An interpretation of the anomalous (1)Pi vibronic structure in the far-UV spectrum of CO

The far-UV spectrum of carbon monoxide presents numerous abnormal (1)Pi rovibronic levels in the region 92,000-105,000 cm(-1) which have been observed by several experimentalists. Yet, and in spite of various attribution attempts carried out over the past two decades, the nature of these levels is poorly understood and they still lack a definitive assignment. The absorption lines in this energy region are characterized by irregular energy level positions and spacings, and odd, smaller than expected, rotational constants. In the current contribution we address this puzzle by relying on recent ab initio calculations of several Rydberg and valence states of CO [G. J. Vazquez, J. M. Amero, H. P. Liebermann, and H. Lefebvre-Brion, J. Phys. Chem. A 113, 13395 (2009)], and on further new calculations in which we compute electronic transition moments between the ground state and several excited (1)Pi states. We focus on the perturbations between the adiabatic (1)Pi states, specifically on the interaction between the second and third potential energy curves, reported in our previous paper. The second adiabatic potential energy curve, which we refer to as E-E('), displays a distorted shape with two minima as a result of an avoided crossing with the third one. We report here the computation of the lowest vibronic levels of a system of two electronic states which undergo a strong Rydberg-valence interaction. Our vibronic calculations proceed as follows: from the second and third computed adiabatic curves we first obtain approximate diabatic curves for the (X (2)Sigma(+))3ppi E (1)Pi Rydberg state and for the valence E(') (1)Pi state. Then we solve a system of 2x2 coupled equations in order to obtain the perturbed vibronic energy levels and wave functions for the interacting E and E(') states. The computed vibronic levels obtained from the coupled equation treatment are compared to the first six observed (1)Pi levels. A good agreement is found with experiment for the four lowest vibronic levels and a reasonable accord for two higher levels.