Effect of a dissipative environment on the dynamics at a conical intersection

Multi-level Redfield theory is applied to study the photoinduced dynamics at a conical intersection which is weakly coupled to a vibrational bath. Based on numerical solutions of the Redfield equations, the effect of vibrational damping on the ultrafast time-dependent electronic decay dynamics and vibrational wave-packet dynamics of the conical intersection are investigated. It is shown that such models typically predict bimodal electronic relaxation dynamics, consisting of an ultrafast (approximate to 10 fs) initial decay driven by the active modes of the conical intersection, followed by slower (sub-picosecond) decay, reflecting vibrational energy relaxation on the lower adiabatic potential-energy surface. It is proposed that few-dimensional models of conical intersections which include vibrational dissipation are widely applicable for the microscopic description of ultrafast radiationless decay processes and the simulation of femtosecond time-resolved spectra in gas-phase and condensed-phase systems. (C) 2000 Elsevier Science B.V. All rights reserved.