Sensitive Linear Surface Optics with Metal–Insulator–Metal Tunnel Contacts

Aluminium–aluminiumoxide–silver tunnel junctions allow to measure light emission by hot electrons and internal photoemission. Adsorption of potassium or oxygen on the silver electrode increases or quenches the light emission considerably. The light intensity increases at negative surface charge on the silver electrode and is suppressed at positive surface charge. Surprisingly, the light emission by hot electrons within the optical skin depth is negligible. The observed intensity is assigned to electron–photon coupling at the surface, especially to inverse photoemission by hot electrons via the time inverted threshold effect postulated by A. Liebsch. This is not in contradiction to time resolved two photon photoemission experiments. Internal photoemission experiments corroborate this interpretation.