SINGLET-TO-TRIPLET ENERGY-TRANSFER VIA (1)PI(1) (3)SIGMA(1+) CURVE CROSSINGS IN GROUP-2 AND 12 METAL ATOM RARE-GAS SYSTEMS/

Of the excited 1PI1 electronic states of van der Waals complexes of Mg , Zn, and Cd atoms (M) with rare-gas (RG) atoms, only the Zn . Xe (1PI 1) and Cd . Xe (1PI1) states predissociate to form the lower-lying tri plet states, Zn (4s4p P-3(2)) and Cd(4s4p P-3(2)) , respectively. It h as been postulated that such predissociations occur by means of potent ial curve crossings between bound 1PI1 levels and repulsive M . RG (3S IGMA1+) states. Since the M . RG (1PI1) states become more bound as th e RG atom becomes more polarizable, from Ne through Xe, and the M . RG (3SIGMA+) states should become more repulsive as the RG atom becomes larger in the same order, the likely reason that only the Zn - Xe and Cd . Xe 1PI1 states pedissociate is that they are the only states whic h have in 1PI1/3SIGMA1+ curve crossings below the energies which are a ccessed spectroscopically. We have carried out ab initio electronic st ructure calculations using various basis sets, and at various levels o f correlation, to examine the repulsive 3SIGMA1+ potential curves of Z n . Ar, Zn . Kr, Zn . Xe, Mg . Ar, and Mg . Xe. These calculations sup port the general mechanism proposed, and show that the likely reason t he Mg . Xe(1PI1) state does not predissociate is because the 1PI1/3SIG MA1+ curve crossing lies slightly above the energy region probed exper imentally. It was necessary to utilize very good quality basis sets an d high levels of correlated calculations to obtain agreement with expe rimental observations. In all cases, there was a regular decrease in t he repulsive character of the M . RG (3SIGMA+) states as the basis qua lity and level of correlation was increased.