Probing electronic and vibrational structures of TaC<i>n</i>−/0 (<i>n</i> = 2–4) using high-resolution photoelectron spectroscopy and theoretical calculations

We report the high-resolution photoelectron spectroscopy of transition metal carbide cluster anions TaCn- (n = 2-4) using a cryogenic ion trap combined with the slow electron velocity imaging (cryo-SEVI) technique. From the vibrationally resolved photoelectron spectra and associated ab initio calculations, the electron affinities of TaCn (n = 2-4) were determined with high precision: 1.818(2), 2.202(5), and 2.431(2) eV, respectively. The electronic and vibrational structures observed in the photoelectron spectra were interpreted using density-functional theory and coupled-cluster singles and doubles with perturbative triples calculations. Both the neutral TaCn clusters and their anions exhibit planar C2v structures, where the Ta atom bridges each C atom. Furthermore, we observed the spin-orbit splitting in the ground state of TaC2 (X̃4B1), with a measured splitting of 256(25) cm-1. This splitting is well explained by the calculated E1/2(±3/2)-E1/2(±1/2) splitting of 216 cm-1, obtained using the MRCI+SOC method.