Dynamics of excited solvated electrons in aqueous solution monitored with femtosecond-time and polarization resolution

Transient pump-probe spectroscopy of equilibrated solvated electrons is carried out in aqueous NaCl solution (5.9 M) in the visible and near-infrared using pulses of 100-170 fs duration and polarization resolution. Excitation is performed by a pump pulse at 620 nm in the blue wing of the electronic absorption band. Transient bleaching occurs in a broad interval around the maximum of the e^- absorption at 705 nm, accompanied by induced absorption at longer wavelengths. No hole-burning features are observed within our experimental time resolution suggesting a time constant τ₁ < 80 fs for rapid solvent relaxation and/or population redistribution among the excited electronic states. The relaxation dynamics involves a first intermediate, a frequency-shifted excited-state p′ with lifetime τ₂ = 190 ± 40 fs. A further time constant τ₃ = 1.2 ± 0.4 ps accounts for the recovery of the ground state. The latter process involves a second intermediate that is assigned as a modified ground-state s″. Evidence for stimulated emission suggests a distinct red shift of the transition p′→ ground state to 800 ± 20 nm, while the transient absorption band of electrons in the s″-level is centered at 780 ± 20 nm. The negligible anisotropy <0.01 of the probe absorption measured during and after the excitation process indicates that the observed distribution of solvent cavities of hydrated electrons is close to spherical symmetry.