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 f or the triplet transitions of the recently predicted molecule BeC has been computed at the multireference single and double excitations conf iguration interaction level with a natural orbital basis and an extend ed 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 c hanges in the transition moment function in that region for transition s involving the (3) Pi states. The C-3 Sigma(-)-X(3) Sigma(-) transiti on moment is practically linear in the interval from 2.5 to 5.0 a(0) w here it shows its largest variation, 0.7881 and -0.4725 au. Transition probabilities expressed in terms of Einstein A(v',v'') coefficients p oint out the C-X(0-0) transition, occurring near the visible/near-IR e dge at 714 nm, as the most intense; in fact, A(00)(C-X)/A(00)(A-X) = 3 5 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 electro nic 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 progressio n close to v = 5 due to the abnormal shape of the A(3) Pi, potential, vibrational and rotational perturbations are likely to bring irregular ities in the lines and/or decrease in intensities for v > 5. The cross ing of the state C-3 Sigma(-) (v = 0) by the states c(1) Pi and d(1) S igma(+) offers also another possible decay mechanism.