Mechanism of a photochemical funnel: a dissipative wave-packet dynamics study

The ultrafast internal conversion and isomerization dynamics of nonadiabatic cis → trans photoreactions are studied. The isomerizing molecule is described by a two-mode two-state model, which involves either an avoided crossing (AC) or a conical intersection (CI, the ‘funnel’) of the potential-energy surfaces. To account for the localization in the cis and trans configuration due to vibrational cooling of the photoproducts, the system is coupled to a harmonic bath via Redfield theory. Although the two models are – except for the topology of the crossing surface – identical, the CI model isomerizes about an order of magnitude faster than the AC model. This high reaction speed and efficiency of the photochemical funnel is caused by a subtle interplay of the highly localized nonadiabatic coupling of the CI and the dissipative dynamics of the molecular system.