Electron Affinity of the Carbon Dimer from Threshold Photodetachment Spectroscopy

Photodetachment spectroscopy of C_{2}^{-} anions across the thresholds to the two lowest electronic states of neutral C_{2} was carried out using rotationally cold trapped ions. The electron detachment was observed to follow p- and s-wave threshold behavior for transitions to the C_{2}X^{1}Σ_{g}^{+}(v^{'}=0) and C_{2}a^{3}Π_{u}(v^{'}=0) states, respectively, in agreement with the threshold laws derived by Wigner [Phys. Rev. 73, 1002 (1948)PHRVAO0031-899X10.1103/PhysRev.73.1002]. The ions were prepared at two different rotational temperatures, using a temperature-variable cryogenic 16-pole wire trap at 8 and 142 K, respectively. By fitting the two thresholds, an accurate value of the adiabatic electron affinity of C_{2} was determined as 26364.2±0.5  cm^{-1}. The energy of the p-wave threshold was found to be particularly insensitive to rotational excitation, due to the absence of P- and R-branch excitations. The new electron affinity deviates significantly from a recent measurement using photoelectron spectroscopy [Laws et al. Nat. Commun. 10, 5199 (2019)NCAOBW2041-172310.1038/s41467-019-13039-y].