Theory of heat transfer between adsorbate vibrational degrees of freedom and ultrafast laser heated hot electrons including vibrational intermode coupling is applied to calculate two-pulse correlation, laser fluence dependence and time dependence of lateral hopping of CO molecules from a step to terrace site on a stepped Pt ͑111͒ surface. The intermode coupling is a key ingredient to describe vibrational heating of the frustrated translation mode responsible for the CO hopping. The calculated results are in good agreement with the experimental results, especially if we scale down the experimentally determined absorbed fluence. It is found that CO hopping is induced by indirect heating of the FT mode by the FR mode with a strong frictional coupling to hot electrons.

Recent low-temperature scanning-tunneling microscopy experiments [T. Kumagai et al., Phys. Rev. B 79, 035423 (2009)] observed the vibrationally induced flip motion of a hydroxyl dimer (OD)2 on Cu(110). We propose a model to describe two-level fluctuations and current-voltage characteristics of nanoscale systems which undergo vibrationally induced switching. The parameters of the model are based on comprehensive density-functional calculations of the system's vibrational properties. For the dimer (OD)2 the calculated population of the high and low conductance states, the I − V , dI/dV , and d 2 I/dV 2 curves are in good agreement with the experimental results and underlines the different roles played by the free and shared OD stretch modes of the dimer.Electron transport through single-molecule junctions has been receiving enthusiastic interest for a development of novel molecular devices. Nonlinear I − V characteristics associated with the vibrationally mediated configurational change with different conductances have been observed in a series of systems such as pyrrolidine on a Cu (001) 1 , H 2 on Cu 2 , CO bridging a Pt contact 3 , and H 2 in Au contacts 4 . In these systems dI/dV spectra show anomalous spikes-in contrast to steps usually observed in inelastic electron tunneling spectroscopy 5 -at the bias voltage related to the vibrational mode energies.Recently Kumagai et al. 6 studied the dynamics of a single hydroxyl (OH, OD) molecule and the dimer (OD) 2 on Cu(110) using a scanning tunneling microscope (STM). The STM images observed for the monomer suggested the possible quantum tunneling of a hydroxyl between two equivalent adsorption configurations on Cu(110), as supported by the density functional theory (DFT) calculations of the transition path and rate of the flipping of OH on Cu (110) 7 . This spontaneous flip motion of hydrogen atoms in the monomer is quenched for the dimer at low temperatures, but can be induced by excitation of the OH/OD stretch mode by tunneling electrons. Time-averaged measurements of the current show a non-linear current (I) increase at the bias voltage (V ) inducing the transition from the low and high conductance states. The appearance of the peak in dI/dV and the peak and dip in d 2 I/dV 2 from transitions between states with distinct conductances have also been reported previously 2-4 .In this paper a combined use of the DFT-based SIESTA 8 , TranSIESTA 9 and Inelastica 10,11 packages permits us to gain insight into the elementary processes that induce the flip motion of the asymmetric dimer. The extensive DFT calculations provide the ground state geometry, vibrational modes, electron-vibration couplings, emission rate of vibrations from tunneling electrons, vibrational damping due to electron-hole pair excitation, and the high and low conductance. These calculated properties allow us to model the population of the high and low conductance states as a function of the bias voltage and the nonlinear I − V characteristics for (OD) 2 on a Cu(110) surface. The experimental resul...

We present a theoretical study of the lateral hopping of a single CO molecule on Cu͑111͒ induced by femtosecond laser pulses by Mehlhorn et al. ͓Phys. Rev. Lett. 104, 076101 ͑2010͔͒. Our model assumes an intermode coupling between the CO frustrated translation ͑FT͒ and frustrated rotation ͑FR͒ modes with a weak and strong electronic friction coupling to hot electrons, respectively, and heat transfer between the FT mode and the substrate phonons. In this model the effective electronic friction coupling of the FT mode depends on the absorbed laser fluence F through the temperature of the FR mode. The calculated hopping yield as a function of F nicely reproduces the nonlinear increase observed above F = 4.0 J / m 2 . It is found that the electronic heating via friction coupling nor the phonon coupling alone cannot explain the experimental result. Both heatings are cooperatively responsible for CO hopping on Cu͑111͒. The electronic heat transfer dominates over the phononic one at high F, where the effective electronic friction coupling becomes larger than the phononic coupling. Real-space and/or real-time monitoring of adsorbate motions and chemical reactions on surfaces are the ultimate techniques to study adsorbate reaction dynamics. The first real-space observation of molecular motion induced by femtosecond laser pulses has been made in combination with a scanning tunneling microscope ͑STM͒ by Bartels et al. 1 for CO on Cu͑110͒. They combined direct imaging of a single CO molecule by STM with femtosecond laser excitation. They found that electronic excitation of the substrate induced by absorption of short laser pulses gives rise to hopping of CO parallel and perpendicular to the close-packed rows, in addition to desorption. STM, which permits a direct imaging of a single molecule before and after laser irradiation, cannot be used to monitor laser-induced adsorbate motions on ultrafast time scale while nonlinear time-resolved optical spectroscopy with unique high surface sensitivity enables adsorbate motions to be monitored on the time scale typically involved in adsorbate dynamics. 2 Recently Mehlhorn et al. 3 have reported on femtosecondlaser-induced hopping of a single CO molecule on Cu͑111͒ using a scanning tunneling microscope. As a function of the absorbed laser fluence F, they observed that the hopping yield Y͑F͒ exhibits a linear increase at low F, followed by a strongly nonlinear increase at high F. They proposed that the linear increase arises from single electronic transitions dynamics induced by electronic transition ͑DIET͒, 4 while the strong increase can be described using a friction model, where hot electrons transfer energy to the frustrated translation ͑FT͒ mode. They assumed the electronic friction el to depend on the electron temperature T el ͑t͒, in accordance with earlier suggestions. 5 However, it was proved that frictional coupling is temperature independent if it originates from electron-hole pair excitation. 6,7 The electronic friction is defined as el = w 1→0 − w 0→1 , where the decay ra...

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Lateral hopping and desorption of a single CO molecule on a Cu(110) surface [Bartels et al., Science 305, 648 (2004)] induced by femtosecond laser pulses are studied using an indirect heat-transfer model. In addition to a direct heating of the reaction coordinate (RC) mode [frustrated translation (FT) mode for hopping and center-of-mass (CM) mode for desorption] by laser-generated hot electrons in the substrate, we consider an indirect heating of the RC mode through intermode coupling between the frustrated rotation (FR) mode and the RC mode. We calculate the transient behavior of the effective temperature of the FT and the CM modes, and of the normalized reaction yield. The experimental result of a ratio of the hopping yield along and across a row on a Cu(110) surface is nicely calculated. Although no information is available for the attempt frequency in a form of the Arrhenius equation for thermally activated reactions, it is predicted under which condition the desorption rate becomes in the same order of magnitude as the hopping rate, although the barrier height for desorption is much higher than for hopping. The present analysis highlights the role of excitation of the FR mode in reactions of a CO molecule as has been confirmed in the real-time observation [Backus et al., Science 310, 1790].