Adsorbate Motion Research Articles

Photo-induced desorption through non-adiabatic couplings

A mechanism describing photo-induced desorption triggered by substrate excitation is presented. In this mechanism adsorbate motion is described quantum mechanically and desorption is considered to occur due to energy transfer from the electronic to the nuclear degrees of freedom via non-adiabatic couplings. Considerable insight is gained into experiments on photo-induced desorption of NO and CO from transition metal surfaces.

The quantum dynamics of interfacial hydrogen: Path integral maximum entropy calculation of adsorbate vibrational line shapes for the H/Ni(111) system

Vibrational line shapes for a hydrogen atom on an embedded atom model (EAM) of the Ni(111) surface are extracted from path integral Monte Carlo data. Maximum entropy methods are utilized to stabilize this inversion. Our results indicate that anharmonic effects are significant, particularly for vibrational motion parallel to the surface. Unlike their normal mode analogs, calculated quantum line shapes for the EAM potential predict the correct ordering of vibrational features corresponding to parallel and perpendicular adsorbate motion.

Assessment of the role of quantal effects in the dynamics of stimulated desorption

We have assessed the roles of various quantum effects in the stimulated desorption spectra and yields of isolated physisorbed species. By an appropriate extension of the semiclassical (Antoniewicz's) and strictly quantum wave-packet squeezing (WPS) models of desorption with dissipative coupling of the ionized adsorbate to the substrate electronic response, we were able to describe quantum effects associated with elastic and inelastic adsorbate motion within a unified model. Using this model we investigated the possibility of removing the long-standing discrepancies between large values of experimental desorption yields from adlayers of Ar atoms on Ru(001) and much smaller ones calculated in Antoniewicz's and WPS models by using empirical atom-surface potential parameters. The desorption rates thus calculated turned out to be rather insensitive to dissipative effects due to a smooth transition between Antoniewicz's and WPS desorption regimes, which could be explained by the properties of the wave packets describing the ionized adparticle motion. This indicated that the desorption rates would be sensitive only to the variations of the neutralization rate and parameters specifying neutral- and ionized-state diabatic potentials. By varying the latter we showed that high enough desorption yields could be reproduced only with extreme forms of the ionized-state potentials. From this we conclude that improved models of stimulated desorption need take into account certain aspects of the many-body lateral adsorbate-adsorbate interactions.

On the lifetime of the parallel vibrational motion of adsorbates on metallic surfaces

We point out that the relationship between the adatom-induced surface resistivity, p s , and the lifetime of the parallel vibrational motion of adatoms, τ, recently derived by Persson can be recast to read as F∫/J∫ = n e el f p s /n a , where F∫, J∫, n e , l f , and n a are the damping force on the adatom, the apparent current density in the metal seen from the adatom, the electron density in the metal, the film thickness, and the 2-dimensional density of adatoms, respectively. This has essentially the same form as the relationship between p , and the indirect (electron-wind) force F w on the adatom located in a current density J W , i.e., F W /J W =n e el f p s /n a , which has been known for a long time in the theory of electromigration. Using a first-principles linear response approach, we show that both the equations hold true if the substrate is translationally invariant in the plane (flat jellium model). We derive a simple expression of the surface resistivity, p s , which can be evaluated from the scattering amplitudes of one-electron wave functions at the Fermi energy alone.

A microscopic theory of desorption induced by electronic transitions

Abstract Desorption induced by electronic transitions (DIET) in two cases is investigated from a microscopic point of view. In case A, where a single electron in a state of the localized kind (localized around adsorbates) is excited into a state of the extended kind (extended into the bosom of the substrate), the shape of the excited-state potential-energy surface (PES) may differ markedly from that of the ground-state PES for adsorbate motion. The Franck-Condon factor then takes a finite value, giving rise to a finite desorption probability. In case B, where a single electron in a state of the extended kind is excited into another state of the extended kind, the shape of the excited-state PES is practically the same as that of the ground-state PES. The Frank-Condon factor is then zero. In such a case, one should take DIET as a single-step (coherent) process and take into account the adsorbate-position dependence of the matrix element for state transitions of the electron system in order to obtain a finite desorption probability.

Simulation of Desorption Kinetics at a Liquid-Solid Interface

ABSTRACTMolecular dynamics simulations are reported for the atomic motion of adsorbates at a liquid-solid interface. The rate for physical desorption is analyzed using the potential of mean force to describe adsorbate and transition state energies.

