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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.

The rotationally resolvedà A 2 B 1 X X 2 A 2 electronic spectra of the allyl radical C 3 H 5 and its isotopic derivative C 3 D 5 have been recorded in the gas phase by cavity ring down spectroscopy through a supersonic planar plasma. A rotational analysis yields accurate molecular constants for the first excited electronic state with T 00 ¼ 24 491:229 2ð80Þ for C 3 H 5 . A C 2v planar structure in the zero-point level is indicated. The obtained rotational constants were employed to determine the C-C bond length 1.48 Å and the CCC bond angle 123 in the 2 B 1 electronic state. Two other bands observed are assigned to the excitation of the 7 (339 cm À1 ) and 9 (101 cm À1 ) vibrational modes in the upper electronic state.

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