Solvation and Thermalization of Electrons Generated by above-the-Gap (12.4 eV) Two-Photon Ionization of Liquid H₂O and D₂O

Abstract

Temporal evolution of transient absorption (TA) spectra of electrons generated by above-the-gap (12.4 eV total energy) two-photon ionization of liquid H2O and D2O has been studied on femto- and picosecond time scales. The spectra were obtained at intervals of 50 nm between 0.5 and 1.7 mum. Two distinct regimes of the spectral evolution were observed: t < 1 ps and t > 1 ps. In both of these regimes, the spectral profile changes considerably with the delay time of the probe pulse. The "continuous blue shift" and the "temperature jump" models, in which the spectral profile does not change as it progressively shifts, as a whole, to the blue, are not supported by our data. Furthermore, no p-state electron, postulated by several authors to be a short-lived intermediate of the photoionization process, was observed by the end of the 300 fs, 200 nm pump pulse. For t < 1 ps, two new TA features (the 1.15 microm peak and 1.4 mum shoulder) were observed for the electron in the spectral region where O-H overtones appear in the spectra of light water. These two features were not observed for the electron in D2O. The 1.4 mum peak observed in D2O may be the isotope-shift analogue of the 1.15 microm feature in H2O. Vibronic coupling to the modes of water molecules lining the solvation cavity is a possible origin of these features. On the sub-picosecond time scale, the absorption band of solvated electron progressively shifts to the blue. At later delay times (t > 1 ps), the position of the band maximum is "locked", but the spectral profile continues to change by narrowing on the red side and broadening on the blue side; the oscillator strength is constant within 10%. The time constant of this narrowing is ca. 0.56 ps for H2O and 0.64 ps for D2O. Vibrational relaxation and time-dependent decrease in the size and sphericity of the solvation cavity are suggested as possible causes for the observed spectral transformations in both of these regimes.

Excitation-energy dependence of the mechanism for two-photon ionization of liquid H2O and D2O from 8.3to12.4eV

“…The change in absorption over the first few picoseconds ͑not visible in the figure͒ is primarily a result of the spectral shift that accompanies solvent reorganization around the electron and depends on the probe wavelength. 30,31 Because the spectral shift does not correspond to population dynamics, we consider only the data for delays longer than 5 ps, after which the shape and position of the electron spectrum remain constant and the signal amplitude reflects the concentration of solvated electrons. All of the traces decay nonexponentially to a fraction of their maximum amplitude, with the amount of decay depending on the excitation energy.…”

Isotope effects in liquid water probed by transmission mode x-ray absorption spectroscopy at the oxygen K-edge

“…To date, the ΔZPE shift and the ΔZPD broadening in the UV absorption spectrum of liquid water are investigated experimentally [19] and theoretically [20]. This shows the importance to accurately quantify these effects to enable a correct interpretation of the fundamental processes upon photoionization of water and the related hydrated electron [21][22][23][24].…”

Numerical Simulation of the Formation of Hydrated Electron States

“…1 and Fig. 2 in comparison with the experimental data from [8] and [12], respectively. Corresponding values of Gaussian parameter σ are given in the Table 1.…”