ELECTRONIC COHERENCE, VIBRATIONAL COHERENCE, AND SOLVENT DEGREES OF FREEDOM IN THE FEMTOSECOND SPECTROSCOPY OF MIXED-VALENCE METAL DIMERS IN H2O AND D2O

We report the first < 20-fs time-resolved pump-probe study on photoind uced intramolecular electron transfer in aqueous solution. The metal-m etal, charge-transfer (MMCT) absorption bands of the mixed-valence com pounds (NH3)(5)(RuCNRuII)-C-III(CN)(5)(-)(RuRu) and (NH3)(5)(RuCNFeII) -C-III(CN)(5)(-) (RuFe) are studied with sufficient time resolution to measure the back-electron-transfer (b-ET) time. In RuRu, the b-ET occ urs in 85 +/- 10 fs in H2O and increases to 122 +/-, 20 fs in D2O. Sim ilar b-ET rates in these solvents are also observed for RuFe. The deut erium isotope effect is shown to originate from the solvent, demonstra ting that hydrogenic solvent motions are directly coupled to the elect ron transfer event. The pump-probe spectroscopy on the MMCT band also provides information on the dynamics of the nuclear degrees of freedom (vibrational and solvent) that are coupled to the MMCT absorption ban d and the b-ET. An oscillatory vibrational response is observed and as signed to resonance impulsive stimulated Raman scattering. Analysis of these oscillations demonstrates that the average vibrational dephasin g time for the observed modes is similar to 300 fs. The early-time beh avior of the pump-probe transient absorption indicates that the optica l dephasing time for MMCT compounds is extraordinarily short (20 fs) d ue to strong solvent-solute coupling. Evidence for fast optical dephas ing is provided by the instrument-response limited coherence coupling signal and the absence of a pump-probe signal corresponding to transie nt hole burning in the solvent coordinate. The combined results indica te that this fast b-ET is an electronically incoherent process; howeve r, vibrational coherence is maintained for some of the degrees of free dom during the b-ET.