No abstract

Femtosecond midinfrared spectroscopy of water (heavy water) after two photon excitation at 9 eV provides clear evidence for two short-lived precursors of the hydrated electron preceding the well-known "wet electron." The measured first intermediate with peak absorption at 2.9 (4.1) microm is proposed to represent an O(H, D):e(-) complex. The subsequent solvation proceeds via e(-):[(H, D)2O](n) complexes at approximately 1.6 (2.0) microm in the electronic ground state, involving an increasing number of water molecules during the first 200 fs and followed by the wet electron.

The spectral substructure of the OH-stretching band of the isotopic mixture HDO in D2O is demonstrated in the temperature range of 273−343 K, using two-color IR spectroscopy with tuneable subpicosecond and picosecond pulses. We derive from time-resolved spectra three major components peaked at approximately 3330 cm-1 (I), 3400 cm-1 (II), and 3450−3500 cm-1 (III). In contrast to I and II, species III displays a distinct temperature dependence of position and bandwidth. The latter varies in the range 90−140 cm-1, representing inhomogeneous broadening above 290 K, as indicated by novel hole-burning observations with a hole width of 45 cm-1 and a lifetime of the holes of ≈1 ps. The species I−III are also characterized by different values of the reorientational time constant in the range of 3−15 ps, depending on temperature, and are attributed to different preferred local environments in the hydrogen-bonded network. Component I observed with decreasing amplitude up to 343 K is close to a frequency characteristic for the ice structure Ih and provides evidence for approximately tetrahedral local geometries in liquid water. From the measured cross-relaxation among the spectral species, a structural relaxation time of 1.5−0.8 ps is deduced in the range 273−343 K. The populational lifetime of the first excited state of the OH-stretching vibration of component II is measured to be 1.0 ± 0.2 ps at room temperature.

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.

The spectral substructure and the dynamics of the OH-and OD-stretching bands of HDO in the diluted isotopic mixtures with D 2 O and for the first time H 2 O are investigated using 2-color IR spectroscopy with tunable subpicosecond pulses. From the observed transient band shapes, we derive four major spectral components within the OH-band of HDO in the solvent D 2 O peaked at approximately 3330 cm -1 (I), 3390 cm -1 (II), 3460 cm -1 (III), and 3520 cm -1 (IV), as well as three corresponding species in the case of the OD-band in the solvent H 2 O. The components display no distinct temperature dependence of position and spectral width in the investigated temperature range 273-343 K. Comparing the OH-stretching mode of HDO:D 2 O to the OD-vibration of HDO:H 2 O, the population lifetime increases significantly from 1.0 ps for OH to 1.8 ps ((0.2 ps) for OD at room temperature, whereas spectral relaxation, assigned to structural changes, seems to slow from 1.0 ( 0.4 ps to 2.0 ( 0.8 ps in the protonated water environment.