Adsorbate Motion Research Articles

Surface Dynamics Studied by Time-Resolved Nonlinear Spectroscopy

Advances in ultrafast laser technologies encourage surface scientists to explore electron and nuclear dynamics at surfaces. Major spectroscopic means are time-resolved nonlinear laser spectroscopy including second harmonic generation (SHG), sum frequency generation (SFG), and two-photon photoemission (2PPE). After introducing fundamental aspects of the nonlinear spectroscopy briefly, this paper describes some recent applications of the nonlinear spectroscopy to ultrafast dynamics at surfaces. Examples include: (1) SFG with tunable infrared pulsed lasers is used for exploring how the vibrational modes of adsorbate couple to bulk phonons when a metal surface is rapidly heated by an ultrafast laser pulse. (2) A vibrational motion of adsorbate on a metal surface is coherently excited by an ultrafast laser pulse and the dephasing of the vibrational mode is probed by time-resolved SHG. (3) Ultrafast electron decay and electron hydration dynamics are explored by 2PPE.

Theory of inelastic tunneling induced motion of adsorbates on metal surfaces

We describe a new way of how inelastic tunneling can induce motions of adsorbates on metal surfaces. The theory shows how the excitation of high frequency internal vibrational modes can induce rotational or translational motion of adsorbates. The probability for adsorbate rearrangements following the vibrational excitation of the high-frequency mode is calculated using a model which includes vibrational relaxation via electron–hole pair excitation, and the anharmonic coupling to the relevant reaction coordinate. The adsorbate rearrangement probability depends on the anharmonic coupling strength, the height of the potential barrier along the reaction coordinate, and the energy of the low-frequency mode. The theory explains why the excitation of the C–O stretch vibration can induce hopping of CO on Pd(1 1 0), but not on Cu(1 0 0).

Theory of vibrational tunneling spectroscopy of adsorbates on metal surfaces

A theoretical model for studying elementary processes in vibrational scanning tunneling microscopy/spectroscopy is presented. General formulae for the elastic and inelastic tunneling currents are derived on the basis of the Anderson Hamiltonian coupled to a vibrational degree of freedom. Our treatment includes the vibrational damping due to electron–hole pair excitations. It is found that the non-equilibrium situation plays a crucial role for opening of the inelastic channel and excitations of adsorbate phonons. Elementary processes for both elastic and inelastic tunneling currents are clarified in a qualitative manner. The distribution function of adsorbate phonons is derived and may be of great importance for the understanding of the recently observed current-induced dynamical motions of adsorbates.

Lateral motion of adsorbate induced by vibrational mode excitation with inelastic tunneling electron

We show a lateral hopping of adsorbates of CO molecules on Pd(1 1 0) induced by the tunneling electron of scanning tunneling microscopy. When tunneling electron with bias voltage of a few tenth of volt is dosed into an isolated adsorbate whose lateral motion is originally frozen at liquid He temperature, hopping of adsorbates to an equivalent site is observed. This phenomenon shows a clear threshold in the bias voltage, and most interestingly the threshold voltage corresponds to the excitation energy of the high-lying vibration mode of the molecule (C–O stretching mode for CO case). We consider a model in which the low-frequency vibrational modes (T- and R-modes), directly related to the lateral hopping mechanism, are excited in accordance with the excitation of the high-lying mode through anharmonic coupling.

IR investigations of surfaces and adsorbates

Synchrotron infrared reflection–absorption measurements on single crystal metal surfaces with adsorbates have led to the determination of many key parameters related to the bonding vibrational modes and the dynamics of adsorbates. In particular, energy couplings between electrons and adsorbate motion have been shown to be a dominant mechanism on metal surfaces. Excellent agreement has been obtained with calculations for many of the observations, and the synergy between theory and experiment has led to a deeper understanding of the roles of electrons and phonons in determining the properties of interfaces and their roles in phenomena as diverse as friction, lubrication, catalysis and adhesion. Nonetheless, as the experiments are pushed harder, to describe such effects as co-adsorbed systems, disagreements continue to challenge the theory and our comprehension also is still evolving.

