Excited state dynamics of liquid water near the surface

Abstract

Abstract. Time resolved photoelectron spectroscopy explores the excited state dynamics of liquid water in presence of cations close to the surface. A transient hydrated electroncation complex is observed.The properties of liquid water are of fundamental importance to all natural sciences. Nevertheless, many properties of water remain poorly understood. This is particularly true for the properties and dynamics of electronic excited states, which have very short lifetimes (sub-100 fs) [1] and high excitation energies in the vacuum ultraviolet (VUV) region. Although the band maximum in bulk liquid water is near 8.4 eV [2], a lowering of the absorption edge can be expected at the surface [3]. By ab initio calculations, we show that, already at photon energies < 8 eV, the proximity of cations enables the direct excitation of a surface specific charge transfer state, leading to the formation of transient hydrated (cation. . .electron) complexes. A surface-solvated electron may thus be temporarily stabilized by subjacent cations.Time-resolved photoelectron spectroscopy of liquids is a new and promising tool to investigate excited state dynamics in solution. It has been applied to study the charge-transfer-to-solvent (CTTS) dynamics of iodide and the solvation and recombination dynamics of solvated electrons [4][5][6]. We now use this technique to study the excited state dynamics of liquid water in the presence of alkali cations at the liquid-to-vacuum interface.A detailed description of the experiment can be found elsewhere [7]. A high-pressure liquid chromatography pump pushes the sample solution (here: solutions of different salts of concentrations in the range of 30-100 mM in H 2 O or D 2 O) through a fused silica nozzle with an inner diameter of 15 µm into a vacuum chamber. A continuous liquid jet in vacuum is formed.To excite liquid water, we irradiate the liquid jet with sub-20 fs pulses at 160 nm (7.75 eV photon energy, several 10 nJ pulse energy), which are generated by non-collinear four wave mixing in an argon gas cell [8]. A delayed pulse at 400 nm (3.1 eV) or 800 nm (1.55 eV) probes the excited state by photoionization. Photoelectrons are collected by a magnetic bottle type time-of-flight spectrometer.A time-resolved photoelectron spectrum is shown in Fig. 1 a-b). After excitation of the sample (at delay ∆τ = 0), the photoelectron signal rises steeply and then decays with transient lifetimes on the fs to ps timescale (depending on solute and solvent). The average binding energy at the temporal overlap is ∼ 2.18 eV in H 2 O and ∼ 2.24 eV in D 2 O (cf. Fig. 1e). The average binding energy changes as function of delay time: it decreases within the first 100 fs and then increases. At long times, the vertical binding energy of this species in water is ∼ 2.5 eV and thus significantly smaller than the binding energy of the solvated electron in bulk water (3.3-3.7 eV, [4, 9-11]). We observe a slight cation dependence of the binding energy, while the anion does not influence the energetics. As shown in Fig. 2, the p...