Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 22, Issue 5, Pages 2677-2684Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c9cp03677f
Keywords
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Funding
- Knut and Alice Wallenberg Foundation [KAW 2014.0370]
- Swedish Research Council
- U.S. Department of Energy, Office of Science, Office of Basic Energy Science [DE-AC02-76SF00515]
- Helmholtz association [VH-NG-1005]
- SIR grant SUNDYN of the Italian MIUR [RBSI14G7TL, CUP B82I15000910001]
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The transient dynamics of carbon monoxide (CO) molecules on a Ru(0001) surface following femtosecond optical laser pump excitation has been studied by monitoring changes in the unoccupied electronic structure using an ultrafast X-ray free-electron laser (FEL) probe. The particular symmetry of perpendicularly chemisorbed CO on the surface is exploited to investigate how the molecular orientation changes with time by varying the polarization of the FEL pulses. The time evolution of spectral features corresponding to the desorption precursor state was well distinguished due to the narrow line-width of the C K-edge in the X-ray absorption (XA) spectrum, illustrating that CO molecules in the precursor state rotated freely and resided on the surface for several picoseconds. Most of the CO molecules trapped in the precursor state ultimately cooled back down to the chemisorbed state, while we estimate that similar to 14.5 +/- 4.9% of the molecules in the precursor state desorbed into the gas phase. It was also observed that chemisorbed CO molecules diffused over the metal surface from on-top sites toward highly coordinated sites. In addition, a new vibrationally hot precursor state was identified in the polarization-dependent XA spectra.
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