Photodissociation of methyl nitrite on Ag(111): Simulation

The photodissociation dynamics of methyl nitrite, CH3ONO, on Ag(111) have been simulated using a description that models 61 cis–methyl nitrite molecules adsorbed on a three-layer block of Ag(111). Based on classical intra- and intermolecular potentials and periodic boundary conditions, molecular dynamics (MD) simulation led to two domain structures at 100 K: those with CONO planes oriented nearly parallel and nearly perpendicular to the Ag(111) surface. To simulate photodissociation dynamics of NO, many NO trajectories were determined, each carried out as follows. At some instant of the MD simulation, a CH3ONO molecule was randomly selected from within the group of 61 and its internal CH3O–NO bond was stretched to a defined dissociation transition state. The nascent NO was given momentum along the direction of the bond broken and NO translational and internal energies were chosen to match those determined experimentally in collision-free gas phase photodissociation. The motion of the whole adsorbate–substrate system was then calculated while following the trajectory of NO. Analyzing the ensemble of NO trajectories, we conclude that, while the initial orientation of the dissociating CH3ONO influences the number of subsequent collisions, the exit direction, and the final translational and internal energy of NO, it does not fully account for the properties of ejected NO. Furthermore, for those molecules lying nearly parallel to the surface, a transition state prepared by simply stretching the O–N bond is often located away from the lowest potential energy exit path due to interactions with nearest neighbor species. As a result, coordinates, e.g., internal twisting, other than the internal CH3O–NO stretching mode are intimately involved in the dissociation channel.

Dynamical phenomena including many body effects at metal surfaces

In this contribution, we give a brief survey of some elementary many body processes observable at surfaces. We begin the survey by discussing how the electron ground state would behave and look like in real space at surfaces. Next, we discuss how these electrons behave when they are perturbed by external fields characterized by ultrashort time scales. We follow this with a discussion of how the dynamics of electrons would then affect the motion of adsorbates on surfaces. Finally, we cite some possible technological applications utilizing this knowledge. We also discuss possible trends or directions of scientific research in this century.

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A Busy March Meeting Is Brewing in Seattle

Five wave mixing: surface-specific transient grating spectroscopy as a probe of low frequency intermolecular adsorbate motion

Surface-specific five wave mixing spectroscopy is used to examine adsorbate dynamics at the fused silica/air interface. Signals whose temporal response is significantly broader than the instantaneous fourth-order electronic polarizability are attributed to low frequency, adsorbate nuclear motion. Based on the dependence of these dynamics on the moment of inertia and their comparison with dynamics observed in liquids, the five wave mixing temporal response has been assigned to adsorbate intermolecular librational motion. Demonstration of this new fourth-order technique allows one, in principle, to extend all third-order nonlinear bulk-phase spectroscopies to surfaces and interfaces with specificity.

Comparing the vibrational properties of low-energy modes of a molecular and an atomic adsorbate: CO and O on Pt(111)

In this work we present a situation where it has been possible to compare the vibrational properties of the low-energy modes of a molecular and an atomic adsorbate on a metal surface. We have, by infrared spectroscopy, studied well-ordered structures of carbon monoxide chemisorbed in two different sites and atomic oxygen in a third site, all three on the same surface, with the same relative coverage and studied under identical conditions. We are able to present a unique test of the antiabsorption mechanism and its applicability to transition metals like platinum. For atomic oxygen we observe a dip in the absorptance spectra associated with the frustrated translation, while in contrast no such dip was observed for the frustrated rotations of CO. This emphasizes that the magnitude of the broadband decrease in reflectivity and the character of the adsorbate motion is very important for the strength of the effect. We also present the first study on the temperature dependence of the antiabsorption, the results being in agreement with theory. For the metal–adsorbate stretch vibrational modes we make relative comparisons between the observed integrated absorptance for the different species. We find a good correspondence between the relative values of the oscillating charge and the binding energy by making simple estimations on the bond geometry and the screening by the metal electrons.

A transition-state theory approach to adsorbate dynamics at arbitrary loadings

There has been much recent interest in using transition-state theory (TST) to extend the time and length scales accessible to molecular-level simulations of adsorbate transport in microsporous materials. However, the vast majority of this work has been performed on systems at infinite dilution. The objective of this paper is to obtain fundamental rate constants for adsorbate motion at nonzero loadings using multidimensional TST. More specifically, we focus on systems where the adsorption of a molecule is not highly localized in a single site, but rather distributed throughout an uncorrugated cage. We develop a theory in which high-dimensional TST integrals are approximated using exact lower-dimensional information. The evaluation of the resulting integrals is performed with an importance sampling method involving the insertion of a single molecule, thus improving the statistical quality of the results. The theory is applied to the motion of methane and xenon in the zeolite ZK4, where hopping between α cages is the rate-limiting event. Our results show that hopping rates increase with loading in the cage, which is consistent with experimental trends in the diffusivity. Agreement between our theory and corresponding molecular dynamics simulations is excellent.

