Invited Paper SS-MoM5
Experimental Evidence of Non-adiabatic Effects in Gas-Surface Interactions

Monday, October 29, 2012, 9:40 am, Room 21

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.