EXCITATION DEPHASING, PRODUCT FORMATION, AND VIBRATIONAL COHERENCE INAN 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 f requency 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 th e several different processes and time scales for the relaxation of co herences and populations in this extended solid. The degenerate Fe-II- Fe-III transitions as well as the octahedral symmetry of the excitatio n in the ''colloidal'' material yields a novel coherent response prior to back electron transfer as observed through the decay of optical an isotropy. Several low frequency Raman active vibrational modes that ar e coupled to the CT coordinate are detected. A sub-picosecond to picos econd 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 mo tion on the ground and CT (i.e. excited) state potential energy surfac es exhibiting five dynamical processes: (1) rapid CT state electronic dephasing, (2) ground state vibrational dephasing, (3) population rela xation, (4) back electron transfer through vibrationally hot levels of the ground state electronic configuration, and (5) formation of the r elaxed CT species.