ENERGY ACTIVATION OF ADIABATIC AND NONADIABATIC ELECTRON-TRANSFER

Low-level and two-level approaches to above-barrier reactions are used to establish the borders between adiabatic and nonadiabatic electron transfer to either stable or unstable products. The multiple curve-cro ssing probability is equal to unity in a narrow ''energy window,'' jus t above the barrier, and slowly decreases with energy over it. When th e reactants are the Langevin oscillators and the energy dispersion per period is less than the-width of energy window, Mel'nikov's one-level theory is appropriate. Otherwise (at higher friction) it must be repl aced by the two-level nonadiabatic theory of Burshtein and Zharikov. T he transition from an energy diffusion regime to transition state theo ry (TST) or perturbation theory results occurs within the Langevin mod el at much,lower friction than within the ''strong collision model'' c onsidered previously.