Theoretical study on the low-lying electronic states of ScO

The scandium monoxide (ScO) molecule is a significant astrophysical species, which exhibits a rich distribution of electronic states. Experimental investigations have identified six $\Lambda$-S electronic states lying below 30000 cm⁻¹ along with their nine $\Omega$ sub-states, including $X^2{\Sigma }^{+}$, ${A^{\prime }}^2{\Delta }_{3/2,5/2}$, $A^2{\Pi }_{1/2,3/2}$, $B^2{\Sigma }^{+}$, $C^2{\Pi }_{3/2,1/2}$, and $D^2{\Sigma }^{+}$, arranged in order of increasing energy. This study employs the multi-reference second-order perturbation theory, incorporating scalar relativistic effects and spin–orbit coupling effects, to compute all the low-lying electronic states of ScO below 40000 cm⁻¹. From the potential energy curves and transition dipole moments of 30 $\Lambda$-S states and the corresponding 65 $\Omega$ states, we have derived spectroscopic constants and radiative lifetimes that generally agree well with the available experimental data. Our theoretical results not only enhance the understanding of the electronic structure of ScO but also serve as a foundation for future spectroscopic investigations of ScO.