PNNL

Abstract

It is well known that the solvent plays a critical role in ultrafast electron transfer reactions. However, solvent reorganization occurs on multiple length scales, and selectively measuring short-range solute-solvent interactions at the atomic level with femtosecond time resolution remains a challenge. We performed femtosecond X-ray scattering and emission measurements following photoinduced charge transfer excitation in a mixedvalence bimetallic (FeIIRuIII) complex in water. The time-dependent X-ray signals, in combination with non-equilibrium molecular dynamics simulations, reveal that the charge transfer excited state has a lifetime of 62 fs and that coherent translational motions of the first solvation shell are coupled to the back electron transfer. The observed coherent translational motions arise from hydrogen bonding changes between the solute and nearby water molecules upon photoexcitation, and have amplitude of tenths of °A, 120-200 cm-1 frequency, and 100 fs relaxation time. This study provides an atomistic view of coherent solvent reorganization mediating ultrafast intramolecular electron transfer.