Resonance-enhanced coherent anti-Stokes Raman scattering in C2

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Abstract
We have obtained resonance-enhanced Coherent Anti-Stokes Raman Scattering (CARS) spectra of the C2 radical formed in a microwave discharge and in an acetylene welding torch. Electronic resonance enhancement was obtained in the Swan system. In order to prepare this work, Swan band emission spectra of C2 had been recorded and assigned using Fourier-transform spectroscopy; this enabled us to determine the optimal conditions for the CARS spectroscopy and to assign the CARS spectra unambiguously. Numerical simulations were performed, with due account for the effects of upper vibrational state populations and for Doppler broadening; the agreement with experimental spectra is fair. We deduce from the spectra C2 densities of about 5 x 1011 cm -3 in the discharge and 1013 cm-3 in the flame. The detection sensitivity is of the order of 1010 cm-3 for our system