Photoionization of oriented CO molecule: Linear dichroism in the angular distribution of photoelectrons

Theoretical investigation of linear dichroism in the angular distribution (LDAD) of photoelectrons obtained in a molecular photoionization process is presented. The developed theory applies to the investigation of both molecules adsorbed on a surface and gas phase molecules when the molecular orientation is fixed by coincidence between a photoelectron and a residual ion resulting from fast molecular dissociation. Several experimental geometries are proposed which are used to unravel the properties of the adsorbed or oriented molecules. The first numerical calculations of LDAD in the case of photoionization of the 4 σ-orbital of fixed-in-space CO molecules are presented. We calculate the differential cross section and the LDAD for two of the geometries of the experiment we have proposed: One allows a simple extraction of theoretical parameters from the experimental data in a kind of complete experiment with molecules; the other allows the determination of molecular orientation in space, for example, with respect to the surface if the nuclear motion of adsorbates is neglected.

Vibrational linewidth of CO adsorbed on Pt(111)

We show that vibrational spectra of molecules chemisorbed on metal surfaces can be affected by anharmonicity much more than usually estimated if various vibrational coordinates are coupled by non-diagonal terms. The strong coverage dependence of the linewidth of the adsorbate-metal vibrations of CO adsorbed on Pt(111) can be attributed to the multiphonon decay, enhanced by a strong coupling of the adsorbate motion to the in-plane displacement of the surrounding surface atoms.

Atomic-scale view of motion on surfaces

Scanning tunneling microscopy has been used to obtain an atomic-scale view of the motion of adsorbates on surfaces. For sufficiently slow diffusion, we have followed this motion in real time. For faster adsorbate motion we were able to observe the partial and transient occupancy of surface sites on the time scale of our measurements. For lateral motion induced by surface processes such as adsorption or chemical reaction, we were able to analyze the final positions of the adsorbates or reaction products in order to measure the distances covered and to elucidate the means by which energy is accommodated to the surface.

Angular motion of molecular adsorbates influenced by surface coverage

Abstract Using classical dynamics, we show that the angular motion of an adsorbate is strongly modified by the surface coverage. Increasing this coverage from submonolayer to monolayer changes the mean angles and rigidifies the structures about the azimuthal angle. Consequently, lateral interaction, including an angular term, should be taken into account in dynamical studies of adsorbates. These conclusions were reached in the particular case of CO/Ar(100) but we believe that they are valid for other systems.

Computational studies of halogen chemistry on rare-gas surfaces. II. Structure of chlorine and bromine submonolayer films on Ar(111) and Xe(111) prepared by molecular beam dosing

The deposition by molecular beam dosing of halogen molecules on rare-gas surfaces has been studied with molecular dynamics simulation. Specifically we have considered films formed by the sequential adsorption reactions: X2g+X2[Θ]ads−Rg(111)[T]→X2(ads)−X2[Θ]ads −Rg(111)[T], where Θ is the film coverage defined by X2[adsorbed]/Rg[surface], T indicates the substrate temperature, X2 is the halogen adsorbate which is either chlorine or bromine, and Rg indicates the rare-gas substrate which is either argon or xenon. The structure of halogen adlayers was studied as a function of coverage for films grown on rare-gas substrates at different temperatures. Chlorine and bromine films on argon exhibit orientational ordering and islanding with increasing coverage. The tendency of the halogen diatoms to align along the surface normal with increasing coverage is strongly enhanced by higher temperatures in the case of chlorine on xenon, moderately enhanced in the case of chlorine on argon, and unchanged in the case of bromine on argon. Chlorine and bromine films form three-dimensional aggregated structures on argon at 15 and 25 K. Chlorine forms a two-dimensional amorphous layer on xenon at 15 K and a highly ordered layer at 50 K. The dynamics of the molecular adsorption event were studied at three different coverages: Θ=0.05, 0.25, and 0.5. At all these coverages, we observed finite possibility of diffusional motion of adsorbate on the surface immediately after it lands: This was not seen at zero coverage.

