Laser spectroscopy and density functional calculations on niobium monocarbide

Abstract

A survey of the jet-cooled Nb12C and Nb13C radicals has been carried out between 13 500 and 18 000 cm−1 using laser-induced fluorescence and resonant two-photon ionization spectroscopy. Several vibronic bands belonging to at least six band systems have been identified. Three of these systems appear to belong to 2Π1/2–2Δ3/2 transitions in which the lower 2Δ3/2 state is the ground electronic state of the molecule. The other three systems also terminate to the same three 2Π1/2 upper states, but originate from a state lying 830 cm−1 above the X 2Δ3/2 state. This state is assigned as the A 2Σ+ state. The ionization potential has been determined to be 56 402±15 cm−1 or 6.9929±0.0018 eV using two-color photoionization efficiency spectroscopy. This value, combined with the ionization potential of Nb and the bond energy of NbC+, yields an improved bond energy of 5.39±0.15 eV for NbC. The (4,0) band of the B 2Π1/2–X 2Δ3/2 system has been studied at a resolution of approximately 0.005 cm−1 using laser-induced fluorescence spectroscopy. The nuclear magnetic hyperfine structure has been resolved in both states, and an analysis confirms that the 2Δ ground state arises from the σ2δ1 electron configuration in which the unpaired δ electron is a pure Nb 4d electron associated with the 4F term arising from the excited 5s24d3 electron configuration. Density functional calculations have been carried out on the lowest 2Δ, 2Σ+, 4Δ, 2Π, and 4(Π,Φ) states of the neutral and the 1Σ+, 3Δ, and 3(Π,Φ) states of the cations. These calculations fully support the experimental evidence for the ground state.