Simultaneous analysis of the Ballik-Ramsay and Phillips systems of C2 and observation of forbidden transitions between singlet and triplet states

6229 lines of the Ballik-Ramsay system (b 3Σ g −–a 3Π u ) and the Phillips system (A 1Π u –X 1Σ g +) of C2 up to v = 8 and J = 76, which were taken from the literature or assigned in the present work, were analyzed simultaneously by least-squares fitting with 82 Dunham-like molecular parameters and spin-orbit interaction constants between the b 3Σ g − and X 1Σ g + states with a standard deviation of 0.0037 cm−1 for the whole data set. As a result of the deperturbation analysis, the spin-orbit interaction constant AbX was determined as 6.333(7) cm−1 and the energy difference between the X 1Σ g + and a 3Π u states was determined as 720.008(2) cm−1 for the potential minima or 613.650(3) cm−1 for the v = 0 levels with Merer and Brown’s N 2 Hamiltonian for 3Π states, which is about 3.3 cm−1 larger than the previously determined value. Due to this sizable change, a new energy-level crossing was found at J = 2 for v = 3 (F1) of b 3Σ g − state and v = 6 of X 1Σ g + state, where the strong interaction causes a nearly complete mixing of the wave functions of the b 3Σ g − and X 1Σ g + states and the forbidden transitions become observable. Using the predictions of our deperturbation analysis, we were able to identify 16 forbidden transitions between the singlet and triplet states at the predicted frequencies with the expected intensities, which verifies our value for the energy difference between the X 1Σ g + and a 3Π u states.