VIBRATIONAL COHERENCE IN ELECTRON-TRANSFER - THE TETRACYANOETHYLENE-PYRENE COMPLEX

Coherent vibrational wave packet motion is created in the excited char ge-transfer state of the electron donor-acceptor complex between tetra cyanoethylene (TCNE) and pyrene by an ultrashort (40 fs) 810 nm pump p ulse. Observations of the dynamics of the TCNE-anion transient absorpt ion 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 sc attering in the ground state and coherent repopulation of the ground s tate surface due to a vibrationally coherent electron transfer reactio n were considered as the cause. Vibrational coherence has also been mo nitored by observing quantum beats in the stimulated emission from the charge transfer state back to the ground state in the near-ir. This o bservation strongly suggests that the electron transfer reaction is in deed vibrationally coherent and that the reaction rate is modulated by this coherence. This interpretation is corroborated by a classical Mo nte Carlo simulation of vibrationally coherent reactions in the invert ed regime. (C) 1996 American Institute of Physics.