Unified model of diffractive and multiphonon He atom scattering from adsorbates: Holstein renormalization of the interactions and the complete Debye-Waller factor

We derive general expressions for the energy and lateral momentum resolved scattering spectrum describing collisions of beams of thermal energy He atoms with adsorbates on a flat substrate surface. Elastic and inelastic components of the projectile-adlayer interaction, of which the latter can be nonlinear in adsorbate displacements, are treated to all powers in the coupling constant g and with full account of the projectile recoil. The established formalism enables a combined treatment of elastic scattering (either diffuse or diffractive) and inelastic excitation of multiple phonons and overtones in the adlayer on an equivalent footing. For nonlinear vibrational coupling the distorted wave matrix elements of the interactions are Holstein-renormalized by zero point motion of adsorbates. In the case of scattering intensities calculated to lowest order in g this gives rise to a direct analog of the standard Glauber\char21{}van Hove Debye-Waller factor. The closed form solutions for the scattering spectra to all orders in g are characterized by a unified or complete Debye-Waller factor which embodies the effects discussed so far only separately in the literature: (i) attenuation of the scattered beam intensities due to zero point motion of adsorbates, and (ii) attenuation of the beam intensity in the elastic channel due to inelastic scattering from adlayer phonons and overtones. The complete Debye-Waller factor acquires a form of an exponentiated sum of Holstein-renormalized scattering intensities and acts to preserve the unitarity of the scattering spectrum in accord with the optical theorem. The developed model facilitates evaluation of the various approximate and limiting forms of the scattering spectra and the associated Debye-Waller factors characteristic of the different scattering regimes and different types of adlayer vibrational dynamics. Potential applications of the model are illustrated by estimating the effect of Holstein renormalization occurring in the unified Debye-Waller factor and scattering intensities for several prototype adlayer systems.

Adsorbate induced surface resistivity from IR-transmission spectroscopy: oxygen on Fe/MgO(001)

In situ IR-transmission spectroscopy from 1300 cm −1 to 5900 cm −1 was performed on flat iron ultrathin films on MgO(001) in UHV before and after exposure to oxygen. Spectra of pure Fe films show an absorption that is strongly determined by film–surface scattering of charge carriers. The adsorbate induced IR-optical surface resistivity is observed by a significant broadband increase of transmission. Applying a proper description of the frequency dependent electron scattering rate of iron and including Persson's surface resistivity model we succeed in explaining this broadband spectral change. We calculate an electron–adsorbate scattering rate, which leads to reasonable values for the dc conductivity of the investigated ultrathin films and for the electron-hole lifetime of adsorbate motion.

Structural and vibrational properties of carbon monoxide adlayers on the copper (001) surface

The structure and vibrational states of a prototype adsorbate-substrate system—carbon monoxide on the copper (001) surface—have been calculated from first principles within local density functional theory. Three CO coverages have been examined: θ=0 (bare surface), 0.5, and 1. These systems are represented by a well converged slab model within which all atomic degrees of freedom are treated on an equal footing. The computed structural relaxations and vibrational frequencies are generally in excellent quantitative agreement with the available experimental measurements. The full monolayer is found to be energetically favorable to the half monolayer plus free CO molecule. This indicates that the maximum stable coverage is greater than θ=0.5, in agreement with experiment. The vibrational analysis reveals that resonant coupling between adsorbate and substrate motions has a profound effect on the vibrational spectra, for example, the low-frequency, in-plane frustrated translational motion of the CO molecules mixes with long-wavelength copper phonons to form a broad resonance peak. This implies a finite lifetime which, for the half-monolayer system, is computed to be 3.0 ps, in excellent agreement with the measured value of 2.3±0.4 ps. For the full-monolayer system, the predicted lifetime is 0.7 ps; however this system is presently inaccessible to experiment. Resonant coupling is also found to affect the Rayleigh wave of the copper (001) surface. At half-monolayer CO coverage, this mode resonantly mixes with bulk copper phonons developing a finite lifetime, which is predicted to be 5.2 ps. To our knowledge, the lifetime of this mode has never been measured. For the fully covered surface, the Rayleigh wave does not form a resonance because the phonon coupling is forbidden by symmetry.

Collective Motion and Structural Order in Adsorbate Vibrational Dynamics

A simple, broadly applicable theory is developed to describe resonant vibrational coupling between adsorbates and a substrate lattice. This is one of the principal mechanisms governing the relaxation of adsorbate vibrations. This theory can be applied to widely varying surface coverages and arbitrary overlayer structures, and it correctly incorporates collective adsorbate motion, which has been shown to have a critical impact on the relaxation dynamics. Vibrational lifetimes predicted by this theory are in excellent quantitative agreement with experiments on adsorbate systems ranging from a diffuse, disordered overlayer to a dense, periodic overlayer. {copyright} {ital 1998} {ital The American Physical Society }

Collision induced desorption of N2 from Ru(001)

