A supersonically expanding argon cascaded arc plasma, with difFerent amounts of hydrogen added (0, 0.7, and 1. 4 vol % H2), was studied using Thomson-Rayleigh scattering and optical emission spectroscopy. %'ith hydrogen added, the electron density profile as a function of the distance from the onset of the expansion shows a large extra ionization loss {compared to the pure argon case), especially after the stationary shock front. This anomalous loss of ionization is attributed to molecular processes, such as associative charge transfer between Ar+ and H2, and dissociative recombination of the resulting ArH+ molecular ion. Spatially resolved emission spectroscopy shows that in the expansion the radial hydrogen emission profiles are broader than the argon profiles. The addition of hydrogen appears to change the characteristic shock behavior of pure argon. For both the argon and the hydrogen system it is shown that the uppermost levels are in Saha equilibrium with their adjacent continuum.
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