Ultrafast adsorbate excitation probed with sub-ps resolution XAS

TL;DR

This study employs ultrafast X-ray absorption spectroscopy with sub-picosecond resolution to observe rapid vibrational dynamics of CO molecules on Ru(0001) surfaces after laser excitation, revealing new insights into adsorbate excitation mechanisms.

Contribution

It demonstrates the capability to resolve ultrafast adsorbate dynamics and suggests a novel coupling mechanism involving non-thermalized electron-hole pairs.

Findings

High excitation of vibrational modes within 200 fs

Thermalization occurs within a few picoseconds

Ultrafast initial excitation likely involves electron-hole pairs

Abstract

We use a pump-probe scheme to measure the time evolution of the C K-edge X-ray absorption spectrum (XAS) from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Due to the short duration of the X-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first ps after the pump can be resolved with unprecedented time resolution. By comparing with theoretical (DFT) spectrum calculations we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the ps regime. The ~100 fs initial excitation of these CO vibrational modes is not readily 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 adsorbate resonances. We suggest that coupling of the adsorbate to non-thermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.