The dynamics of collision-induced desorption (CID) of N2 from Ru(001) exposed to hyperthermal rare gas colliders generated in a supersonic atomic beam source have been studied. Low coverage of 0.01 ML N215 at crystal temperature of 96 K was chosen to represent a CID process of a practically isolated molecule, neglecting the effect of lateral N2–N2 interactions. The cross sections for CID of nitrogen molecules, σdes(Ei,θi), as a function of the kinetic energy and angle of incidence of Ar and Kr colliders have been measured. It was found that σdes(Ei,θi=0°) changes monotonically in the range 0–25 Å2 for beam energy in the range of 0.5–5.5 eV and is insensitive to the type of collider (Ar, Kr) as well as to the adsorbate isotope (14N2, 15N2). The threshold energy for desorption has been determined to be 0.50±0.10 eV, which is twice the binding energy of N2 to Ru(001). The cross section for CID at a fixed collider’s energy rises approximately four times as the incidence angle θi increases from 0° to 70° relative to the surface normal. Neither normal nor total energy scaling of the cross section could describe the results. The σdes(θi) scales reasonably well, however, with the tangential energy of the collider for angles above 30°. Classical molecular dynamics simulations were performed to gain better understanding of the CID process. Threshold energy and angular dependence of the cross section were reproduced very well. The predominant CID mechanism was concluded to originate from a direct rare gas–nitrogen collision, in which impulsive-bending and the motion along the surface are coupled to the adsorbate motion which leads to desorption.

Continuum elastic theory of adsorbate vibrational relaxation

An analytical theory is presented for the damping of low-frequency adsorbate vibrations via resonant coupling to the substrate phonons. The system is treated classically, with the substrate modeled as a semi-infinite elastic continuum and the adsorbate overlayer modeled as an array of point masses connected to the surface by harmonic springs. The theory provides a simple expression for the relaxation rate in terms of fundamental parameters of the system: γ=mω̄02/AcρcT, where m is the adsorbate mass, ω̄0 is the measured frequency, Ac is the overlayer unit-cell area, and ρ and cT are the substrate mass density and transverse speed of sound, respectively. This expression is strongly coverage dependent, and predicts relaxation rates in excellent quantitative agreement with available experiments. For a half-monolayer of carbon monoxide on the copper (100) surface, the predicted damping rate of in-plane frustrated translations is 0.50×1012 s−1, as compared to the experimental value of (0.43±0.07)×1012 s−1. Furthermore it is shown that, for all coverages presently accessible to experiment, adsorbate motions exhibit collective effects which cannot be treated as stemming from isolated oscillators.

Lateral Manipulation of Single Adsorbates and Substrate Atoms With the Scanning Tunneling Microscope

The stability and precision of modern scanning-tunneling-microscope (STM) systems allow positioning of the tip on a subnanometer scale. This advancement has stimulated diverse efforts on surface modifications in the nanometer and even atomic range, as recently reviewed by Avouris. The lateral movement of individual adatoms and molecules in a controlled manner on solid surfaces and the construction of structures on a nanoscale were first demonstrated by Eigler and collaborators at 4 K. The reason for operating the STM at low temperatures (apart from increased stability and sensitivity of the STM setup itself) is the necessity to freeze the motion of single adsorbates, which are very often mobile at ambient temperatures. By selecting strongly bonded adsorbate/substrate combinations and large molecules, it was possible to extend the lateral manipulation technique even to room temperature. In the case of large molecules, not only their translational motion but also internal flexure of the molecule during the positioning process must be considered. In general, different physical and chemical interaction mechanisms between tip and sample can be exploited for atomic-scale manipulation. We will concentrate in the following on lateral manipulation where solely the forces that act on the adsorbate because of the proximity of the tip are used. This means that long-range van der Waals and short-range chemical forces can be used to move atoms or molecules along the surface. No bias voltage or tunneling current is necessary. Apart from this technique, additional advances using the effects caused by the electric field generated by the bias voltage between tip and sample and by the current flowing through the gap region can be used for atomic or molecular modification.

Dynamical studies of surface species — observing librational motions of adsorbates

A review of the use of the ESDIAD (electron stimulated desorption ion angular distribution) method for the measurement of the librational motion of two adsorbates on single crystal metal surfaces is presented. Both molecular rotation and anisotropic librational motions are measured. A new type of ESDIAD measurement technique is introduced in which time-of-flight selection allows the lateral momentum of an adsorbate to be measured.

A Model Calculation for Photo-Stimulated Desorption

A model calculation is presented for photo-stimulated desorption of adsorbates from a metal surface. In the model, the electron system of the material is described by a two-band tight-binding Hamiltonian. Interband electronic excitation is induced by laser pulse irradiation and causes a change in adsorbate trajectory. Close investigations are made of the dependence of the adsorbate trajectory on the photon energy and the time-width of laser pulse within one-photon process. For that purpose, an effective excited-state potential for adsorbate motion is introduced tentatively. When the electronic excitation is spatially localized in the neighborhood of the surface, substantial deviation of shape from the ground state potential can be seen in the effective excited-state potential, and gives rise to adsorbate desorption. It can be expected from the obtained results that the desorption probability increases as the photon energy is increased and as the time-width of laser pulse is decreased.