| AVS 60th International Symposium and Exhibition | |
| Surface Science | Thursday Sessions |
| Session SS2-ThA |
| Session: | Surface Dynamics and Non-adiabatic Processes |
| Presenter: | J.L. LaRue, SLAC National Accelerator Lab |
| Authors: | J.L. LaRue, SLAC National Accelerator Lab M. Dell’Angela, Univ. of Hamburg, Germany M. Beye, HZB für Mat. und Energie/Elektronenspeicherring BESSY II, Germany R. Coffee, SLAC National Accelerator Lab A. Föhlisch, HZB für Mat. und Energie/Elektronenspeicherring BESSY II, Germany J. Gladh, Stockholm Univ., Sweden T. Katayama, SLAC National Accelerator Lab S. Kaya, SLAC National Accelerator Lab J.A. Sellberg, SLAC National Accelerator Lab A. Møgelhøj, Technical Univ. of Denmark D. Nordlund, SLAC National Accelerator Lab J.K. Nørskov, SLAC National Accelerator Lab H. Öberg, Stockholm Univ., Sweden H. Ogasawara, SLAC National Accelerator Laboratory H. Öström, Stockholm Univ., Sweden L.G.M. Pettersson, Stockholm Univ., Sweden W.F. Schlotter, SLAC National Accelerator Lab. W. Wurth, Univ. of Hamburg, Germany M. Wolf, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany A. Nilsson, SLAC National Accelerator Lab. |
| Correspondent: | Click to Email |
The femtosecond dynamics of CO adsorbed on a Ru(0001) surface after excitation by an optical laser have been studied using the Linac Coherent Light Source (LCLS) free electron x-ray laser. Changes in the occupied and unoccupied electronic structure were monitored using ultrafast soft x-ray absorption and emission spectroscopy. We found that the CO molecules, upon excitation by the optical laser, are excited to a weakly adsorbed precursor state occurring on a time scale of >2 ps, and at longer timescales, desorbed from this precursor state. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process. Within the first couple picoseconds, the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, each of these states can be selectively probed and the optical laser fluence dependent transient populations can be determined. At high optical laser fluences, a large fraction of the molecules (30%) were trapped in the transient precursor state that precedes desorption. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate. About half the molecules in the precursor state desorbed from the surface while the other half return to the chemisorbed state. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells - chemisorbed and precursor state separated by an entropy barrier - explain the anomalously high prefactors often observed in desorption of molecules from metals.