Lattice model and simulation of dynamics of adsorbate motion in zeolites

Abstract Previous Monte Carlo simulations of the distribution of adsorbates in zeolites have suggested that adsorbates are confined to a lattice of sites in the micropore. In this work, we examine the effect of the structure and energetics of the adsorption site lattice on the mobility of small molecules in cage-like micropores using Monte Carlo lattice dynamics (MCLD) simulations. In MCLD, motion is modeled as a series of activated site-to-site hops over energetic and entropic barriers whose magnitudes can be functions of zeolite structure, sorbate chemistry, and loading. The topology of the lattice is such that adsorbate hops are of two types: (i) between sites in a given case, and (ii) between sites in neighboring cages. For comparison, an analytical model of adsorbate self-diffusivity is constructed by applying a random walk theory to this lattice. This model is exact at low loading and approximates the dynamics well even for a crowded lattice. We also compare MCLD to molecular dynamics (MD) simulations of methane adsorbed in zeolite A and observe qualitative agreement between the two approaches. However, the computational cost of MCLD is an order of magnitude lower than MD.

Transient hyperthermal diffusion following dissociative chemisorption: a molecular dynamics study

Abstract The non-equilibrium diffusion of highly hyperthermal atoms resulting from dissociative adsorption of a diatomic molecule is investigated using molecular dynamics and an effective-medium model potential. The parameters of the potential are chosen to describe O2 on Al(111) and the simulation results are discussed in relation to recent STM measurements for this system [Brune et al., Phys. Rev. Lett. 68 (1992) 624]. The transient displacement along the surface is found to be about one order of magnitude larger than the distance to the nearest adsorption site. The traveling distance is limited mainly by the rapid randomization of the adsorbate motion, and not primarily by the rate of energy transfer to the substrate degrees of freedom.

Chemisorption of sulfur on nickel: A study of cluster convergence in the linear combination of Gaussian-type orbitals local density functional approach

Chemisorption of sulfur at the (100), (110), and (111) surface of nickel has been studied, using the linear combination of Gaussian-type orbitals local density functional (LCGTO-LDF) method. Employing various cluster models consisting of 11 to 29 substrate atoms, adsorption at the experimentally known sites has been considered. Besides the equilibrium distance and the force constant of the vertical adsorbate motion, the dynamical dipole moment was evaluated and it turned out to provide a sensitive probe of cluster convergence. The influence of atoms from the third substrate layer on the various observables is found to be considerable in some cases. With increasing cluster size, bond lengths are stabilized to 0.02 Å, frequencies to 20 cm−1, and dipole moments to 0.1 D. The converged results agree very well with experiment. Adsorption induced population changes are restricted to only four to six neighboring substrate atoms of the modifier atom. However, energy resolved charge density differences reveal a possible mechanism for the transmission of the long-range electronic effects caused by the adatom.

Stochastic excitation and dissociation of adsorbate in a laser field

Abstract A classical dynamics study of the interaction between an absorbate and an infrared laser field is presented. The resonance overlap criterion is found to provide reasonable estimates of the transition of the adsorbate motion from the quasi-periodic to the chaotic regime when compared with numerical calculations. Out study shows that exciting the system with a second laser will lower the threshold intensity for the laser desorption process, and the resonance overlap criterion provides an explanation of this observation.

Theoretical studies of surface diffusion and of photo-induced surface processes

Abstract Theoretical study of three types of processes associated with adsorbate-substrate interactions will be discussed. The first process considered is the surface diffusion mechanism of large adsorbates. It will be demonstrated that the conventional random walk mechanism, associated with the surface diffusion of small adsorbates, has to be largely modified when description of the motion of large adsorbates along the surface is desired. The results of these calculations shows good qualitative agreement when compared with experimental data. In the second study reported here, the branching ratio between desorption and dissociation for large model adsorbates on a laser heated substrate is examined. It is found that the branching ratio is very sensitive to a number of quantities including the adsorbate size, adsorption geometry and the relative strength between the adsorbate-surface bond and the intramolecular bond which is broken during the dissociation process. The last group of calculations investigate the dynamics of photodissociation products and their relation to the orientation and alignment of the adsorbates on the solid surface. The calculations are carried out for the HBr/LiF(001) system. Quantities such as the photoreaction probability, the angular and energy distributions of the scattered photofragment are shown to depend strongly on the initial alignment of the adsorbate. In addition, the quantum nature of the H photoproduct is examined using a 2-D quantum mechanical model. It is shown that for some systems the angular distribution of the scattered H-atom may exhibit a complicated diffraction pattern which can be related to the structure of the interface between the adsorbate layer and the substrate.

