges ≪1200 A, the efficiency was obtained indirectly by comparison with values for Pt and Au. The values obtained are tabulated below. $$\begin{array}{cccccc}\mathrm{Wave-}& \mathrm{Borosili-}& \mathrm{Soda}& & & \\ \mathrm{length\; A}& \mathrm{cate\; glass}& \mathrm{glass}& \mathrm{Au}& \mathrm{Pt}& \mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}\mathrm{Remarks}\\ \text{2537\hspace{0.17em}}\mathrm{A}& {10}^{\text{−4}}& {\text{6×10}}^{\text{−4}}& {10}^{\text{−2}}& \dots & \mathrm{Vacuum}\\ \text{≫2300\hspace{0.17em}}\mathrm{A}& {10}^{\text{−4}}& \dots & \dots & {10}^{\text{−3}}& \mathrm{Glow\; discharge}\\ \text{≪1250\hspace{0.17em}}\mathrm{A}& {10}^{\text{−3}}& \dots & {10}^{\text{−1}}& {10}^{\text{−1}}& \text{0.1\hspace{0.17em}}\mathrm{to}\text{\hspace{0.17em}10\hspace{0.17em}}\mathrm{mm\; Hg}\end{array}$$