Two-photon photodetachment spectroscopy of cold C6− probing three excited 2Π g states

We report on an experimental investigation of the photoabsorption of cold (∼30 K) C6− anions in the range 420–650 nm by laser two-photon photodetachment on a stored beam of anions in a merged beams interaction setup. The recorded photoabsorption spectrum shows evidence for photoabsorption through the lowest 2Πg states (labeled C, D, and E) with new assignments of the term values of T0.C = 1.9363 eV, T0.D = 2.2992 eV, and T0.E = 2.7029 eV. The detailed structures of the photoabsorption spectrum suggest strong vibronic coupling in C6− and that the photoabsorption is dominated by combination bands of the lowest energy totally symmetric vibrational mode (ν3) and at least the lowest energy asymmetric bend vibrations (ν9a and ν9b), but likely more bending modes contribute. From the dynamical information on the detachment process, available in the storage ring experiment, we suggest a small revision of the electron affinity of C6 to being EAC6 = 4.036 eV (previous value 4.180 eV). Our results generally compare very well to previous experimental results, however, with a modified assignment facilitated by a higher photon energy resolution and the experimental conditions in an ion storage ring. Our results are not consistent with a recent computational study and likely call for renewed theoretical–computational studies of the photoabsorption dynamics of the C6− prototype linear molecule.