Reassignment of the Vibronic Structure in the Absorption Spectrum of Carbon Cluster Anion C6– Exhibiting Fast Radiative Cooling

Linear carbon cluster anions, such as C6–, have been considered to be promising candidate interstellar molecules. Recent experiments have demonstrated that in a collision-free vacuum environment, C6 exhibits fast radiative cooling from its highly vibrationally excited states through inverse internal conversion (IIC). Since IIC is driven by vibronic coupling, understanding of vibronic structures of C6– is of theoretical significance. Here, we utilize time-dependent density functional theory to calculate the absorption spectrum of C6–, allowing us to reassign the peaks in the experimental spectrum to the vibronic progressions associated with the C2Πg ← X2Πu and D2Πg ← X2Πu electronic transitions. A vibronic coupling density analysis reasonably explains the strong vibronic peaks and validates the reassignment. We also discuss the potential softening caused by quadratic vibronic coupling in the excited states and the partially allowed vibronic transitions. Additionally, we show that a pseudo-Jahn–Teller distortion occurs along the ν4(σu+) antisymmetric stretching mode due to the pseudo-degeneracy of the A and B states.