Dissociation of Molecular Hydrogen by Electron Impact

The dissociation of molecular hydrogen by electron impact was studied by observing the rate of pressure decrease in a closed system when the dissociation fragments were trapped on molybdenum trioxide. An onset potential of 8.8 (±0.2) eV was obtained, indicating that a large part of the dissociation must proceed through direct excitation of the repulsive b 3Σu+ state of the molecule. From the onset potential to 95 eV, the upper energy limit of our observations, the dissociation rate increased rapidly at first, then more slowly above 14 eV passing through a flat maximum at 40 eV and then slowly decreasing up to the highest energy investigated. Reasons are given for supposing that the observed cleanup of hydrogen at high electron energies contains an effect due to dissociative neutralization of molecular ions. With this assumption the cross section for excitation of the molecule to triplet states is derived by subtracting the molecular ionization cross section from the observed total dissociation cross section. In this way we obtain a cross section for dissociation into neutral atoms which has a maximum of 9×10−17 cm2 at an energy of 16.5 eV, followed by a rapid decrease with increasing electron energy.