Resonance and environmental fluctuation effects in STM currents through large adsorbed molecules

Scanning tunnelling microscopy (STM) involving electron tunnelling through large adsorbate molecules with discrete electronic levels accessible at low bias voltage, exhibits conceptual and physical analogies to other thermal and optical multi-level electronic processes. The analogies are most conspicuous if the adsorbate levels are strongly coupled to the environmental molecular, conformational or solvent nuclear motion but interact weakly with the conducting substrate and tip. These conditions would be appropriate for example for adsorbed large transition metal complexes or redox metalloproteins. In these limits electronic-vibrational coupling induces resonance between the local adsorbate level and either the substrate or the tip levels, and the STM current-voltage relations can be approached by methods known from the theory of related electronic transitions such as long-range molecular electron transfer and multi-photon optical processes. We provide a new theoretical frame for STM processes in this limit. The formalism rests on perturbative coupling of the adsorbate levels to the substrate and tip. Specific models incorporate strong coupling to the adsorbate and environmental nuclear motion, vibrational relaxation features, and the continuous electronic spectra of the substrate and tip. All these features are directly and transparently reflected in the current-voltage relations of the STM process.

Monte‐Carlo Simulation of Surface Diffusion and Monolayer Completion

physica status solidi (b)Volume 166, Issue 1 p. K11-K14 Short Note Monte-Carlo Simulation of Surface Diffusion and Monolayer Completion G. Gładyszewski, G. Gładyszewski Laboratoire de Métallurgie Physique, associé au C.N.R.S., U.R.A. 131, Université de Poitiers Search for more papers by this authorL. Gładyszewski, L. Gładyszewski Institute of Physics, MCS University, Lublin Search for more papers by this author G. Gładyszewski, G. Gładyszewski Laboratoire de Métallurgie Physique, associé au C.N.R.S., U.R.A. 131, Université de Poitiers Search for more papers by this authorL. Gładyszewski, L. Gładyszewski Institute of Physics, MCS University, Lublin Search for more papers by this author First published: 1 July 1991 https://doi.org/10.1002/pssb.2221660136 40, Av. du Recteur Pineau, F-86022 Poitiers Cédex, France. Pl. M. Curie-Skłodowskiej 1, PL-20-031 Lublin, Poland. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume166, Issue11 July 1991Pages K11-K14 RelatedInformation

Adsorbate migration on a solid surface: The connection between hopping dynamics and the atom-surface interaction energy

The migration of an adsorbed atom at moderate temperatures is described in terms of uncorrelated jumps between lattice sites which lead to diffusion. It is widely believed that a jumping rate coefficient and therefore a diffusion coefficient can be defined only if energy exchange with the moving lattice or collisions with randomly distributed impurities give the motion of the adsorbate a random character. In this paper we examine systematically a suggestion of Haug, Wanhstrom, and Metiu, who conjectured that coupling between the adsorbate motion along the surface and its motion perpendicular to it can provide the necessary randomization and, in particular, make possible the definition of a hopping rate coefficient. We calculate the flux–flux correlation functions needed for describing the dynamics of single and double jumps by using a set of simple, but reasonably realistic, adsorbate-surface interactions. In all these calculations the lattice atoms are held fixed. We show that in spite of this, the correlation functions converge and rate constants can be defined for many of the potentials. We study in detail those features of the potential energy surface (PES) that lead to convergence and also how the shape of the PES influences the amount of recrossing (i.e., the accuracy of the transition state theory) and multiple jumping. Our results indicate that it is possible to develop a correction to the transition state theory which includes the effect of thermal fluctuations and calculates the recrossing correction by holding the lattice atoms fixed. This saves substantial computer time.

Optical high resolution study of quinizarin absorbed on γ-alumina powder: indication of low frequency adsorbate motions

Abstract Quinizarin adsorbed on γ-alumina was investigated with the optical high resolution methods fluorescence line narrowing (FLN) and persistent spectral hole burning (PSHB). Fluorescence excitation spectra showed an absorption red shift of about 35 nm for adsorbed quinizarin. The FLN spctra indicate a coupling of low frequency adsorbate motions to the electronic transitions of the adsorbed molecules. Such motions may represent an effective dephasing mechanism leading to an increased width of spectral holes when compared to matrixdoped molecules.