The splitting of low-lying states for hydroxyl molecule under spin--orbit coupling

The splitting of potential energy curves for the states X^2\Pi _3/2, ^2\Pi _1/2 and A^2\Sigma ^ + of hydroxyl OH under spin--orbit coupling (SOC) has been calculated by using the SO multi-configuration quasi-degenerate perturbation theory (SO-MCQDPT). Their Murrell--Sorbie (M--S) potential functions have been derived, then, the spectroscopic constants for X^2\Pi _3/2,^ 2\Pi _1/2 and A^2\Sigma ^ + have been derived from the M--S function. The calculated dissociation energies for the three states are D_0OH(X^2\Pi _3/2)=34966.632cm^-1, D_0OH(^2\Pi _1/2)=34922.802cm^-1, and D_0OH(A^2\Sigma ^ + )=17469.794cm^-1, respectively. The vertical excitation energy \nu ^2\Pi _1/2 ( \nu = 0 ) \to X ^2\Pi _3/2 ( \nu = 0 ) = 139.6\rm cm^-\rm 1. All the spectroscopic data for the X^2\Pi _3/2 and ^2\Pi _1/2 are given for the first time except the dissociation energy of X^2\Pi _3/2.