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CARS spectra are reported for N2, O2, H2, D2, C2H4, and various mixtures cooled by supersonic jet expansion. Analysis of the Q branch structure of the diatomic molecules permits direct measurement of rotational populations, temperatures, and densities as a function of position in the free jet zone. Good agreement is obtained with temperature results for N2 from electron beam fluorescence studies and with densities calculated from the customary isentropic expansion equations. The detection sensitivity was sufficient that, for the first time, broadband CARS spectra were obtained for a molecular beam (D2). A simple relaxation model of the expansion process yields rotational collision numbers of 10±4 And 14±4 for N2 and O2 but these values may be high because of condensation heating. The model was not successful in treating the rotational relaxation of D2 and H2 but temperature measurements show qualitatively that D2 ⋅⋅⋅ X transfer efficiency increases in the order X = D2<He<Ar<CH4.

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