Spectroscopic evidence for a gold-coloured metallic water solution.

Insulating materials can in principle be made metallic by applying pressure. In the case of pure water, this is estimated1 to require a pressure of 48 megabar, which is beyond current experimental capabilities and may only exist in the interior of large planets or stars2-4. Indeed, recent estimates and experiments indicate that water at pressures accessible in the laboratory will at best be superionic with high protonic conductivity5, but not metallic with conductive electrons1. Here we show that a metallic water solution can be prepared by massive doping with electrons upon reacting water with alkali metals. Although analogous metallic solutions of liquid ammonia with high concentrations of solvated electrons have long been known and characterized6-9, the explosive interaction between alkali metals and water10,11 has so far only permitted the preparation of aqueous solutions with low, submetallic electron concentrations12-14. We found that the explosive behaviour of the water-alkali metal reaction can be suppressed by adsorbing water vapour at a low pressure of about 10-4 millibar onto liquid sodium-potassium alloy drops ejected into a vacuum chamber. This set-up leads to the formation of a transient gold-coloured layer of a metallic water solution covering the metal alloy drops. The metallic character of this layer, doped with around 5 × 1021 electrons per cubic centimetre, is confirmed using optical reflection and synchrotron X-ray photoelectron spectroscopies.

Photoelectron spectra of alkali metal-ammonia microjets: From blue electrolyte to bronze metal.

Experimental studies of the electronic structure of excess electrons in liquids-archetypal quantum solutes-have been largely restricted to very dilute electron concentrations. We overcame this limitation by applying soft x-ray photoelectron spectroscopy to characterize excess electrons originating from steadily increasing amounts of alkali metals dissolved in refrigerated liquid ammonia microjets. As concentration rises, a narrow peak at ~2 electron volts, corresponding to vertical photodetachment of localized solvated electrons and dielectrons, transforms continuously into a band with a sharp Fermi edge accompanied by a plasmon peak, characteristic of delocalized metallic electrons. Through our experimental approach combined with ab initio calculations of localized electrons and dielectrons, we obtain a clear picture of the energetics and density of states of the ammoniated electrons over the gradual transition from dilute blue electrolytes to concentrated bronze metallic solutions.

Deeply cooled and temperature controlled microjets: Liquid ammonia solutions released into vacuum for analysis by photoelectron spectroscopy.

A versatile, temperature controlled apparatus is presented, which generates deeply cooled liquid microjets of condensed gases, expelling them via a small aperture into vacuum for use in photoelectron spectroscopy (PES). The functionality of the design is demonstrated by temperature- and concentration-dependent PES measurements of liquid ammonia and solutions of KI and NH4I in liquid ammonia. The experimental setup is not limited to the usage of liquid ammonia solutions solely.

Valence and Core-Level X-ray Photoelectron Spectroscopy of a Liquid Ammonia Microjet.

Photoelectron spectroscopy of microjets expanded into vacuum allows access to orbital energies for solute or solvent molecules in the liquid phase. Microjets of water, acetonitrile and alcohols have previously been studied; however, it has been unclear whether jets of low temperature molecular solvents could be realized. Here we demonstrate a stable 20 µm jet of liquid ammonia (-60 °C) in a vacuum, which we use to record both valence and core-level band photoelectron spectra using soft X-ray synchrotron radiation. Significant shifts from isolated ammonia in the gas-phase are observed, as is the liquid-phase photoelectron angular anisotropy. Comparisons with spectra of ammonia in clusters and the solid phase, as well as spectra for water in various phases potentially reveal how hydrogen bonding is reflected in the condensed phase electronic structure.

The influence of aqueous solvent on the electronic structure and non-adiabatic dynamics of indole explored by liquid-jet photoelectron spectroscopy.

Understanding how the electronic structure of an aqueous solute is intricately bound up with the arrangement of a host liquid provides insight into how non-adiabatic photochemistry takes place in the condensed phase. For example, the presence of water provides additional solute-solvent interactions compared to non-polar solvents: changing the stability of ionized products and modifying the energies of low-lying excited valence states, as well as moving the point of intersection between potential surfaces. Thus, the locations and topography of conical intersections between these surfaces also change. The overall impact of the aqueous environment can be to modify the intricate photochemical and non-radiative pathways taking place after photoexcitation. Time-resolved photoelectron spectroscopy (TRPES) in a liquid micro-jet is implemented here to investigate the influence of water on the electronic structure and dynamics of indole, the chromophore of the amino acid tryptophan. TRPES is used to establish ultrafast relaxation pathways that vary as a function of excitation wavelength. In our experiment, aqueous indole was excited with femtosecond pulses centered at 292 nm and 266 nm. The vertical excitation energy of aqueous indole is extracted and found to be lowered by 0.5 eV in water relative to the gas phase. In the TRPES study, the spectral signature of 1La and evidence of solvated electron formation on an ultrafast timescale are observed. Our data also points to a possible contribution of the dissociative πσ* state, which can be accessed by a conical intersection (CI) with the 1La state.

Translational, rotational and vibrational relaxation dynamics of a solute molecule in a non-interacting solvent.

Spectroscopically observing the translational and rotational motion of solute molecules in liquid solutions is typically impeded by their interactions with the solvent, which conceal spectral detail through linewidth broadening. Here we show that unique insights into solute dynamics can be made with perfluorinated solvents, which interact weakly with solutes and provide a simplified liquid environment that helps to bridge the gap in our understanding of gas- and liquid-phase dynamics. Specifically, we show that in such solvents, the translational and rotational cooling of an energetic CN radical can be observed directly using ultrafast transient absorption spectroscopy. We observe that translational-energy dissipation within these liquids can be modelled through a series of classic collisions, whereas classically simulated rotational-energy dissipation is shown to be distinctly faster than experimentally measured. We also observe the onset of rotational hindering from nearby solvent molecules, which arises as the average rotational energy of the solute falls below the effective barrier to rotation induced by the solvent.