Article pubs.acs.org/JPCC
Trapped Electrons at the Amorphous Solid Water/Vacuum Interface as Possible Reactants in a Water Splitting Reaction Sarah B. King,* Daniel Wegkamp, Clemens Richter, Martin Wolf, and Julia Staḧ ler* Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany S Supporting Information *
ABSTRACT: The dynamics and energetics of electrons in water impacts diverse fields, from electrochemistry to astrochemistry. In the present experiments, long-lived, low-energy, electronic states are observed at the amorphous solid water (ASW)/vacuum interface with decay times on the order of seconds, orders of magnitude longer than solvated electron states observed in the ASW bulk. The formation, relaxation, and reactivity of these trapped electrons were investigated using two-photon photoemission of >15 bilayers of amorphous D2O adsorbed on the Cu(111) surface. The decay time of the trapped electron signal is approximately 16 s. This extraordinarily long lifetime allows for a reaction between trapped electrons and the ASW surface, producing hydroxide anions and molecular hydrogen at the interface. This reaction is observed by a decrease of the trapped electron population and a concomitant increase of the work function during ultraviolet light exposure. The low-energy electron reactivity at the ASW/vacuum interface has profound implications for astrochemistry due to the prevalence of ASW in space.
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INTRODUCTION The energetics and dynamics of the bound states of electrons in water have been an active area of scientific debate in the last 10 years.1−7 Papers have claimed to observe both equilibrium bulk solvated electrons as well as surface-bound hydrated electrons.3,6,8,9 While theory and experiment agree on the existence of bulk solvated electrons, the hydrated electron on the liquid water/vacuum interface has been particularly debated;10,11 the binding energies and lifetimes of metastable water electronic states can be sensitive to the water temperature and preparation method.12,13 The possibility of such electronic states at the water/vacuum interface is important to atmospheric- and astrochemistry, where solar radiation and electrons can interact with various solid phases of water.1,14−16 The majority of water found in the universe is believed to be in an amorphous solid phase, not the thermodynamically stable crystalline phase.16,17 Amorphous solid water (ASW) is found in cometary objects, satellites, and rings, as well as on the surfaces of dust grains that are the precursors to planetary materials.15 The ASW surface and solid-phase reactions in general are important in the production of reactive molecular species and the increase in molecular diversity that is crucial for life.18 In the atmosphere, charged species on the surface of melted ice grains in clouds are involved in how thunderclouds are electrified, an important consideration in global weather phenomena.14 The role of vacuum ultraviolet light and highenergy electrons in reactions with ASW has been investigated in detail,19−24 however, the influence of low-energy electrons (
