Electron-Mediated Phonon-Phonon Coupling Drives the Vibrational Relaxation of CO on Cu(100)

Abstract

We bring forth a consistent theory for the electron-mediated vibrational intermode coupling that clarifies the microscopic mechanism behind the vibrational relaxation of adsorbates on metal surfaces. Our analysis points out the inability of state-of-the-art nonadiabatic theories to quantitatively reproduce the experimental linewidth of the CO internal stretch mode on Cu(100) and it emphasizes the crucial role of the electron-mediated phonon-phonon coupling in this regard. The results demonstrate a strong electron-mediated coupling between the internal stretch and low-energy CO modes, but also a significant role of surface motion. Our nonadiabatic theory is also able to explain the temperature dependence of the internal stretch phonon linewidth, thus far considered a sign of the direct anharmonic coupling.

Local temperatures out of equilibrium

“…It has been found that two distinct processes give rise to these changes: electron-hole pair excitations result in the frequency shifts, while electron-mediated vibrational mode coupling gives rise to linewidth changes. Since the electron temperature affects the electron-hole pair excitations and the electron-mediated vibrational mode coupling depends on both electron and phonon temperatures [447], the electron and phonon temperatures play an important role in laser-induced ultrafast dynamics.…”

Adiabatic and nonadiabatic energy dissipation during scattering of vibrationally excited CO from Au(111)

“…This nonadiabatic energy dissipation mechanism has been characterized by various theoretical models, [8][9][10][11][12][13][14][15][16][17][18] cumulating with the latest first-principles calculations that quantitatively reproduced the observed lifetime. [19,20] It was thus a surprise when Shirhatti et al reported a long lifetime (~10 2 ps) for trapped CO(ν=1) in the scattering of vibrationally excited CO(ν=2) from Au (111).[21] It was postulated that physisorption might be involved, given the relatively low desorption temperature of CO from Au (111).[22] Indeed, a recent density functional theory (DFT) study by Lončarić et al did find such a physisorption well for CO on Au(111),[23] using the Bayesian Error Estimation Functional method with van der Waals corrections (BEEF-vdW).[24] The lifetime of physisorbed CO(ν=1) was calculated within first-principles many-body perturbation theory and found to be consistent with the experimental value.[21] The long vibrational lifetime was attributed to the weaker couplings with EHPs because of the large distance between the adsorbate and surface. The same argument has also been used to explain the vibrationally hot precursor CH4 on the Ir(111) surface.…”

Ultrafast Transient Dynamics of Adsorbates on Surfaces Deciphered: The Case of CO on Cu(100)

“…These two features are known to be instrumental for explaining the long wavelength part of the phonon spectrum in metallic bulk systems [21,23,25,26]. The processes behind π [1] ν depend on the electron distribution, while π [2] ν depends on both electron and phonon distributions [19]. In our theory, the first-and second-order contributions depend indirectly on time via the electron T e (t) and lattice T l (t) temperatures describing the excited electron and phonon distributions.…”

“…The former term accounts for electron-hole pair excitation processes [20] including nonthermal charge transfer to unoccupied states (e.g., antibonding CO states). The latter term is responsible for the EMPPC [19]. These two features are known to be instrumental for explaining the long wavelength part of the phonon spectrum in metallic bulk systems [21,23,25,26].…”

“…Our firstprinciples formalism accounts for electron-phonon coupling processes up to second order, including electronhole pair excitations (the first-order NC) as well as vibrational intermode coupling due to the indirect interaction with hot electrons. This so-called electron-mediated phonon-phonon coupling (EMPPC) allows the interaction of phonons with incommensurate frequencies, even in the absence of direct anharmonic coupling [19].Here we apply this theoretical framework to investigate the early stage dynamics of nonthermal CO adsorbates (i.e., a molecular overlayer) on Cu(100). This system, which is considered as the benchmark system for electroninduced dynamics at surfaces, has recently been monitored with time-resolved sum-frequency generation spectroscopy with unprecedented subpicosecond resolution [11].…”