Computational study on the electronic states of tungsten nitride molecule

The state-averaged complete active space self-consistent field (SA-CASSCF) method and rotated multi-state complete active space second-order perturbation theory (RMS-CASPT2) are applied to investigate the low-lying Λ–S and Ω electronic states of tungsten nitride, WN. The potential energy curves of the low-lying states are computed, and the fitted spectroscopic constants are in good agreement with the available experimental results. The A4Π-X4Σ- transition is focused to compare with the previously observed emission spectra of WN in the region of 500–580 nm. With the inclusion of the spin–orbit coupling, the ground Λ–S state X4Σ- and the excited state A4Π split into 4Σ−1/2, 3/2 and 4Π−1/2, 1/2, 3/2, 5/2, respectively; and according to the present computed results, in conjunction with the results from previous studies, a complete set of transition data for the WN molecule is obtained. Our computed results indicate that the observed spectra should be assigned to the fine structure of A4Π-X4Σ- transition in WN. Furthermore, spectra information, including transition energy and transition dipole moment, arising from the transitions from the other excited states to ground states is also predicted. It is expected that the present work would promote the further experimental studies on absorption or emission spectra of WN.