Asymmetry and coverage dependence in two-pulse correlation measurements of CO photodesorption from Pd(111): Insights from theory

Abstract

Two-pulse correlation experiments performed using pulses of different intensities on Pd(111) with different CO coverages showed that the CO photodesorption probability depends on whether the strong or the weak pulse arrives first on the surface, with this difference being particularly large for the low-covered surface. Motivated by these experiments, we perform molecular dynamics simulations using a multicoverage potential energy surface that was previously constructed with the embedded atom neural network method. The process is modeled by combining the two-temperature model (2TM)—to describe the laser-excited electrons and phonons—and Langevin dynamics with electronic time-dependent temperature Te—to model the coupling of the nuclear degrees of freedom with the laser-excited electrons. We show that improving the energy balance description in 2TM—by including ab initioTe-dependent electronic heat capacity and electron–phonon coupling constant—is key to reproduce the asymmetry of the photodesorption probability Pdes between positive and negative time delays. Furthermore, we also explore possible reasons for the usual underestimation of Pdes at zero delay given by state-of-the-art calculations. In particular, we improve the description of the energy exchange between CO and the metal surface at high Te by including in the simulations a Te-dependent friction coefficient. The prediction for Pdes at zero delay in this case increases by an order of magnitude, reducing its discrepancy with the experimental value. Altogether, our results hint at the importance of accounting for the temperature dependence of the electronic structure and properties in describing the extreme conditions generated in 2PC experiments.