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 car ried out in neat water and in an aqueous NaCl solution (5.9 M) in the visib le and near infrared using pulses of 100-170 fs duration and polarization r esolution. Excitation is performed by a pump pulse at 620 nm in the blue wi ng of the electronic absorption band of the e(-) promoting electrons from t he Is ground state to the highest of the ap-substates. Transient bleaching is observed in a broad interval around the maximum of the band at 720 nm, a ccompanied by induced absorption at longer wavelengths. No holeburning feat ures are observed within our experimental time resolution suggesting a time constant tau(1) < 80 fs for rapid solvent relaxation and/or population red istribution among the excited ap-substates. The relaxation dynamics clearly involves a first intermediate that is strongly proposed to be a modified e xcited state p' and the lifetime of which is determined to be tau(2) = 190 +/- 40 fs. After 500 fs an isosbestic point develops in the transient spect rum that is related to a second intermediate that is assigned to a modified ground state s ". A further time constant tau = 0.9 +/- 0.15 ps accounts f or the final recovery of the population to the original ground state is. Ev idence for stimulated emission in the probe absorption of the first interme diate allows its assignment as p' and suggests a distinct fed shift of the transition p' --> ground state to 760 nm, while the transient absorption ba nd of electrons in the longer-lived s "-level is centered at 810 nm. The ne gligible net anisotropy < 0.01 of the probe absorption measured during and after the excitation process indicates that the observed distribution of so lvent cavities of hydrated electrons is close to spherical symmetry. Compar ison with similar observations for NaCl solution is also reported.