Slow photoelectron velocity-map imaging of cold C₇ ^- and C₉ ^-

High-resolution anion photoelectron spectra of cryogenically cooled C₇ ^- and C₉ ^- clusters obtained using slow photoelectron velocity-map imaging are presented, providing insight into the vibronic structure of neutral C₇ and C₉. These spectra yield accurate measurements of vibrational frequencies for the neutral clusters as well as electron affinities of 3.3517(4) and 3.6766(14) eV for C₇ and C₉, respectively. In the C₇ ^- spectrum, transitions involving the previously unreported v₁ and v₂ symmetric stretching modes, as well as the v₉, v₁₀, and v₁₁ asymmetric bending modes, are assigned. Spin-orbit splitting is observed for several transitions in this spectrum, giving an energy difference of 28(6) cm^-1 between the Π1/2g2 and Π3/2g2 spin-orbit levels of the C₇ ^- anion. In the spectrum of C₉ ^-, transitions involving the previously unreported symmetric stretch v₁ and the asymmetric bend v₁₁ are observed. In both spectra, several features are assigned to Franck-Condon forbidden transitions involving the doubly degenerate v₁₀ and v₁₁ modes of C₇ and the v₁₃ and v₁₄ modes of C₉. The appearance of these transitions is attributed to Herzberg-Teller coupling between the electronic states of the neutral clusters. Additional FC-forbidden transitions to states previously observed in gas-phase infrared experiments are observed and attributed to vibronic coupling between the electronic states of the anion, resulting in non-totally symmetric character in the anion's full vibrational ground state. Finally, consideration of the energy dependence of detachment cross sections and Dyson orbital analyses reveal that addition of more carbon atoms to the linear chain results in photodetachment from delocalized molecular orbitals with increasing nodal structure, leading to threshold photodetachment cross sections that differ considerably from simple symmetry considerations.