Probing the transient adsorbate excitation with sub-ps resolution x-ray absorption spectroscopy: Is non-thermalized electron-hole pairs responsible for the ultrafast initial excitation?

Understanding the details of adsorbate dynamics is essential for describing heterogeneous catalytic reactions. In particular, how energy is transferred between the bulk substrate and adsorbate vibrational modes can be critical to alter selectivity in catalysis. In this experiment, which was carried out at DIPROI beamline of FERMI, we performed experimental C K-edge XAS of CO/Ru(0001) after pulsed optical laser excitation and we focused on the ultrafast dynamics occurring within the first picosecond. Since CO chemisorbs on Ru(0001) in an upright geometry at the on-top site where the C atom coordinated toward the Ru atom, the strongest moment for the C K-edge 1s → 2p* transition can be observed when the electric field vector (E-vector) is parallel to the surface. Therefore, using linearly polarized X-rays with the E-vector oriented either in-plane (lin H) or out-of-plane (lin V) we can monitor how the electronic structure and molecular orientation temporally changes.
Fig. 1a shows the spectral changes in lin H XAS spectra. Previous theoretical studies suggested that the internal C-O stretch (IS) and the frustrated CO rotation (FR) modes are the modes most likely to be significantly excited directly by thermal metal electron-hole pair excitations. Our DFT theoretical studies of the first order electron-phonon coupling for CO/Ru confirm this pattern, and simulated XAS let us identify the IS excitation as inducing the initial decrease of the peak intensity and the FR excitation as causing the redshift of the peak after 100 fs. To study the time evolution in detail, the intensities are integrated over two selected energy regions as function of delays (shown in Fig. 1b). For comparison, the two-temperature model is also shown and immediately indicates that the initial response in the lin H signal (blue dotted line) at low energy side region is faster than the rise of the phonon temperature (black line), and thus must be electron-driven, while the lin V signal (green dots) increase is due to more complicated dynamics involving both electrons and phonons.
The mechanism behind the very fast initial IS and FR excitation is still unclear. We suggest that the most likely reason for the rapid initial excitation is due to the highly excited non-thermal e-h pair distribution, initially created in the substrate by laser absorption, giving a stronger coupling to adsorbate motion. Since the LUMO 2p* resonance for CO/Ru is found around 5 eV above the Fermi level while the optical laser energy is only 3.1 eV (i.e., 400 nm), significant direct population of this resonance is unlikely to give efficient energy transfer. However, the non-thermal e-h pair distribution could still excite adsorbate modes more efficiently than thermal e-h pairs, since e-h pair states far from the Fermi level may couple more strongly than close to it. Extending the electron-phonon coupling to second order opens up an entirely new important class of energy transfer processes, so-called electron-mediated phonon-phonon couplings (EMPPC). However, it is unlikely that their addition can account for the ultrafast initial mode excitations responsible for the changes in the XAS, especially since XAS is a local probe at the C, so the spectrum is insensitive to the relative phase of vibrations unlike optical probes. 
We summarize our main observations in Fig. 1c. We can tentatively identify an initial excitation of the IS within 100 fs of the laser pulse, followed by excitation of the FR mode during the subsequent 100 fs. Thermalization of the substrate phonons, and heating of the adsorbate translational modes, proceeds over the first ps, consistent with the commonly used two-temperature model. The initial excitations are significantly faster than what can be accounted for in non-adiabatic friction models, pointing to a lack of current theoretical descriptions of the coupling between highly excited electron distributions and adsorbate dynamics. We instead suggest that non-thermalized electron-hole pairs within the first ~100 fs have a stronger coupling to the adsorbate and preferentially excite the IS and FR modes within this time scale. A detailed explanation poses a challenge for future theoretical and experimental work.
 

Figure 1.  (a) Measured lin H spectra, averaged over the indicated pump-probe delay ranges, together with calculated spectra with excited internal stretch (IS) and frustrated rotation (FR) modes. (b) Time evolution of the integrated spectral intensity, minus the unpumped intensity, in the main peak region (red) and low energy region (green), respectively. The electron (T_e) and phonon (T_ph) temperatures from the two-temperature model are shown in the down panel. (c) Illustration of the system evolution during the first picosecond. Atom colors: O red, C grey, Ru green. 

This research was conducted by the following research team:

Elias Diesen,¹Hsin-Yi Wang,²Simon Schreck,²Matthew Weston,²Hirohito Ogasawara,³Jerry LaRue,⁴Fivos Perakis,²Martina Dell’Angela,⁵Flavio Capotondi,⁶Luca Giannessi,⁶Emanuele Pedersoli,⁶Denys Naumenko,⁶Ivaylo Nikolov,⁶Lorenzo Raimondi,⁶Carlo Spezzani,⁶Martin Beye,⁷Filippo Cavalca,²Boyang Liu,²Jörgen Gladh,^2,3Sergey Koroidov,²Piter S. Miedema,⁷Roberto Costantini,^5,8Tony F. Heinz,^3,9Frank Abild-Pedersen,¹Johannes Voss,¹Alan C. Luntz,¹Anders Nilsson²

¹ SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, California 94025
² Department of Physics, AlbaNova University Center, Stockholm University, Stockholm, Sweden
³ SLAC National Accelerator Laboratory, Menlo Park, California 94025
⁴ Schmid College of Science and Technology, Chapman University, Orange, California 
⁵ CNR-IOM, Trieste, Italy
⁶ FERMI, Elettra-Sincrotrone Trieste SCpA, Trieste, Italy
⁷ DESY Photon Science, Hamburg, Germany
⁸ Physics Department, University of Trieste, Trieste, Italy
⁹ Department of Applied Physics, Stanford University, Stanford, California 

Contact persons:

Elias Diesen, email: ediesen@stanford.edu