Vibrational relaxation in H2 molecules by wall collisions: Applications to negative ion source processes

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Abstract
In the volume of a hydrogen discharge, H2 molecules, excited to high vibrational levels (v″>6), are formed either by fast-electron collisions or from H+2 ions that are accelerated across the discharge-wall potential that undergo Auger neutralization prior to impact with the discharge chamber wall. We have used computer molecular dynamics to study the deexcitation and reexcitation of vibrationally excited H2 molecules undergoing repeated wall collisions. The initial translational energies range from thermal to 100 eV and the initial vibrational states range from v″=2 to v″=12. The average loss or gain of vibrational, rotational, translational, and total molecular energies and the survival rates of the molecules have been evaluated. At thermal energies vibrational deexcitation is the predominant process, and a consistent picture emerges of rapid energy redistribution into all the molecular degrees of freedom and a slower rate of loss of total molecular energy to the wall. At higher translational energies (1–100 eV) a substantial fraction of the molecules survives with large (v″>6) vibrational energy. This vibrational population provides a contribution to the total excited vibrational population comparable to that from the fast-electron collision process. Implications of these results for negative ion generation will be discussed.

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