On Intensity Alterations in C2 “Swan” Emission Spectra

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Abstract
The presence of intensity alternations in C2 Swan emission spectra of flames is analyzed. It is argued that the nonequilibrium rotational energy distribution is caused by collision‐induced electronic transitions between the radiating A 3 Π g state and the vibrationally excited A′ 3 Σ g − or A″ 3 Σ u + states of C2. The selection rules are derived on the basis of the parity properties of the relevant states and the various terms of the multipole expanded interaction potential. The selection rules lead to an unambiguous choice between the collision‐induced transitions A″ 3 Σ u + →A 3 Π g or A 3 Π g →A′ 3 Σ g − . On the basis of available experimental information of the pressure dependence of the effect, additional evidence is given that a formation process is responsible for the occurrence of the intensity alternations. Examples of intensity alternations in Si2 and N2 are briefly discussed.