Raman observations of quantum interference in the $ν_1 / 2ν_2$ Fermi dyad region of carbon dioxide
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Coherent anti-Stokes Raman spectra (CARS) were obtained for CO$_2$ in a positive column discharge. The intensities of the Raman transitions to the $ν_1 / 2ν_2$ Fermi dyad decrease significantly when the discharge is turned on because of equalization of the lower and upper state populations by electron impact excitation, in a manner similar to saturation. The strength of the 1285 cm$^{-1}$ transition is observed to decrease by a factor of 20 greater than the 1388 cm$^{-1}$ line due to a quantum interference decreasing the vibrational relaxation rate of the upper state of the 1285 cm$^{-1}$ transition. This interference is verified by measurements of the decay rates from the state. Experiments ruling out Stark effects and polarization effects are described. Supporting the observed rapid vibrational excitation by electrons, a strong hot band transition $20^00 \leftarrow 02^20$ at 1425.61 cm$^{-1}$ was observed. These observations are compared with recent measurements of the Raman spectra of CO$_2$ heated in a flame.
10 pages, 14 figures, 29 references
10 pages, 14 figures, 29 references