Adsorbate-substrate System Research Articles

First-principles investigation of a symmetry incompatible adsorbate-substrate system: PF3 on Cu(001)

The structural relaxation of the adsorption of PF3 on Cu(001), resulting in a c(4 × 2) overlayer, was studied using first-principles Density Functional Theory (DFT) calculations in the generalized gradient approximation (GGA). Unlike previous experimental results, we found no tilting of the molecules on the surface, with the C3v axis of PF3 perfectly perpendicular to the surface. Then using Density Functional Perturbation Theory (DFPT), we calculated the phonon spectra of the Cu (001): PF3 c(4 × 2) at two high symmetry points in the surface Brillouin zone, Γ¯ and M¯. By plotting the projected density of states, we found modes localized on the surface and determined how they changed upon adsorption. Our results indicate considerable mixing with bulk phonon modes at certain back-folded q-points of the bare-surface Brillouin zone. These modes have dominant vertical vibration on the topmost layer. Modes with surface covered atoms that move with the molecule as a unit can undergo softening up to 1.5 meV. However, modes with dominant motion on the second layer remain unperturbed. In addition, we studied molecular external modes (i.e. modes that arise from free motions of the PF3 molecule) that are in the range of bulk and surface modes of Cu. Depending on the symmetry of the underlying structure, PF3 vibrations can remain isolated and induce a localized surface mode or mix with bulk modes and become a resonance.

Effect of substrate relaxation on adsorption energies: The example of α-Fe2O3(0001) and Fe3O4(111)

Adsorption energies calculated from total energy differences of the adsorbate-substrate and isolated adsorbate and substrate systems consist of chemical contribution which results from the electron charge redistribution, and mechanical (relaxation) energy component resulting from the changes in the atomic configuration of the adsorbate-substrate system during adsorption. In this work Au and CO adsorption on the α-Fe2O3(0001) and Fe3O4(111) surfaces is used as model systems for the separation of these two contributions by critical analysis of published data and additional density functional theory calculations in order to analyze how the calculated adsorption energies are influenced by substrate relaxation effects. It is shown that a reliable estimation of these effects is obtained from the difference between desorption and adsorption energies. The scatter in the calculated adsorption energies of Au and CO on iron oxides is explained.

Interpretation of x-ray absorption spectroscopy in the presence of surface hybridization.

X-ray absorption spectroscopy (XAS) yields direct access to the electronic and geometric structure of hybrid inorganic-organic interfaces formed upon adsorption of complex molecules at metal surfaces. The unambiguous interpretation of corresponding spectra is challenged by the intrinsic geometric flexibility of the adsorbates and the chemical interactions with the interface. Density-functional theory (DFT) calculations of the extended adsorbate-substrate system are an established tool to guide peak assignment in X-ray photoelectron spectroscopy of complex interfaces. We extend this to the simulation and interpretation of XAS data in the context of functional organic molecules on metal surfaces using dispersion-corrected DFT calculations within the transition potential approach. For the prototypical case of 2H-porphine adsorbed on Ag(111) and Cu(111) substrates, we follow the two main effects of the molecule/surface interaction onto the X-ray absorption signatures: (1) the substrate-induced chemical shift of the 1s core levels that dominates in physisorbed systems and (2) the hybridization-induced broadening and loss of distinct resonances that dominate in more chemisorbed systems.

Surface Structure and Electron Transfer Dynamics of the Self-Assembly of Cyanide on Au{111}

A vibronic resonance between Au{111} surface states and adsorbed CN vibrations has been predicted, which we target for study. We have formed stable monolayers of cyanide on Au{111} and observe a hexagonal close-packed lattice with a nearest neighbor distance of 3.8 ± 0.5 A. Cyanide orients normal to the surface attached via a Au–C bond. We show that the substrate–molecule coupling is particularly strong, leading to ultrafast electron transfer from the cyanide molecules to the Au{111} substrate as measured by resonant Auger spectroscopy using the core–hole clock method. The CN/Au{111} system is a simple example of a strongly interacting adsorbate–substrate system and will be the subject of a number of further studies, as discussed.

A quantum mechanical study of water adsorption on the (110) surfaces of rutile SnO₂ and TiO₂: investigating the effects of intermolecular interactions using hybrid-exchange density functional theory.

