Electronic spectra of radicals in a supersonic slit-jet discharge by degenerate and two-color four-wave mixing.

Four-wave mixing techniques have been used for the measurement of electronic transitions of cold transient species generated in a supersonic slit-jet discharge expansion. The origin band of the d(3)Pi(g)-a(3)Pi(u) system of C(2) and A(2)Pi(3/2)-X[combining tilde](2)Pi(3/2) electronic transition of HC(4)S were recorded. A signal-to-noise ratio of 10(4) in the spectra was achieved, resulting in detection limits of 10(10) cm(-3) for these two molecules. Application of selective two-color resonant four-wave mixing is used for the spectral assignment utilizing the double-resonance nature of the method. The combination of these techniques with a slit source proves to be a sensitive approach for the detection of transient molecules in a molecular beam discharge.