R. Laenen et al., Femtosecond internal conversion and solvation dynamics of the solvated electrons in neat water and aqueous solution, B POL CHEM, 47(4), 1999, pp. 283-301
Citations number
51
Categorie Soggetti
Chemistry
Journal title
BULLETIN OF THE POLISH ACADEMY OF SCIENCES-CHEMISTRY
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.