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The hydrated electron , an electron in aqueous solution, was discovered as a product of water radiolysis by ionizing radiation  and has since become a species of central interest in the physical sciences. It plays a major role in radiation chemistry and biology, because hydrated electrons can be formed by ionizing radiation in living cells, and their high reactivity leads to free radical formation with significant potential for genetic damage. From a more fundamental perspective,  represents the simplest quantum mechanical solute, thereby motivating many experimental and theoretical studies that have focused on understanding its spectroscopy, reactivity, and relaxation dynamics subsequent to electronic excitation. A parallel experimental and theoretical effort has focused on gas phase water cluster anions, (H₂O)_n¯, in which an electron is bound to a known number of water molecules. The cluster studies have provided valuable insights into the nature of , but have also raised the issue of how the properties of finite clusters can be extrapolated to bulk aqueous solutions. For example, extrapolation of the vertical binding energies (VBE's) of water cluster anions to n®¥ yields an estimated value of 3.4±0.2 eV for the VBE of the bulk hydrated electron, based on work with clusters up to n=69 by Coe et al.¹. Recent work by Siefermann et al.² on hydrated electrons in liquid water jets yielded the first actual measurement of this quantity, finding remarkable agreement (3.3 eV) with the cluster extrapolation. A similar value has been reported by Tang et al.³

We have carried out a systematic study of the photoelectron spectroscopy of hydrated electrons in liquid water jets using multiple precursors and photodetachment wavelengths.⁴  Hydrated electrons were generated in and detached from liquid microjets using two photons from a single nanosecond laser pulse at 266 or 213 nm. Solutions of 50 to 250 mM potassium hexacyanoferrate(II) or potassium iodide were used to provide precursor anions.   Results at 266 nm are shown in the figure below.  All of our experimental conditions yield similar results, giving a mean vertical binding energy of 3.6±0.1 eV at a temperature of ~280 K, a slightly higher value than in recent reports.

            (1)        Coe, J. V.; Lee, G. H.; Eaton, J. G.; Arnold, S. T.; Sarkas, H. W.; Bowen, K. H.; Ludewigt, C.; Haberland, H.; Worsnop, D. R. J. Chem. Phys. 1990, 92, 3980.

            (2)        Siefermann, K. R.; Liu, Y. X.; Lugovoy, E.; Link, O.; Faubel, M.; Buck, U.; Winter, B.; Abel, B. Nature Chemistry 2010, 2, 274.

            (3)        Tang, Y.; Shen, H.; Sekiguchi, K.; Kurahashi, N.; Mizuno, T.; Suzuki, Y. I.; Suzuki, T. Phys. Chem. Chem. Phys. 2010, 12, 3653.

            (4)        Shreve, A. T.; Yen, T. A.; Neumark, D. M. Chem. Phys. Lett. 2010, 493, 216.