Observation of dipole forbidden transitions of C2

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Higher vibrational levels of C2 were observed through visible band systems. Molecular constants of the c-state of C2 were precisely determined from room-temperature spectra of the d-c band system. We found that the c(v=1) state shows a level-crossing with the a(v=7) state. The perturbation analysis between the c(v=1) and a(v=7) states has been carried out. Accurate rovibronic level energies which have been determined from the experiment indicate that the singlet A(v=2) state is located very close to the triplet a(v=7) state, interacting with each other. However, the direction of the observed level shifts in the A(v=2) state cannot be explained with the literature value of the single-triplet energy gap, T0(a-X)=610.4 cm-1. It is concluded that the singlet-triplet energy gap should be modified by more than 3 cm-1 from the literature value.