RADIATIONLESS TRANSITIONS BETWEEN THE FIRST EXCITED TRIPLET-STATE ANDTHE SINGLET GROUND-STATE IN ETHYLENE - A THEORETICAL-STUDY

A theoretical analysis is reported for the first triplet (T) to ground state (N) radiationless transition of the ethylene molecule. This wor k is based on ab initio calculations of the potential energy surfaces for the T and N states taking into account CC and CH bond stretching a s well as HCH bending and torsion of the CH2 groups. The radiationless transition rates are calculated for different molecular vibrational l evels using the Fermi-Wentzel golden rule formalism. The interaction m atrix element [T\V-int\N] is approximated by the electronic spin-orbit coupling matrix element between the N and T states, integrated over t he torsional coordinate; the effect of the three totally symmetric nor mal modes on the calculated rates is accounted for by their Franck-Con don factors. The transition rates calculated in this work are on the o rder of 10(6)-10(8) s(-1) if one or two levels of the totally symmetri c modes are excited, which is in good agreement with the global values predicted earlier by Salem and Rowland.