Periodic hybrid-exchange density functional theory calculations are used to explore the first layer of water at model oxide surfaces, which is an important step for understanding the photocatalytic reactions involved in solar water splitting. By comparing the structure and properties of SnO2(110) and TiO2(110) surfaces in contact with water, the effects of structural and electronic differences on the water chemistry are examined. The dissociative adsorption mode at low coverage (1/7 ML) up to monolayer coverage (1 ML) on both SnO2 and TiO2(110) surfaces is analysed. To investigate further the intermolecular interactions between adjacent adsorbates, monolayer adsorption on each surface is explored in terms of binding energies and bond lengths. Analysis of the water adsorption geometry and energetics shows that the relative stability of water adsorption on SnO2(110) is governed largely by the strength of the chemisorption and hydrogen bonds at the surface of the adsorbate-substrate system. However on TiO2(110), a more complicated scenario of the first layer of water on its surface arises in which there is an interplay between chemisorption, hydrogen bonding and adsorbate-induced atomic displacements in the surface. Furthermore the projected density of states of each surface in contact with a mixture of adsorbed water molecules and adsorbed hydroxyls is presented and sheds some light on the nature of the crystalline chemical bonds as well as on why adsorbed water has often been reported to be unstable on rutile SnO2(110).

Calculation of two-dimensional potential energy surfaces of CO on a rutile(110) surface: ground and excited states

Being one of the most important catalytically active metal oxide surfaces, the aim of this study was the description of the laser-induced photodesorption of the CO adsorbate from a TiO2(110) surface. As a first step, this paper presents two-dimensional potential energy surfaces of a CO molecule on a rutile TiO2(110) surface. Focussing on the desorption mechanism taking place in this adsorbate–substrate system, the quantum chemical and quantum dynamical calculations regarding the desorption coordinate Z and the polar angle θ allowed a first insight into the mechanistic processes in the CO–TiO2(110) system which are relevant for laser-induced photodesorption. For the electronic ground state X1A1 the adsorption minimum was found for the polar angle θ = 0°, which corresponds to a linear coordination of the CO adsorbate with the carbon atom down to the substrate surface. This is in contrast to the electronically excited state A3B2, where the adsorption minimum was found for the polar angle θ = 180°, which describes a linear coordination with the oxygen atom of the CO molecule on top of the rutile TiO2(110) surface. Moreover, this paper shows exemplary quantum dynamical calculations which simulate the laser-induced photodesorption as a first step to understand the desorption process in detail. Hence, higher dimensional calculations regarding more than 2 degrees of freedom of the CO molecule on the substrate surface are needed to get a complete description of this complex adsorbate–substrate system.

Calculation of thermal effects in the photodesorption of NO from NiO(100)

The influence of temperature in the photodesorption of NO from a NiO(100)-surface is studied with a two-dimensional quantum wave packet approach. The complete process including laser-induced excitation and subsequent relaxation is treated explicitly from first principles. The electronic quenching caused by the interaction of the excited adsorbate–substrate system with electron–hole-pairs in the surface is treated with the surrogate Hamiltonian approach. We have implemented a parallelization scheme of the wave packet propagation based on a one-dimensional data decomposition to perform simulations in a reasonable computing time. The results are compared with mixed quantum-classical simulations and with experimental measurements. Both desorption yield and mean velocity of the desorbing molecules were enhanced with increasing temperature. The calculated rotational temperatures are consistent with experimental results.

Atomic Geometry and Adsorption of Cu(100)/H Surface

The structural and adsorption properties of Cu(100)/H surface were studied using density-functional theory (DFT) and projector-augmented wave (PAW) method. It was concluded that atomic hydrogen was adsorbed on the fourfold hollow (FFH) site with a perpendicular distance of 0.052nm from the outmost Cu layer for a Cu(100)c(2×2)/H geometry. The bond length between H atom and substrate was calculated to be about 0.189nm. In this adsorbatesubstrate system the surface work function was predicted to be about 4.47eV, which was almost identical to that of a clean Cu(100) surface. The total-energy calculations showed that the chemisorption energy of atomic hydrogen in the case of Cu(100)c(2×2)/H surface was about 2.37eV with respect to an isolated atomic hydrogen as reference. The hydrogen adsorption on Cu(100) surface yielded the hybridization between surface Cu atoms and adsorbed H, and generated the surface localized states at -0.8eV relative to Fermi energy E(subscript F). This system was modeled at different coverages using p(1×1), p(2×2), and p(3×3) geometries of hydrogen atoms adsorbed on the FFH sites of the Cu(100) surface. The corresponding equilibrium geometries were obtained by total energy and Helleman-Feynman force conjugate-gradient optimizations. In the regime of lower H coverages, the hybridization between adsorbed hydrogen and substrate displayed a type of Cu(S)-H-Cu(S-1) mixing.

