The Scattering of Low Velocity Hydrogen Ions in Water Vapor

Abstract

The elastic scattering of H⁺, H₂⁺, and H₃⁺ in water vapor has been measured in the velocity range 2 to 130 volts. For H⁺ the potential law for 3 to 15 volts (r_a = 2.74 to 1.77A) is $\mathrm{V=-}\frac{{\text{6.33×10}}^{\text{−32}}}{r^4}$ . Assuming an equilibrium distance of 1.013A the proton affinity of H₂O is calculated to be 6.5 electron volts. Above 15 volts (r_a < 1.77A) the coefficient of r becomes very large (about 20). For H₂⁺ in H₂O between 3 and 20 volts (r_a = 3.14 to 2.33A) $\mathrm{V=-}\frac{{\text{193×10}}^{\text{−56}}}{r^7}$ . Above 20 volts the coefficient of r increases to a much larger value (about 13). H₃⁺ in H₂O shows a potential law $\mathrm{V=-}\frac{{\text{8.7×10}}^{\text{−16}}}{r^2}$ between 5 and 20 volts (r_a = 4.27 to 2.33A). This is about what would be expected on the basis of the interaction of a dipole and a charge. Between 20 and 30 volts the coefficient of r becomes 3 and from 30 to 60 volts it is much larger (about 8). Neutralization is experienced with both H⁺ and H₂⁺ in H₂O but not with H₃⁺. These neutralization cross sections are reported as a function of velocity. A design is given of a Knudsen gauge found satisfactory for the measurement of the pressure of water vapor. A new technique is described which was found necessary for the removal of insulating films on the metallic surfaces. These films are apparently carbonaceous and formed by the action of the ion beam on traces of organic compounds in the vapor. The technique used is an electric discharge in oxygen.