Molecular origins of the ultrafast relaxation of a photoexcited hydrated electron

Recent experiments have shown conclusively that the photoexcited hydrated electron has a non-radiative lifetime of ~50 fs. However, theoretical studies have been unable to rationalize such an ultrafast timescale and a molecular mechanism has remained elusive. To address this, we simulated the excited-state dynamics of this species with our recently developed quantum-mechanics/molecular-mechanics technology. Analyzing the evolution of the hydrated electron's ground and excited-state wavefunctions within a charge-transfer picture reveals that the lobes of the excited p state undergo a rapid separation which closes the energy gap to the ground state and promotes internal conversion on an ultrafast timescale. The lobe separation is seen to correlate with hydrogen bond rearrangements in the first few solvation shells of the electron.