Atomic and molecular hydrogen densities in a plasma expansion

We treat the kinetics of the three dominant species in a hydrogen plasma expansion, which was studied using various laser spectroscopic techniques. Whereas the ground-state H/sub 2/ molecules expand like a normal gas, the behaviour of H atoms and ro-vibrationally excited molecules is considerably altered as a consequence of their reactivity. The hydrogen atoms are primarily lost via surface association. In the source, H atoms are lost on the surface of the nozzle, reducing the atomic flux, and in the expansion, the H atoms diffuse out due to large H atom concentration gradients between the plasma and the background that exists as a result of surface association on the vessel walls. The loss process of H atoms in the nozzle, surface association, is the main production process of H/sub 2/ molecules with high rotational and vibrational excitation. The behaviour of these ro-vibrationally excited H/sub 2/ molecules in the expansion shows the relaxation process from the high temperature in the upstream region to the low temperature of the downstream region. The lower rotational levels (J <5) adapt to the local temperature as expected, but the higher levels (J > 7) do not because of the large energy spacing between the hydrogen rotational levels. The consequence of this high rotational excitation is demonstrated in the dissociative attachment of electrons to H/sub 2/