Results from emission spectroscopy measurements on an Ar/SiH4plasma jet which is used for fast deposition of amorphous hydrogenated silicon are presented. The jet is produced by allowing a thermal cascaded arc plasma in argon (I=60 A,V=80 V, Ar flow=60 scc/s and pressure 4 × 104Pa) to expand to a low pressure (100 Pa) background. In the resulting plasma SiH4is injected in front of the stationary shock front. Assuming a partial local thermal equilibrium situation for higher excited atomic levels, emission spectroscopy methods yield electron densities (∼ 1018m−3), electron temperatures (∼5000 K) as well as concentrations of H+, Si+, and Ar+particles. The emission spectrum of the SiH radical, theA 2&Dgr;–X 2&Pgr; electronic transition, is observed. Numerical simulations of this spectrum are performed, resulting in upper limits for the rotational and vibrational temperatures of 4000 and 5600 K, respectively. The results can be understood assuming that, in the expansion, charge exchange and dissociative recombination are dominant processes in the formation of species in excited states, notably Si+.