AB-INITIO CI STUDY OF THE ELECTRONIC-SPECTRUM OF BISMUTH IODIDE EMPLOYING RELATIVISTIC EFFECTIVE CORE POTENTIALS

A relativistic CI treatment including spin-orbit coupling has been car ried out for the low-lying electronic states of bismuth iodide, employ ing effective core potentials for both atoms. The X(3) Sigma(-) ground state is computed to have a zero-field splitting of 5096 cm(-1), 1086 cm(-1) less than the most recent measured value. The a (1) Delta stat e is predicted to have a T-e value of 12336 cm(-1), and it is suggeste d on the basis of correlation effects that the true value should lie a bout 1000 cm(-1) lower. This conclusion is also based in part on the f inding that the computed BO+ T-e value of 24148 cm(-1) overestimates t he measured result by 759 cm(-1). The latter state is shown to arise f rom an avoided crossing between the (1) Sigma(+) and (5) Pi Lambda-S s tates, which produces only a relatively shallow well and a slight barr ier to dissociation. Because the 3 Pi state is repulsive, no other low -lying Omega = 0(+) state is found in the spectrum, similarly as in Sb I but in contrast to BiF. Due to the much greater spin-orbit effects i n BiI, the composition of the lowest two excited 0(+) states in terms of (1) Sigma(+) and (3) Pi Lambda-S states is notably different than i n SbI and this fact is important in understanding why the T-e value of the lowest bound 0(+) states of these two systems are so different. T ransition probabilities have also been computed for various pairs of v ibrational states. The radiative lifetime of the X(2)1 fine structure component is calculated to be 20.7 ms, which agrees well with a recent measured value of 20 +/- 4 ms by Fink and Shestakov. In agreement wit h Colin et al.'s empirical rule, it is found that the b-X(2) transitio n is stronger than b-X(1), and this result also confirms an earlier th eoretical analysis of this general phenomenon given by the authors.