Simulations of the femtosecond laser-induced desorption of CO from Cu(100) at 0.5 ML coverage

Understanding the Photoinduced Desorption and Oxidation of CO on Ru(0001) Using a Neural Network Potential Energy Surface

The study of ultrafast photoinduced dynamics of adsorbates on metal surfaces requires thorough investigation of laser-excited electrons and, in many cases, the highly excited surface lattice. While ab initio molecular dynamics with electronic friction and thermostats (Te, Tl)-AIMDEF addresses such complex modeling, it imposes severe computational costs, hindering quantitative comparison with experimental desorption probabilities. In order to bypass this limitation, we utilize the embedded atom neural network method to construct a potential energy surface (PES) for the coadsorption of CO and O on Ru(0001). Our results demonstrate that this PES not only reproduces the short-time ab initio dynamics but is also able to yield statistically significant data for long lasting trajectories that correlate well with experimental findings. Furthermore, the analysis of the laser-induced dynamics reveals the existence of a dynamic trapping state that acts as a precursor for CO desorption, and it is not observed under thermal conditions. Altogether, our results validate the underlying theoretical framework, providing robust support for the description of not only the photoinduced desorption but also the oxidation of CO in terms of nonequilibrated but thermal hot electrons and phonons.

Multicoverage Study of Femtosecond Laser-Induced Desorption of CO from Pd(111)

We study the strong coverage dependence of the femtosecond laser-induced desorption of CO from Pd(111) using molecular dynamics simulations that consistently include the effect of the laser-induced hot electrons on both the adsorbates and surface atoms. Adiabatic forces are obtained from a multicoverage neural network potential energy surface that we construct using data from density functional theory calculations for 0.33 and 0.75 monolayer (ML). Our molecular dynamics simulations performed for these two trained coverages and an additional intermediate coverage of 0.60 ML reproduce well the peculiarities of the experimental findings. The performed simulations also permit us to disentangle the relative role played by the excited electrons and phonons on the desorption process and discover interesting properties of the reaction dynamics as the relevance that the precursor physisorption well acquires during the dynamics as coverage increases.

Disentangling the role of electrons and phonons in the photoinduced CO desorption and CO oxidation on (O,CO)-Ru(0001)

The role played by electronic and phononic excitations in the femtosecond laser induced desorption and oxidation of CO coadsorbed with O on Ru(0001) is investigated using ab initio molecular dynamics with electronic friction. To this aim, simulations that account for both kind of excitations and that only consider electronic excitations are performed. Results for three different surface coverages are obtained. We unequivocally demonstrate that CO desorption is governed by phononic excitations. In the case of oxidation the low statistics does not allow to give a categorical answer. However, the analysis of the adsorbates kinetic energy gain and displacements strongly suggest that phononic excitations and surface distortion also play an important role in the oxidation process.

Why Ultrafast Photoinduced CO Desorption Dominates over Oxidation on Ru(0001)

CO oxidation on Ru(0001) is a long-standing example of a reaction that, being thermally forbidden in ultrahigh vacuum, can be activated by femtosecond laser pulses. In spite of its relevance, the precise dynamics of the photoinduced oxidation process as well as the reasons behind the dominant role of the competing CO photodesorption remain unclear. Here we use ab initio molecular dynamics with electronic friction that account for the highly excited and nonequilibrated system created by the laser to investigate both reactions. Our simulations successfully reproduce the main experimental findings: the existence of photoinduced oxidation and desorption, the large desorption to oxidation branching ratio, and the changes in the O K-edge X-ray absorption spectra attributed to the initial stage of the oxidation process. Now, we are able to monitor in detail the ultrafast CO desorption and CO oxidation occurring in the highly excited system and to disentangle what causes the unexpected inertness to the otherwise energetically favored oxidation.

On-Surface Chemistry Induced by Long-Lived Excitons: (NO)2 Dissociation on C60

Solid-state excitonic excitations play an increasingly important role in optoelectronic and light harvesting processes due to their ubiquitous presence in dipolar two-dimensional materials. Here we show that long-lived solid-state excitons induce chemical reactions in adsorbed molecules and thus convert light into chemical energy. For the model system (NO)₂ dimer adsorbed on ordered c(4×4) C₆₀ films, time-of-flight measurements following UV laser excitation reveal a slow and a fast dissociative desorption channel, which are assigned to intersystem crossing and internal conversion, respectively, by time-dependent density functional theory calculations.

Electrons and Phonons Cooperate in the Laser-Induced Desorption of CO from Pd(111)

Femtosecond laser induced desorption of CO from a CO-covered Pd(111) surface is investigated with ab initio molecular dynamics with electronic friction that incorporates effects due to the excited electronic and phononic systems, as well as out-of-phase coadsorbate interactions. Our simulations show evidence of an important electron-phonon synergy in promoting CO desorption that has largely been neglected in other similar systems. At the saturated coverage of 0.75 ML, effects due to CO-CO interadsorbate energy exchange are also important. Our dynamics simulations, in concert with site-specific desorption energy calculations, allow us to understand the large coverage dependence of the desorption yields observed in experiments.

