THEORETICAL-STUDY OF THE ENERGIES AND LIFETIMES OF THE LOW-LYING STATES OF BISMUTH FLUORIDE

A series of CI calculations has been carried out for various low-lying electronic states of the bismuth fluoride molecule by employing relat ivistic effective core potentials including spin-orbit effects. It is found that the lowest 0+ excited state (A0+) of this system contains a large contribution from the pi --> sigma* 3PI lambda-s state, especi ally at bond distances which are equal to or greater than the equilibr ium value for the X(1)0+ ground state. The B0+ state at somewhat highe r energy is found to contain the largest portion of the b1SIGMA+ chara cter arising from the pi4pi2 configuration, in disagreement with earl ier calculations by Balasubramanian who only reports a single excited state of this (0+) symmetry. Results of the latter study for the X(1)0 +, X(2)1 and a2 states of lower energy are in good agreement with thos e of the present work, however. The relatively large r(e) value observ ed for the A0+ state is quite consistent with the present theoretical description, as well as the correspondingly lower vibrational frequenc y compared to that of X(1)0+. Radiative lifetimes have also been obtai ned for a number of excited states and the results are found to be in reasonably good agreement with recent measured data. An explanation is also provided for the anomalous mu0/mu1 ratio for the A0+ --> X trans itions, again based on the large amount of 3PI character in the upper state. The present data also provide a clear assignment for the A' and A'' states recently found for this system (OMEGA = 1 and 0-) respecti vely. The lifetime of the X(2)1 state is computed to be 1.05 ms and a zero-field splitting of 6937 cm-1 is obtained, both of which results a re in good agreement with the corresponding measured values of 1.4 ms and 6768 cm-1. The next-lowest-energy state (a2) is predicted to have a lifetime which is 80 times larger, a result which is consistent with the failure to date to observe emission bands initiating from this st ate.