Orbital angular momentum in triatomic molecules

The effects of orbital angular momentum on the details of the spin and rotational fine structure of the [Xtilde] 2 B 1 and à 2 A 1 states of NH2 and H2O+ are considered. It is found that the erratic spin-orbit splittings and asymmetry parameters of the à 2 A 1 states can be reproduced with good accuracy by calculations that also account for the regular spin and rotational structures of the ground states; the only input parameters are the shapes of the Born-Oppenheimer potential curves, the bond lengths and the spin-orbit coupling constants. The same calculations give an almost quantitative explanation of various perturbations in the à 2 A 1 state of NH2. These perturbations are caused by the ground state and result from the presence of orbital angular momentum. Three types have been documented; one is direct spin-orbit interaction between the à and [Xtilde] states, and the others involve rotational asymmetry, which is found to be an important mechanism for causing perturbations between electronic states.