Vibrational coherence in electron transfer: The tetracyanoethylene-pyrene complex

Coherent vibrational wave packet motion is created in the excited charge-transfer state of the electron donor-acceptor complex between tetracyanoethylene (TCNE) and pyrene by an ultrashort (40 fs) 810 nm pump pulse. Observations of the dynamics of the TCNE-anion transient absorption and the disappearance of the bleach of the ground state absorption show that the electron-transfer reaction back to the ground state of the complex occurs on a 250 fs-1.5 ps time scale. The bleach recovery signal shows clear oscillations and both impulsive stimulated Raman scattering in the ground state and coherent repopulation of the ground state surface due to a vibrationally coherent electron transfer reaction were considered as the cause. Vibrational coherence has also been monitored by observing quantum beats in the stimulated emission from the charge transfer state back to the ground state in the near-ir. This observation strongly suggests that the electron transfer reaction is indeed vibrationally coherent and that the reaction rate is modulated by this coherence. This interpretation is corroborated by a classical Monte Carlo simulation of vibrationally coherent reactions in the inverted regime. (C) 1996 American Institute of Physics.