Resonant two-photon ionization spectroscopy of jet-cooled tantalum carbide, TaC

The optical spectrum of diatomic TaC has been investigated for the first time, with transitions recorded in the range from 17 850 to 20 000 cm?1. Six bands were rotationally resolved and analyzed to obtain ground and excited state parameters, including band origins, upper and lower state rotational constants and bond lengths, Fermi contact parameter bF for the ground state, and lambda doubling parameters for the excited states. The ground state of TaC was found to be X ??+, originating from the 1??2??1?3?1 electronic configuration, in which only the valence orbitals arising from the Ta(5d+6s) and C(2s+2p) orbitals are listed. All of the rotationally resolved and analyzed bands were found to originate from the ground state, giving B"0=0.489 683(83) cm?1, r"0=1.749 01(15) ?, and b"F=0.131 20 (36) cm?1 (1? error limits) for 181Ta 12C. Comparison of the Fermi contact parameter to the atomic value shows that the 3? orbital is approximately 75% Ta 6s in character. The other group 5 transition metal carbides, VC and NbC, have long been known to have 1??2??1rr?1?1, ?? ground states, with low-lying 1??2??1rr?3?1, ??+ excited states. The emergence of a different ground state in TaC, as compared to VC and NbC, is due to the relativistic stabilization of the 6s orbital in Ta. This lowers the energy of the 6s-like 3? orbital in TaC, causing the 1??2??1rr?3?1, 2?+ state to fall below the 1??2??1rr?1??, ?? state.