Capture of Electrons From Mercury Atoms By Positive Ions of Helium

Abstract

In a recent paper¹ we gave an account of some experiments on the determination of the mobility of ions in helium gas at a pressure of 360 mm. of mercury. We found that the mobility of the positive ions decreased when small traces of other impurities were introduced into the apparatus, and we interpreted the results as due to an ‘exchange’ phenomenon similar to that observed by Kallmann and Rosen in the case of high-speed positive ions. On this view, when a helium ion ‘collides’ with an impurity molecule there is a certain probability that an electron will be captured from the impurity by the ion. The impurity ion so formed will not lose its charge in collisions with other helium atoms, because the ionisation potential of helium is greater than that of any impurity and the speed of the ions in our experiment is much too small to supply the energy required for the transition. For this reason, a very small concentration of impurity is sufficient to change completely the rate at which the positive charge is carried through the gas.