# Stripping in collisions between hydrogen molecules

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## Abstract

The cross sections for the production of fast molecular ions in collisions of H[lowered 2] on H[lowered2] and D[lowered 2] on D[lowered 2] have been measured at lab kinetic energies between 50 and 500 eV. The cross section for the H[lowered 2] reaction ranges from 2.2 X 10[raised -17] cm[squared] at 500 eV to 5.0 X 10[raised -19] cm[squared] at 50 eV. The D[lowered 2] cross section varies from 1.7 X 10[raised -17] cm[squared] to 3.8 X 10[raised -19] cm[squared] in the same energy range. The angular dependence of the reaction products was found to be highly peaked in the forward direction so that for the geometry used, practically all fast ions left the collision chamber for energies above 100 eV. The angular loss was found to be within reasonable limits at the lowest energies. The total stripping cross sections for H[lowered 2] and D[lowered 2], when plotted vs. 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 cut-offs 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 pseudo-crossings 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 = .43 A, 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 ([gamma]) were measured for H[lowered 2] and D[lowered 2] impinging on a brass surface. The [gamma] values were found to vary from .77 and .65 at 500 eV to .045 and .020 at 50 eV for H[lowered 2] and D[lowered 2], respectively.