Femtosecond internal conversion and solvation dynamics of the solvated electrons in neat water and aqueous solution

Transient pump-probe spectroscopy of equilibrated solvated electrons is carried out in neat water and in an 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 of the e^- promoting electrons from the 1s ground state to the highest of the 2p-substates. Transient bleaching is observed in a broad interval around the maximum of the band at 720 nm, accompanied by induced absorption at longer wavelengths. No holeburning 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 2p-substates. The relaxation dynamics clearly involves a first intermediate that is strongly proposed to be a modified excited state p and the lifetime of which is determined to be τ₂ = 190 ± 40 fs. After 500 fs an isosbestic point develops in the transient spectrum that is related to a second intermediate that is assigned to a modified ground state s″. A further time constant τ = 0.9 ± 0.15 ps accounts for the final recovery of the population to the original ground state 1s. Evidence for stimulated emission in the probe absorption of the first intermediate allows its assignment as p′ and suggests a distinct red shift of the transition p′ → ground state to 760 nm, while the transient absorption band of electrons in the longer-lived s″-level is centered at 810 nm. The negligible net 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. Comparison with similar observations for NaCl solution is also reported.