The Action of Mercury Metastable Atoms on a Tungsten Surface

The electron emission from metal surfaces due to the action of the $2{}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ metastable atoms was studied under varying conditions. The metastable atoms were produced by electron impact and diffused to the surface. The sensitivity of any metal surface was found to vary markedly with the vacuum conditions in the experimental tube and the extent to which the surface had been cleaned and outgassed. After thorough outgassing and cleaning each of the metals tested acquired practically the same sensitivity, which furthermore for any metal remained very nearly constant over long periods. Heating of a surface after it had acquired this equilibrium or "normal" sensitivity always resulted in temporary changes of the sensitivity following which it returned to its normal value. Investigation of these temporary changes of the sensitivity for a tungsten surface, with different mercury vapor pressures, showed that a deposit of mercury atoms was essential for the formation of a surface sensitive to mercury metastable atoms. The nature of these changes further indicated that the residual gas (oxygen) also played an important part and that the normal value of the sensitivity was due to the establishment of a very stable, complex arrangement of mercury and oxygen atoms upon the tungsten. Carefully dried oxygen was admitted to the experimental tube, and the observations of the changes in sensitivity as this oxygen was removed by progressive outgassing confirmed the conclusions as to the part played by oxygen. Transient values of the sensitivity fifty to a hundred times larger than the normal value were observed at one stage of this outgassing process when the relative concentrations of oxygen and mercury atoms were such as to make possible the formation of monatomic films. From the values of these high sensitivities it is concluded that in the case of the normal surface not more than one or two metastable atoms out of a hundred caused the emission of an electron. The formation of the sensitive surface was found to be uninfluenced by the metastable atoms. No correlation was found between the changes of response of a surface to mercury metastable atoms and to the radiation from an external source.