Adlayer structure dependent ultrafast desorption dynamics in carbon monoxide adsorbed on Pd (111)

We report our ultrafast photoinduced desorption investigation of the coverage dependence of substrate-adsorbate energy transfer in carbon monoxide adlayers on the (111) surface of palladium. As the CO coverage is increased, the adsorption site population shifts from all threefold hollows (up to 0.33 ML), to bridge and near bridge (>0.5 to 0.6 ML) and finally to mixed threefold hollow plus top site (at saturation at 0.75 ML). We show that between 0.24 and 0.75 ML this progression of binding site motifs is accompanied by two remarkable features in the ultrafast photoinduced desorption of the adsorbates: (i) the desorption probability increases roughly two orders magnitude, and (ii) the adsorbate-substrate energy transfer rate observed in two-pulse correlation experiments varies nonmonotonically, having a minimum at intermediate coverages. Simulations using a phenomenological model to describe the adsorbate-substrate energy transfer in terms of frictional coupling indicate that these features are consistent with an adsorption-site dependent electron-mediated energy coupling strength, ηel, that decreases with binding site in the order: three-fold hollow > bridge and near bridge > top site. This weakening of ηel largely counterbalances the decrease in the desorption activation energy that accompanies this progression of adsorption site motifs, moderating what would otherwise be a rise of several orders of magnitude in the desorption probability. Within this framework, the observed energy transfer rate enhancement at saturation coverage is due to interadsorbate energy transfer from the copopulation of molecules bound in three-fold hollows to their top-site neighbors.

Disentangling Multidimensional Nonequilibrium Dynamics of Adsorbates: CO Desorption from Cu(100)

Hot carriers at metal surfaces can drive nonthermal reactions of adsorbates. Characterizing nonequilibrium statistics among various degrees of freedom in an ultrafast time scale is crucial to understand and develop hot carrier-driven chemistry. Here we demonstrate multidimensional vibrational dynamics of carbon monoxide (CO) on Cu(100) along hot-carrier induced desorption studied by using time-resolved vibrational sum-frequency generation with phase-sensitive detection. Instantaneous frequency and amplitude of the CO internal stretching mode are tracked with a subpicosecond time resolution that is shorter than the vibrational dephasing time. These experimental results in combination with numerical analysis based on Langevin simulations enable us to extract nonequilibrium distributions of external vibrational modes of desorbing molecules. Superstatistical distributions are generated with mode-dependent frictional couplings in a few hundred femtoseconds after hot-electron excitation, and energy flow from hot electrons and intermode anharmonic coupling play crucial roles in the subsequent evolution of the non-Boltzman distributions.

Two-dimensional infrared surface spectroscopy for CO on Cu(100): Detection of intermolecular coupling of adsorbates

Surface-specific infrared signals obtained by subjecting the system to two infrared laser pulses are calculated for an admixture of CO and isotopic CO on Cu(100) by using molecular dynamics simulation based on a stability matrix formalism. The two-dimensional profiles of the signals in the frequency domain show both diagonal and cross peaks. The former peaks mainly arise from the overtones of the CO and isotopic CO, while the latter represent the couplings between those. As temperature is increased, the phases of cross peaks in a second-order infrared response function change significantly, while those of diagonal peaks are unchanged. The authors show that the phase shifts are originated from the potential anharmonicities due to the electronic interaction between adsorbates. Using a model with two dipole moments, they find that the frustrated rotational mode activated with temperature has effects on the anharmonicity. These results indicate that two-dimensional infrared surface spectroscopy reveals the anharmonic couplings between adsorbates and surface atoms or between adsorbates which cannot be observed in first-order spectroscopy.

Real-Space Observation of Molecular Motion Induced by Femtosecond Laser Pulses

Femtosecond laser irradiation is used to excite adsorbed CO molecules on a Cu110 surface; the ensuing motion of individual molecules across the surface is characterized on a site-to-site basis by in situ scanning tunneling microscopy. Adsorbate motion both along and perpendicular to the rows of the Cu110 surface occurs readily, in marked contrast to the behavior seen for equilibrium diffusion processes. The experimental findings for the probability and direction of the molecular motion can be understood as a manifestation of strong coupling between the adsorbates' lateral degrees of freedom and the substrate electronic excitation produced by the femtosecond laser radiation.

Ultrafast laser excitation of CO/Pt(111) probed by sum frequency generation: coverage dependent desorption efficiency

CO photodesorption from Pt(111) induced by femtosecond laser pulses is probed by IR+visible sum frequency generation (SFG). Steady state analysis of SFG spectra at varying CO pressure and laser fluence allows one to measure a approximately 5 orders of magnitude decrease of the photodesorption rate constant when CO coverage decreases from 0.37 to 0.07 monolayer. We ascribe this effect in the framework of the Menzel-Gomer-Redhead mechanism to electron delocalization in the CO layer. The lifetime of electronic excitation decreases when coverage decreases.

Chemical dynamics at metal surfaces

Theoretical aspects of dynamical processes at metal surfaces are reviewed. Experimental challenges to theory are presented and progress toward meeting these challenges is appraised. Topics include adsorbate vibrational energy flow, inelastic molecule-surface scattering, adsorption, transient mobility, dissociation, desorption, photochemistry, and electron-induced chemistry at metal surfaces. Experimental examples cited illustrate the richness of dynamical phenomena to be understood and the necessity of developing multidimensional, beyond Born-Oppenheimer, formulations of adsorbate dynamics. Classical mechanical and quantum mechanical treatments of dynamics are contrasted. The importance of including phonon and electron-hole pair dissipation in theories of adsorbate dynamics is emphasized, and strategies for doing this in classical and quantum treatments are presented.