Perturbation Facilitated Optical Optical Double Resonance Investigation Of The Quintet Manifold Of C2 By Applying Two-color Four-wave Mixing

The potential of four-wave mixing spectroscopy for deperturbation studies has been demonstrated by an analysis of the spin-orbit and $L$-uncoupling interaction between the $d\,^3\Pi_g,v=4$ and the $b\,^3\Sigma_g^-, v=16$ states of C$_2$.\footnote{P. Bornhauser, G. Knopp, T. Gerber, and P.P. Radi, Journal of Molecular Spectroscopy 262, 69 (2010).} The double-resonance method provides unambiguous assignments of perturbed transitions by intermediate level labeling. Furthermore, the sensitivity of the method unveiled extra transitions that originate from the perturbing $b\,^3\Sigma_g^-, v=16$ state. A following study\footnote{P. Bornhauser, Y. Sych, G. Knopp, T. Gerber, and P.P. Radi, J. Chem. Phys. 134, 044302 (2011). } has successfully applied the method to deperturb the $d\,^3\Pi_g,v=6$ state of the dicarbon and lead to the discovery of the first high-spin state of C$_2$. The energetically lowest quintet ($^5\Pi_g$) %and the additionally perturbing $b\,^3\Sigma_g^-, v=19$ state% has been characterized by applying a conventional Hamiltonian. The detailed study unraveled major issues of the so-called high-pressure band of C$_2$ which were initially observed back in 1910\footnote{A. Fowler, Monthly Notices of the Royal Astronomical Society 70, 484 (1910).} and later observed in numerous experimental environments. In this work we take into account our recent studies on tri-carbon\footnote{Y. Sych, P. Bornhauser, G. Knopp, Y. Liu, T. Gerber, R. Marquardt, and P.P. Radi, J. Chem. Phys. 139, 154203 (2013).} where we used perturbation-facilitated two-color resonant four-wave mixing spectroscopy to access the (dark) triplet manifold of C$_3$ from the singlet $\tilde{X}\;^1\Sigma_g^+$ ground state \textit{via} ``gate-way'' levels (i.e. singlet-triplet mixed levels). In a similar way, we performed for this study perturbation-facilitated optical-optical double-resonance experiments to access the first excited quintet state of C$_2$ \textit{via} ``gate-way states'' in the perturbed $d\,^3\Pi_g,v=6$. The newly found $^5\Pi_u$ state is characterized at rotational resolution by performing a least-squares fit of the observed transitions to a $^5\Pi_u$ - $^5\Pi_g$ Hamiltionian. The work represents a rare case of a successful analysis of a quintet manifold of a molecule exhibiting a singlet ground state ($^1\Sigma_g^+$)