AVS2012 Session SS-MoM: Nonequillibrium and Nonlinear Processes

The hydrogen evolution reaction is one of the key reactions in catalysis whereby protons from a liquid phase discharge at a metal surface and form atomic hydrogen. A correlation between the reaction rate and the chemisorption energy of the metal-hydrid species (so called Volcano plot) was motivated for decades by an exponentially increasing reaction rate originated from a stronger chemisorption energy by arguments from the equilibrium thermochemistry. Progress in the study of metal-gas phase interactions points to electronically excited states, when atoms adsorb on a metal or chemical reactions with molecules occur. Up to now metal-liquid interfaces are not in the focus of research activities considering non equilibrium processes in the course of interfacial chemical reactions. We show experimental concepts how reactions on metal-liquid interfaces can be reviewed with respect to the existence of chemically induced electronic excitations. In a theoretical model we show that even small deviations from the electronic equilibrium may change the rate of the discharge reaction of protons on metal surfaces.

Interactions of molecules at metal surfaces can result in nonadiabatic electronic energy exchange with the metal. This complicates theoretical strategies designed to simulate surface reactivity, most of which today are based on the assumption that the electronic motion can be treated adiabatically, i.e. within the Born-Oppenheimer approximation. One widely applied electronically nonadiabatic theory that makes the leap beyond the Born-Oppenheimer approximation is “electronic friction”. In this method coupling of adsorbate motion to metal electrons is treated as a weak perturbation involving frictional forces modifying the molecular dynamics in a systematic and simple way.

Recent experiments on multiquantum vibrational excitation at metal surfaces suggest that at least for certain systems, multi quantum transitions involve energy transfer between the molecule and a single electron hole pair of the solid. These processes might better be described as an electron transfer reaction than as friction. These results suggest that theoretical approaches that go beyond electron weak coupling and electronic friction will be needed to properly treat electronically nonadiabatic effects in surface chemistry.

The pertubation of the electronic system during gas-metal interactions can cause significant electronic excitations with lifetimes on the femtosecond timescale [1]. Such non-adiabatic processes occur when electronic states are injected below the Fermi level such rapidly that the occupation of the states by resonant charge transfer is delayed. According to Zener’s criterion [2] this happens more likely in cases of fast nuclear motion and of low coupling between gas particle and metal states. Experimental evidence of chemically induced electronic excitations is gained by detecting exoelectron emission into the vacuum, surface chemiluminescence and internal hot hole or hot electron chemicurrents. The latter method uses thin-film electronic devices with internal potential barriers as high-pass energy filters. Metal-semiconductor (Schottky) diodes are the most prominent examples for sensitive detectors of both, hot charge carriers and chemiluminescence photons. Independent measurements of the phenomena uncover the various excitation mechanisms. An empty state below the Fermi level may inject a hot hole into the band of occupied electronic metal states or can be filled after Auger relaxation leading to an excited electron in the metal surface. For the oxidation of metals these two fundamental processes can be experimentally distinguished. In addition, it is shown that rapid state injection does not necessarily imply the dissociation of the oxygen molecule as peroxide formation also leads to a significant excitation of the electronic system. The non-adiabatic energy transfer can be associated with a rapid intermolecular motion of the oxygen atoms during the reactive collision. The interaction of chlorine with potassium will be discussed as an example for a strong chemiluminescence reaction. By use of K/Ag/Si-multilayer Schottky diodes the coupling between emitted photons and Ag surface plasmons leads to an enhanced photocurrent in the device at a typical Ag film thickness of around 50 nm. Competing effects in the devices due to adiabatic energy dissipation, e.g., local heating of the system, are discussed. In the experiments such can be either certainly excluded or clearly separated from the non-adiabatic signatures.

[1] B.I. Lundqvist et al. in : Handbook of Surface Science, Vol. 3, Eds.: E. Hasselbrink and B.I. Lundqvist (North-Holland, Amsterdam, 2008), pp. 430-524.

[2] C. Zener, Proc. R. Soc. Lond. A 137 (1932) 696.