Stripping in collisions between hydrogen molecules below 500 eV

The cross sections for the production of forward scattered molecular ions in collisions of H₂ on H₂ and D₂ on D₂ have been measured at lab kinetic energies between 50 and 500 eV. The cross section for the H₂ reaction ranges from 2.2 × 10⁻¹⁷ cm² at 500 eV to 5.0 × 10⁻¹⁹ cm² at 50 eV. The D₂ cross section varies from 1.7 × 10⁻¹⁷ cm² to 3.8 × 10⁻¹⁹ cm² in the same energy range. The angular dependence of the ions was found to be highly peaked in the forward direction for energies above 100 eV. The cross sections for H₂ and D₂, when plotted versus incident velocity, are found to approach the same values at the highest velocities. The two cross sections diverge toward lower velocities as they approach their respective cutoffs, where each process becomes energetically impossible. These cross sections have been found to fit a theoretical model based on ideas first presented by Demkov and Komarov, in which ionization is presumed to take place via a series of pseudocrossings of the initial state into the continuum of the adiabatic states of the collision complex. For the interaction of two hydrogen molecules, the internuclear separation of each molecule is assumed fixed and the adiabatic states are defined in terms of the distance between the two molecular centers (R). Best agreement between the observed cross sections and those predicted from the model are obtained assuming the transition region to be centered around R=0.43 \AA{}, 16.5 eV above the energy of the ground state at large R. For the purpose of monitoring the neutral-beam intensity, the secondary-electron emission coefficients (γ) were measured for H₂ and D₂ impinging on a brass surface. The γ values were found to vary from 0.77 and 0.65 at 500 to 0.045 and 0.020 at 50 eV for H₂ and D₂, respectively.