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

Abstract

It is generally assumed that the hydrated electron occupies a quasi-spherical cavity surrounded by only a few water molecules in its equilibrated state. However, in the very moment of its generation, before water has had time to respond to the extra charge, it is expected to be significantly larger in size. According to a particle-in-a-box picture, the frequency of its absorption spectrum is a sensitive measure of the initial size of the electronic wavefunction. Here, using transient terahertz spectroscopy, we show that the excess electron initially absorbs in the far-infrared at a frequency for which accompanying ab initio molecular dynamics simulations estimate an initial delocalization length of ≈40 Å. The electron subsequently shrinks due to solvation and thereby leaves the terahertz observation window very quickly, within ≈200 fs.

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Figure 1: Terahertz response to photoionization of neat water.
Figure 2: Averaged radius of gyration rg (blue) and lowest excitation energy (green) of an excess electron attached to neat water system as a function of time.
Figure 3: Correlation between radius of gyration rg of the spin density and first excitation energy (blue dots).
Figure 4: Schematic depiction of excess electron formation and localization process in water.

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Acknowledgements

The authors thank S. Bradforth and P. Kužel for discussions. The work was supported in part by the Swiss National Science Foundation (SNF) through the NCCR MUST. P.J. acknowledges the Czech Science Foundation (grant P208/12/G016) for support and thanks the Academy of Sciences for the Praemium Academie award. F.U. and P.J. also acknowledge the computing time granted by the John von Neumann Institute for Computing (NIC) and provided on the supercomputer JUROPA at Jülich Supercomputing Centre (JSC).

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J.S. and P.H. designed the experiments and analysed the data. J.S. and S.A. performed the experiments. P.J. and F.U. designed and analysed the computational part of the study and FU performed the calculations. P.H., P.J. and J.S. co-wrote the paper. J.S. and F.U. contributed equally to this work.

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Correspondence to Peter Hamm or Pavel Jungwirth.

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Savolainen, J., Uhlig, F., Ahmed, S. et al. Direct observation of the collapse of the delocalized excess electron in water. Nature Chem 6, 697–701 (2014). https://doi.org/10.1038/nchem.1995

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