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 H2 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 H2 molecules with high rotational and vibrational excitation. The behaviour of these ro-vibrationally excited H2 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 H2