Photoinduced dynamics of CO on Ru(0001): Understanding experiments by simulations with all degrees of freedom.

Real-time pump-probe experiments are powerful tools for monitoring chemical reactions but often need parallel theoretical modeling to disentangle different contributions. Monitoring x-ray spectra of photoinduced dynamics of CO on Ru(0001) provided a strong indication for a transient "precursor state" of unidentified nature to various subsequent outcomes. So far, the precise nature of the postulated precursor has also remained elusive in state-of-the-art ab initio molecular dynamics models, including single-moving CO molecules. In the present work, we have constructed a density functional theory-based machine learning interatomic potential energy surface that is valid for all ionic degrees of freedom of the system, comprising many molecules at various coverages and moving surface atoms. Our Langevin dynamics with electronic friction based on the new potential energy surface identified the precursor state as dynamically trapped molecules around 6 Å from the surface that arise from adsorbate-adsorbate interactions. We have compared our results to experimental observations and calculated the dependence of reaction probabilities on pump laser fluence and initial surface coverage.