The Motion of Slow Positive Ions in Gases

Abstract

Positive caesium ions with velocities of 3.5 to 600 equivalent volts have been studied in hydrogen and helium. No absorption of the ions in the apparatus used is found for pressures below 0.01 mm of mercury. For the higher velocities this absence of absorption persisted for pressures as high as 0.05 mm of mercury. In all cases the ions were retarded in passing through the gas and there is an approximate probability distribution of ion energies about an average retarded value. The retardation in all cases is found to be proportional to the pressure. For ${\mathrm{Cs}}^{+}$ ions in helium the percent loss of initial energy varies from 11.5 for 400 volt ions to 55 for ions of 3.5 volts initial energy. Similar results are found for hydrogen. Values of the constant, $h$, of the error curve which are characteristic of a particular distribution are given for various velocities and pressures. An interpretation of the absence of scattering and the retardation of the ions is given on the basis of elastic collisions between ion and gas molecule. It is found that no absorption of the ${\mathrm{Cs}}^{+}$ ions would be expected until the pressures are high enough to give rise to multiple scattering. The data on retardation give values of the radius of the ${\mathrm{Cs}}^{+}$ ion as: 1.78A in helium and 3.09A in hydrogen. The slowing up of the ions offers an explanation of the large absorption of ${\mathrm{Cs}}^{+}$ in helium found by Ramsauer and Beeck. Probability considerations show that for increasing velocities the energy loss per collision found in the present experiments is less than that to be expected on the basis of elastic collisions.