Ab initio configuration interaction calculations of the predissociation of rovibrational levels of the C 3Πg and d 1Πg 3sσ Rydberg states of the oxygen molecule

Abstract

Ab initio configuration interaction calculations have been carried out for seven low-lying 3,1Πg states of the oxygen molecule. Three different types of nonadiabatic couplings have been considered: spin-orbit, radial, and rotational. The complex scaling method has been employed to compute rovibrational level locations and predissociation linewidths with a basis of 200 Hermite polynomials for each of 13 different Ω electronic states. The calculations correctly predict that the v=2 level has the narrowest linewidth for the O216CΠg3 state, while v=4 is narrowest for O218. Marked variations in the linewidths of the different Ω components of the C state are explained by the fact that the π*→3sσ Rydberg and σ→π* valence Πg3 states have different occupations of the π* orbital, causing opposite orderings of their respective Ω levels. Rotational coupling is found to be important for high J values of the C state. The d 1Πg 3sσ state shows even more unusual effects by virtue of the fact that there is a sharply avoided crossing between the corresponding Rydberg diabatic state with a bound σ→π* 1Πg valence state. The calculations find irregular spacings in the d-state vibrational manifold, wide variations in linewidth for different v,J levels, and a large change in the rotational constant in successive vibrational levels, all of which effects have been earlier demonstrated in experimental work. Satellite lines are indicated for both the v=2 and 3 levels as a result of the interaction with the bound Πg1 valence state, whereby experimental verification exists only for v=2. The v=3 state has not yet been successfully identified due to the broadness of the d-X spectrum in the energy range of interest. The observed temperature dependence of the linewidths of the two features near the expected location of the v=2 level can also be understood on the basis of these calculations. Finally, the change in the predissociation mechanism for the d state from spin-orbit to radial as v changes from 0 to 2 which has been deduced experimentally is also verified in the present theoretical treatment.