SINGLET-TO-TRIPLET ENERGY-TRANSFER WITHIN M(RG)(N) VAN-DER-WAALS CLUSTERS (M=MG, ZN RG=AR, KR, XE)

Singlet-to-triplet energy transfer within M(RG)(n) van der Waals clust ers has been examined, where M Mg, Zn; RG = Ar, Kr, and/or Xe; and n = 1, 2, 3. For n = 1, the Breckenridge-Malmin mechanism for such energy -transfer processes, whereby the potential curve for a repulsive M . R G((3) Sigma(1)(+)) state crosses the potential curve for an attractive M . RG((1) Pi(1)) state, has been shown to be consistent with both fu ll-collision and half-collision experimental results as well as ab ini tio calculations of the M . RG((3) Sigma(+)) potential curves. Only fo r RG = Xe and M = Zn in the n = 1 case is singlet-to-triplet energy tr ansfer efficient, due to the more attractive character of the (1) Pi(1 ) states as the polarizability of the RG atom increases from Ar to Kr to Xe, and the more repulsive character of the (3) Sigma(1)(+) states as the M(p sigma)-RG(p sigma) repulsion of the RG atom increases in th e same ordering. By using pairwise atom-atom potentials obtained from spectroscopic measurements, ab initio calculations, and reasonable est imates, we have examined the possibilities of singlet-to-triplet energ y transfer within excited states of the analogous M(RG)(2) and M(RG)(3 ) clusters. Detailed considerations lead to the conclusion that B-1(1) /B-3(2) surface crossings should cause predissociation to triplet prod ucts in C-2v MgXe2, ZnKr2, and ZnXe2. Singlet-to-triplet predissociati on is also shown to be likely in the MgXe3, ZnKr3, and ZnXe3 clusters, not by surface crossings in C-3 upsilon symmetry, but via B-1(1)/B-3( 2) crossings in a T-shaped C-2 upsilon geometry in which one RG-RG bon d is broken and a near-linear RG-M-RG linkage is created.