Anomalous transient blueshift in the internal stretch mode of CO/Pd(111)

In time-resolved pump-probe vibrational spectroscopy the internal stretch mode of polar molecules is utilized as a key observable to characterize the ultrafast dynamics of adsorbates on surfaces. The adsorbate nonadiabatic intermode couplings are the commonly accepted mechanisms behind the observed transient frequency shifts. Here, we study the CO/Pd(111) system with a robust theoretical framework that includes electron-hole pair excitations and electron-mediated coupling between the vibrational modes. A mechanism is revealed that screens the electron-phonon interaction and originates a blueshift under ultrafast nonequilibrium conditions. The results are explained in terms of the abrupt change in the density of states around the Fermi level, and are instrumental for understanding the dynamics at multicomponent surfaces involving localized and standard s or p states.The authors acknowledge financial support by the Gobierno Vasco-UPV/EHU Project No. IT1569-22 and the Spanish Ministerio de Ciencia e Innovación (Grant No. PID2019-107396GB-I00/AEI/10.13039/501100011033). R.B. also acknowledges European Union-NextGenerationEU, Ministry of Universities and Recovery, Transformation and Resilience Plan, through a call from Polytechnic University of Catalonia. D.N. additionally acknowledges financial support from the Croatian Science Foundation (Grant No. UIP-2019-04-6869). This research was conducted in the scope of the Transnational Common Laboratory (LTC) QuantumChemPhys - Theoretical Chemistry and Physics at the Quantum Scale. Computational resources were provided by the DIPC computing center.Peer ReviewedPostprint (author's final draft