CHARACTERIZATION OF THE 1 5Πu - 1 5Πg BAND OF C2 BY TWO-COLOR RESONANT FOUR-WAVE MIXING AND LIF

newcommand{cdimb}{ensuremath{text{b};^3Sigma_text{g}^-}} newcommand{cdimd}{ensuremath{text{d};^3Pi_text{g}}} newcommand{cdimQlo}{ensuremath{text{1};^5Pi_text{g}}} newcommand{cdimQup}{ensuremath{text{1};^5Pi_text{u}}} The application of two-color resonant four-wave mixing (TC-RFWM) in combination with a discharge slit-source in a molecular beam environment is advantageous for the study of perturbations in C$_2$. Initial investigations have shown the potential of the method by a detailed deperturbation of the cdimd, $v=4$ state.footnote{P. Bornhauser, G. Knopp, T. Gerber, and P.P. Radi, Journal of Molecular Spectroscopy 262, 69 (2010)} The deperturbation of the cdimd, $v=6$ state unveiled the presence of the energetically lowest high-spin state of C$_2$. This dark state gains transition strength through the perturbation process with the cdimd, $v=6$ state yielding weak spectral features that are observable by the high sensitivity of the TC-RFWM technique. The successful deperturbation study of the cdimd, $v=6$ state resulted in the spectroscopic characterization of the quintet (cdimQlo) and an additional triplet state (cdimb, $v=19$). footnote{P. Bornhauser, Y. Sych, G. Knopp, T. Gerber, and P.P. Radi, Journal of Chemical Physics 134, 044302 (2011)} More recently, investigations have been performed by applying unfolded TC-RFWM to obtain further information on the quintet manifold. The first high-spin transition (cdimQup - cdimQlo) has been observed textit{via} an intermediate ``gateway'' state exhibiting both substantial triplet and quintet character owing to the perturbation between the cdimQlo, $v=0$ and the cdimd, $v=6$ states. The high-lying quintet state is found to be predissociative and displays a shallow potential that accommodates three vibrational levels only. footnote{Bornhauser, P., Marquardt, R., Gourlaouen, C., Knopp, G., Beck, M., Gerber, T., van Bokhoven, JA, and Radi, P. P., Journal of Physical Chemistry, submitted} Further studies of the high-spin system will be presented in this contribution. By applying TC-RFWM and laser-induced fluorescence, data on the vibrational structure of the cdimQup - cdimQlo , system is obtained. The results are combined with high-level ab initio computations at the multi-reference configuration interaction (MRCI) level of theory and the largest possible basis currently implemented in the 2012 version of MOLPRO