Dissociation of Hydrogen by Electrons

Variation with pressure of the rate of dissociation of hydrogen by electron impact.—Electrons were driven down the axis of a tube containing hydrogen. Atomic hydrogen formed by dissociation was condensed on the walls of the tube which was immersed in liquid air. The rate of dissociation was calculated from the rate of change in pressure as measured by a special hot wire gauge which was so designed that the galvanometer deflection was strictly proportional to the pressure from less than ${10}^{-5\mathrm{}}$ mm to 0.27 mm. The deflections were recorded photographically. With both 100 volt electrons and 20 volt electrons the rate of diminution of pressure is directly proportional to the pressure. Hence the dissociation of hydrogen by electron impact cannot be a secondary effect, the result of a collision between a neutral hydrogen molecule and an ionized or excited molecule, unless the mean free path of the ionized or excited molecule is less than $\frac{1}{300}$ that of the normal molecule. This latter is a most unlikely possibility. The dissociation is therefore very probably produced as a direct result of the impact of an electron on the molecule, a result which is strongly supported by the studies of intensities of hydrogen lines at different pressures by Blackett and Frank, and by Lowe.