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 (≃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.