Fundanrenrcei Ondcrzoek der Materic. lmtirctrrr voor Aroom-en Mo?encu[~ sica. h-nrislaau JOT. 109s SJ Amsterdam. The Xeti~erlands Kcceired 30 May 1983 The tunnehng xiidths of high-energy metsstable states lying in the classical irregular region of the H&on-Heiles potential energy surface \xcre calculated via complex analytical continuation of stabilization graphs. Mode specificity is demonstrated in that the lifetimes of normal-mode, local-bond-mode. *and mode-mixed states at nearby energies differ by an order of mayitudc. Unitnolecutar decomposition rate theory is traditionally formulated in two ways. In the statistical RR01 [ l-31 approach where the excess (above dissociation threshold) energy is assumed to have random distribution among the nuclear modes of motion. the unitnolecular rate constant is d smooth monotonicly increasing function of the energy. On the other hand is the mode-specific theory of Slatcr [-I] which postulates a non-uniform energy distribution leading to rate constants that depend on the metastable state prepared by the excitation process. Experimentally _ it is known that the breakup of van der Waals molecules (e.g. I, : He + 1, + He) follows the mode-specific Slater-theory [5]. but the decomposition of ordinary chemical bonds usually shows statistical behavior [6] _ Theoretical indications that mode-localized packets which would presumably lead to mode-specific breakup. might esist in polyatoniic molecules came from classicltl dynamic studies on model systems_ These studies show that two types of classical motion appear in coupled oscillator systems [7.S]: the mode-localized regular quasiperiodic motion which is confined to specific