Photoelectric and Metastable Atom Emission of Electrons from Surfaces in the Rare Gases

Abstract

The causes of conductivity observed at considerable distances beyond the end of a rare gas discharge are investigated at pressures of 0.5-2 mm. In pure gases, currents from negatively charged electrodes are found to be mainly caused by photoelectric action thereon of the resonance or other extreme ultraviolet radiations. For similar discharge conditions, currents are roughly 10-fold greater in Ne than in He and 20-fold greater in Ne than in A. When traces of impurities (ionizable by metastable atoms) are added, the currents are increased because of the ionization of the former by metastable atoms (volume effect). Under these circumstances strong irradiation with light of the gas in question (which is found to destroy more than half the metastable atoms formed) causes marked decreases in the currents; but the currents in pure gases are only negligibly so reduced. Electron currents from a thin metal disk are several times greater when the plane of the disk is perpendicular to the direction of the radiation than when it is parallel thereto. The fraction of the currents in Ne which may be caused by the emission of electrons from surfaces by metastable atoms is 10 percent for the parallel position of the above disk and 3 percent for the perpendicular position. This fraction is still smaller in A but may be considerably higher in He. The fraction of the scattered resonance quanta which are converted into metastable atoms before reaching the walls may be as high as 50 percent in He but is probably 5 percent in Ne. The small value of this fraction in Ne is in accord with other evidence for the abnormally long free paths of resonance radiation in this gas. The efficiency of metastable atom emission of electrons from surfaces under the present conditions is found to be 1 and is probably not much greater in order of magnitude than the photoelectric efficiency of the radiations concerned. The effective cross section for the ionization of A by metastable Ne atoms is estimated. A convenient method of detecting and measuring small traces of impurities in Ne and He is described. Evidence is given that, on account of wall reflections, visible radiation, and probably also (but for gaseous absorption) extreme ultraviolet radiation, passing down a glass tube will, over a considerable range, decrease approximately with the inverse first power of the distance.