A THEORETICAL-STUDY OF THE TRANSITION MOMENTS, EINSTEIN-A COEFFICIENTS, AND RADIATIVE LIFETIMES OF THE LOWEST TRIPLET-STATES OF BEC

The dependence of the transition moment on the internuclear distance for the triplet transitions of the recently predicted molecule BeC has been computed at the multireference single and double excitations configuration interaction level with a natural orbital basis and an extended set of cartesian gaussians, The avoided crossing between the states A(3) Pi and B-3 Pi at similar to 2.9 a(0) is responsible for abrupt changes in the transition moment function in that region for transitions involving the (3) Pi states. The C-3 Sigma(-)-X(3) Sigma(-) transition moment is practically linear in the interval from 2.5 to 5.0 a(0) where it shows its largest variation, 0.7881 and -0.4725 au. Transition probabilities expressed in terms of Einstein A(v',v'') coefficients point out the C-X(0-0) transition, occurring near the visible/near-IR edge at 714 nm, as the most intense; in fact, A(00)(C-X)/A(00)(A-X) = 35 and A(00)(C-X)/A(00)(B-X) = 1.9. The transitions A-X are expected to occur in the near IR (similar to 1064 nm) and those of the B-X bands in the visible at similar to 640 nm and show Franck-Condon broadening. Radiative lifetimes for the lowest vibrational levels of each electronic state are of the order of 500 ns (C), 15 mu s (A), and 1.0 mu s (B). Besides the interruption in the regular course of a band progression close to v = 5 due to the abnormal shape of the A(3) Pi, potential, vibrational and rotational perturbations are likely to bring irregularities in the lines and/or decrease in intensities for v > 5. The crossing of the state C-3 Sigma(-) (v = 0) by the states c(1) Pi and d(1) Sigma(+) offers also another possible decay mechanism.