Ultrafast surface dynamics probed with time resolved photoemission

摘要

Time resolved core level photoemission (trXPS) allows real-time atom-specific investigation of ultrafast surface dynamics. Core levels contain information on the chemical state and the structure of the surface as well as the local charge distribution around specific atoms. Monitoring their evolution after optically exciting the surface, can give valuable information on the electronic (few picosecond time scale) and lattice dynamics (several picosecond timescale). We have performed a trXPS experiment at the free-electron laser FLASH at DESY in Hamburg on a clean Ir(111) surface measuring the temporal evolution of the 4f core levels of Ir(111) after optically exciting the sample. The spectral changes due to X-ray and optical laser induced space charge effects which occur in trXPS experiments with high fluence pump and probe pulses have been fully characterized and controlled during the measurements. At early time scales after the optical excitation we observe time-dependent energy shifts and intensity changes which can be partially attributed to the formation of sidebands. Furthermore, we can clearly identify contributions which result from a change in the surface electron density which then relaxes on a time scale on the order of 2 ps. (C) 2015 Elsevier B.V. All rights reserved.