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

TL;DR

This study investigates an unexpected transient blueshift in the vibrational mode of CO on Pd(111) surfaces, revealing a novel electron-mediated screening mechanism under ultrafast conditions.

Contribution

It introduces a theoretical framework that accounts for electron-hole pair excitations and uncovers a new screening mechanism causing blueshift during ultrafast surface dynamics.

Findings

Identification of a screening mechanism causing blueshift

Explanation based on changes in the density of states near the Fermi level

Enhanced understanding of vibrational dynamics on multi-component surfaces

Abstract

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 adsorbates non-adiabatic 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 non-equilibrium 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 dynamics at multi-component surfaces involving localized and standard $s$ or $p$ states.