Detection of CH in an expanding argon/acetylene plasma using cavity ring down absorption spectroscopy

Cavity ring down (CRD) absorption spectroscopy is used to measure the methylidyne (CH) radical in an Ar/C2H2 plasma. The rotational spectrum of the A 2Δ (v′=0) ← X 2Π (v′′=0) transition around 430 nm is recorded to determine the total CH ground state density, both as a function of the current through the arc producing the low-pressure Ar plasma and as a function of the injected acetylene flow. Total ground state densities between 5×1015 and 8×1016 m−3 are detected. The trends show that the methylidyne radical plays a minor role in the growing mechanism of hydrogenated amorphous carbon films and is predominantly formed in the charge exchange/dissociative recombination channel starting from the C2H radical.