Relaxation Dynamics and Genuine Properties of the Solvated Electron in Neutral Water Clusters

Abstract

We have investigated the solvation dynamics and the genuine binding energy and photoemission anisotropy of the solvated electron in neutral water clusters with a combination of time-resolved photoelectron velocity map imaging and electron scattering simulations. The dynamics was probed with a UV probe pulse following above-band-gap excitation by an EUV pump pulse. The solvation dynamics is completed within about 2 ps. Only a single band is observed in the spectra, with no indication for isomers with distinct binding energies. Data analysis with an electron scattering model reveals a genuine binding energy in the range of 3.55-3.85 eV and a genuine anisotropy parameter in the range of 0.51-0.66 for the ground-state hydrated electron. All of these observations coincide with those for liquid bulk, which is rather unexpected for an average cluster size of 300 molecules.

Figure 1

Evolution of the photoelectron signal as a function of the pump–probe time delay. (a) Vertical electron binding energy (VBE) and (b) full width at half-maximum (fwhm). Uncertainties are estimated not to exceed ±0.2 and ±0.3 eV for the VBE and fwhm, respectively. Lines connecting data points are intended as a guide to the eye. Data sets 1 and 2 represent independent measurement series, providing an impression of the reproducibility of the experiment.

Figure 2

(a) Experimental binding energy spectrum recorded at a pump–probe delay of ∼2 ps (circles). Dashed line spectrum: Simulation with genuine eBE of the liquid from Luckhaus et al. (dashed line in panel b). Full line spectrum: Simulation with a single Gaussian fit for the genuine cluster eBE (full line in panel b).

Figure 3

Observable β values as a function of genuine β values (β^g) simulated for different models of the electron scattering cross sections (average cluster size ⟨n⟩ = 300; see ref  for simulation details). Lines in between data points represent linear interpolations. The horizontal dashed line represents the experimental β value of 0.19. The gray shaded area indicates the range of genuine β parameters bracketed by cluster models iii and iv, which is consistent with the experimental β value measured for water clusters.