EXCITATION DEPHASING, PRODUCT FORMATION, AND VIBRATIONAL COHERENCE IN AN INTERVALENCE CHARGE-TRANSFER REACTION

The ultrafast dynamics associated with optically induced intervalence charge-transfer reactions in solution are investigated in the Fe-II-Fe-III complex Prussian blue. The experimental measurements, including frequency and polarization dependent pump probe studies, are performed using ultrashort (i.e. 20 fs) pulses generated by a home-built cavity-dumped Ti:sapphire laser. Complicated time-domain waveforms reflect the several different processes and time scales for the relaxation of coherences and populations in this extended solid. The degenerate Fe-II-Fe-III transitions as well as the octahedral symmetry of the excitation in the ''colloidal'' material yields a novel coherent response prior to back electron transfer as observed through the decay of optical anisotropy. Several low frequency Raman active vibrational modes that are coupled to the CT coordinate are detected. A sub-picosecond to picosecond time scale transient absorption feature is shown to reflect the formation of a new product state (i.e. relaxed CT state) that persists for tens of picoseconds. The results are interpreted as wavepacket motion on the ground and CT (i.e. excited) state potential energy surfaces exhibiting five dynamical processes: (1) rapid CT state electronic dephasing, (2) ground state vibrational dephasing, (3) population relaxation, (4) back electron transfer through vibrationally hot levels of the ground state electronic configuration, and (5) formation of the relaxed CT species.