Ultrafast Adsorbate Excitation Probed with Subpicosecond-Resolution X-Ray Absorption Spectroscopy

Elias Diesen, Hsin-Yi Wang, Simon Schreck, Matthew Weston, Hirohito Ogasawara, Jerry LaRue, Fivos Perakis, Martina Dell’Angela, Flavio Capotondi, Luca Giannessi, Emanuele Pedersoli, Denys Naumenko, Ivaylo Nikolov, Lorenzo Raimondi, Carlo Spezzani, Martin Beye, Filippo Cavalca, Boyang Liu, Jörgen Gladh, Sergey Koroidov, Piter S. Miedema, Roberto Costantini, Tony F. Heinz, Frank Abild-Pedersen, Johannes Voss, Alan C. Luntz, Anders Nilsson
Year of publication: 
Physical Review Letters

We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrumfrom CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the shortduration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamicsduring the first picosecond after the pump can be resolved with unprecedented time resolution. Bycomparing with density functional theory spectrum calculations, we find high excitation of the internalstretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization ofthe system in the picosecond regime. The∼100fs initial excitation of these CO vibrational modes is notreadily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g.,electronic frictions based on first order electron-phonon coupling or transient population of adsorbateresonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsiblefor the ultrafast initial excitation of the modes.

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