Observation Of The Forbidden Transitions Between The A1πu And B3σg− States Of C2

In the last symposium,\footnote{W. Chen, J. Tang, and K. Kawaguchi, 68th OSU International Symposium on Molecular Spectroscopy, WJ11 (2013).} we reported that a global fit simultaneously for the Phillips band system ($A^1\Pi_u-X^1\Sigma_g^+$) and the Ballik-Ramsay band system ($b^3\Sigma_g^--a^3\Pi_u$) was carried out to deperturb the spin-orbit interaction between the $X^1\Sigma_g^+$ state and the $b^3\Sigma_g^-$ state of \chem{C_2}. As the result, the energy gap between the $a^3\Pi_u$ state and the $X^1\Sigma_g^+$ state was obtained as 720.0 $\wn$, which is quite larger than the previous value\footnote{C. Amiot, J. Chauville, and J. -P. Maillard, J. Mol. Spectrosc. \underline{75}, 19 (1979).} of about 716.7 $\wn$ (converted from 718.3 $\wn$ after the definition of the Hamiltonian for the $^3\Pi$ state is corrected by adding one $B$ value on all the diagonal elements as the one\footnote{J. M. Brown and A. J. Merer, J. Mol. Spectrosc. \underline{74}, 488 (1979).} we use widely today). This newly determined singlet-triplet energy gap showed that the $X^1\Sigma_g^+ (v=6)$ level and the $b^3\Sigma_g^- (v=3)$ level cross at $J=2$ with the energy difference of only 0.07 $\wn$ before the spin-orbit interaction is considered, which makes the singlet-triplet mixing nearly 50-50\%. Therefore, we thought that the forbidden transitions related to this mixing should be observable. When rechecking the previously observed FTIR emission spectrum in the study of the CH radical,\footnote{P. N. Ghosh, M. N. Deo, and K. Kawaguchi, Astrophys. J. \underline{525}, 539 (1999).} where the emission spectrum of \chem{C_2} appeared to be very strong, we found that the allowed $A^1\Pi_u (v=4)-X^1\Sigma_g^+ (v=6)$ transitions of \chem{C_2} around 3950 $\wn$ were accompanied by the forbidden $A^1\Pi_u (v=4)-b^3\Sigma_g^- (v=3)$ transitions with $J''=2$, and two such forbidden transitions were identified clearly with the similar intensities as the corresponding allowed transitions. The observation of the forbidden transitions exactly at the predicted positions means that our deperturbation analysis was successful