Model studies of mode specificity in unimolecular reaction dynamics

Energies and lifetimes (with respect to tunneling) for metastable states of the Henon-Heiles potential energy surface [V(x,y) = 1/2 x{sup 2} - 1/3 x{sup 3} + 1/2 y{sup 2} + xy{sup 2}] have been computed quantum mechanically (via the method of complex scaling). This is a potential surface for which the classical dynamics is known to change from quasiperiodic at low energies to ergodic-like at higher energies. The rate constants (i.e. inverse lifetimes) for unimolecular decay as a function of energy, however, are seen to be well described by standard statistical theory (microcanomical transition state theory, RRKM plus tunneling) over the entire energy region, This is thus another example indicating that mode-specificity in unimolecular reaction dynamics is not determined solely by the quasiperiodic/ergodic character of the intramolecular mechanics