Non-adiabaticity in surface chemical reactions

Chemical reactions at surface may dissipate energy exciting electron-hole pairs in the metal substrate. Direct detection of the chemically induced hot charge carriers may be achieved by measuring the tunnel current in Ta-TaOx-Au tunnel junctions when the Au top electrode is exposed to an atomic hydrogen beam. A current of 1 nA cm−2 was observed during a hydrogen exposure with a flux of 0.1 ML s−1. The transient is related to the reaction kinetics and allows us to identify the elementary reaction steps causing the electronic excitations which are monitored by the observed current. Using Pt as top electrode material a markedly different transient is observed. Applying a bias voltage to the sensor allows spectroscopy of the electronic excitations. The experiments provide detailed insights into the non-adiabaticity of various reaction steps at a surface.