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

Abstract

Transient absorption measurements monitor the geminate recombination kinetics of solvated electrons following two-photon ionization of liquid water at several excitation energies in the range from 8.3 to 12.4 eV. Modeling the kinetics of the electron reveals its average ejection length from the hydronium ion and hydroxyl radical counterparts and thus provides insight into the ionization mechanism. The electron ejection length increases monotonically from roughly 0.9 nm at 8.3 eV to nearly 4 nm at 12.4 eV, with the increase taking place most rapidly above 9.5 eV. We connect our results with recent advances in the understanding of the electronic structure of liquid water and discuss the nature of the ionization mechanism as a function of excitation energy. The isotope dependence of the electron ejection length provides additional information about the ionization mechanism. The electron ejection length has a similar energy dependence for two-photon ionization of liquid D 2 O, but is consistently shorter than in H 2 O by about 0.3 nm across the wide range of excitation energies studied.

Direct observation of the collapse of the delocalized excess electron in water

“…It is remarkable that this number is very close to the ejection length, which has independently been estimated from the recombination kinetics (see Supplementary Figure 4) along the lines of Ref. [21].…”

“…Comparing the recombination kinetics with Refs. [20,21], we estimate that 800-nm pumping result in an effective excitation energy of 12.4 eV and an average ejection length of ≈38Å. At this energy the electron reaches the conduction band vertically [19], or even states above the vacuum level (which we expect to relax very quickly in bulk solution).…”

“…According to the current knowledge, there exists a threshold in the photodetachment between molecularbased mechanisms and bulk excitation to delocalized states [17,21]. With high enough energies given to the ejected electron, the conduction band can be directly reached, leaving the nuclear positions unchanged.…”

“…With high enough energies given to the ejected electron, the conduction band can be directly reached, leaving the nuclear positions unchanged. Given the inhomogeneity in the water hydrogen bonding network, the threshold energy for this vertical ionization mechanism is ≈9.8-9.9 eV [21], about 0.5-1 eV below the vacuum level (it should be added however that other works have estimated a much smaller width of the conduction band based on an extrapolation from clusters [25]). Below this threshold, the conduction band is not directly accessible and e aq is formed via mechanisms involving nuclear degrees of freedom in the sense of solvation, or directly of the excited water molecule.…”

“…Femtosecond pump-probe experiments have been used to unravel details of the equilibration processes following the trapping by the solvent [5][6][7][8][9][10][11][12]. In addition, picosecond measurements on the recombination kinetics of e − aq have described the ejected electron's migration lengths from its geminate partners [13][14][15][16][17][18][19][20][21][22]. These studies evoke a picture of an excess electron that is at first delocalized in the conduction band and then localizes to the hydrated electron e − aq within about a picosecond.…”