Surface condition and electron emission from cold cathodes in vacuum and in noble gas glow discharge

An optogalvanic method is used to measure photoemission coefficient gamma(ph) in a gas discharge He (i is the current density, exposed to the resonant radiation of helium atoms. The range of working current j/P-He(2) (j is the current density, and P-He is the gas pressure) extends from 2 to 1000 mu A/(cm(2) Torr), and field strength E/N at the cathode varies from 0.45 to 13 kTd. Up to j/P-He(2) = 10 mu A/(cm Torr), photoemission coefficient gamma(ph) grows and then tends toward saturation at a level of gamma(ph) = 0.30 +/- 0.01. Under the no-discharge conditions, gamma(ph) = 0.35 +/- 0.05. It is concluded that the emissivity of cold cathodes in a gas discharge is governed by adsorption of the working gas on the cathode surface and its implantation into the cathode. With allowance for this factor, the contribution of photoemission to the discharge current is reconsidered. It is shown that, for cathodes with diameter d(c) >> l(c) (l(c) is the length of the cathode layer), a normal or weakly abnormal glow discharge in noble gases is largely of a photoelectron character. In light noble gases, the photoelectron character of the discharge persists even for a strongly abnormal discharge. The energy dependences of coefficients gamma of kinetic and potential emission in helium are calculated with allowance for implantation of helium atoms into the cathode and compared with published data. The influence of particle implantation on gamma in a vacuum is estimated.