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.
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