The ground and low-lying electronic states of OsO.

The significant electron correlation and relativistic effect in the 5d transition atoms lead to a complex electronic structure of their compounds. Even for the osmium monoxide (OsO) molecule, the ground electronic state has been posing a considerable challenge for both theoretical and experimental investigations so far. Here, we report the observation and identification of the electronic states of OsO in low energy through laser-induced fluorescence and dispersed fluorescence spectra. There are six excitation bands rotationally resolved in the energy range of 15 000-23 100 cm-1, thus enabling the determination of the ground X3Π2 state and the rotational constants in the upper and lower states. The spin-orbit splitting and vibrational frequency of the ground state and three low-lying Ω states are obtained from the emission spectra. In addition, by performing high-level ab initio calculations, we obtained detailed spectroscopic properties for numerous Λ-S and Ω electronic states below 25 000 cm-1, which support the spectral assignments derived from experimental measurements. The results provide quantitative evidence for the significant relativistic effect on the bonding of OsO, as well as a foundation for describing its electronic structure.