AB-INITIO CALCULATIONS OF RADIATIONLESS TRANSITIONS BETWEEN EXCITED AND GROUND SINGLET ELECTRONIC STATES OF ETHYLENE

General expressions for internal conversion (IC) rate constant calcula tions have been derived by taking into account displacements, distorti ons, and rotation (mixing) of normal modes. The electronic part of the rate constant has been computed through the nb initio calculations of vibronic coupling. The corresponding expressions for the simplest two -mode case as well as for the general n-mode case have been derived. W e demonstrate the effect of rotated (mixed) normal modes on the IC rat e constants based on a model consisting of one promoting and two mixed modes. The dynamics of excited states of C2H4 has been investigated b ased on the internal conversion mechanism. The calculated rate of inte rnal conversion show that the lifetimes of the excited pi-3p and pi-pi states of C2H4 are on the picosecond scale. We predict that if the m olecule is excited to a Rydberg pi-3p state, it relaxes to the ground state via the cascade mechanism, i-3p-->pi-3s(B-1(3u))-->pi-pi(B-1(1u ))-->(1)A(g). (C) 1998 American Institute of Physics.