Competition between relaxation and decay of a core-level electron ionized state in a charge transfer system

In a weak charge transfer (CT) system, such as an adsorbate–substrate system a core-level electron ionized state in an adsorbate observed by photoelectron spectroscopy (PES) as the smallest binding energy state, may be a metastable state. On time scale of core–hole decay it may relax further to the most stable state by CT core–hole screening via transfer of an electron from the substrate to an unoccupied adsorbate discrete level pulled down below the Fermi level of the substrate by an attractive core–hole potential. The latter state is a CT shakedown state, which is not observable by PES but its presence is visible in a decay spectrum, such as Auger-electron spectroscopy (AES) spectrum. The CT shakedown state resembles a resonantly excited core–hole state lying below the core–hole state of the smallest binding energy. If this is so, the AES spectrum measured at far above the core-level electron ionization threshold has then an additional spectral feature, which resembles the autoionization spectrum by the core–hole decay of the resonantly excited core–hole state. Such an example is the N 1s AES spectrum of N 2 molecule physisorbed on graphite reported by Björneholm et al. [Q. Björneholm, A. Nilsson, A. Sandell, B. Hernnäs, N. Mårtensson, Phys. Rev. Lett. 68 (1992) 1892]. The competition between relaxation and core–hole decay of a core-level electron ionized state in a weak CT system is formulated by a many-body theory and discussed. The present theory provides a theoretical foundation of the interpretation by Björneholm et al. They determined the CT time from the intensity ratio of the AES spectrum by the decay of the CT shakedown state to that by the decay of the core–hole state observed by PES as the smallest binding energy state. However, the CT time cannot be determined, unless the branching ratio of the former decay rate is equal to that of the latter one.

Two-dimensional surrogate Hamiltonian investigation of laser-induced desorption of NO∕NiO(100)

The photodesorption of NO from NiO(100) is studied from first principles, with electronic relaxation treated by the use of the surrogate Hamiltonian approach. Two nuclear degrees of freedom of the adsorbate-substrate system are taken into account. To perform the quantum dynamical wave-packet calculations, a massively parallel implementation with a one-dimensional data decomposition had to be introduced. The calculated desorption probabilities and velocity distributions are in qualitative agreement with experimental data. The results are compared to those of stochastic wave-packet calculations where a sufficiently large number of quantum trajectories is propagated within a jumping wave-packet scenario.

Diffusion of rhodium layers on the stepped surface of a tungsten micro-crystal

We have investigated the spreading of rhodium at coverages of 0.25, 0.5, 1, 2 and 3 ML over the curved surface of a field emitter tip using field electron microscopy. We have found that the activation energy of spreading as well as the prefactor for the diffusivity depend strongly on the thickness of the layer diffusing, due to a change in interactions in the adsorbate–substrate system. The derived average activation energy for spreading first decreases from E dif = 1.32 eV/atom at Θ = 0.25 ML to E dif = 0.71 eV at Θ ≈ 2 ML and than rises again to E dif = 1.20 eV at Θ ≈ 3 ML. The prefactor for the diffusivity D 0 also decreases with increasing coverage from 0.5 to 1 ML, and stays almost constant for multilayer diffusion in a range of few orders of magnitude lower than for single atom diffusion. We register typical spreading behavior with a sharp moving boundary in the (0 1 1)–(0 0 1) zone of the tip and an unusual picture of diffusion in the (0 1 1)–(1 1 2) region of the tip. In the second region diffusion proceeds without a sharp boundary, independent of the thickness of the moving layer. We think that such an unusual picture can be caused by the change in composition of the second and next layers of adsorbing material due to the early stage of faceting observed in this region of the tip at higher temperature. The results are compared with data for diffusion of individual Rh atoms and small clusters; to understand the observed diffusion we propose taking account of the atomic surface structure of the substrate, modified by strong interactions of the Rh adsorbate with the W micro-crystal surface.

Microscopic understanding of an ultrafast photochemical surface reaction: H ads + H ads → H 2,gas on Ru(0 0 1)

As a prototypical surface reaction initiated by excitation with an ultrafast light pulse, the associative desorption of hydrogen from a Ru(0 0 1) surface has been studied. Femtosecond laser techniques reveal detailed information on the underlying reaction mechanism, the pathway and time scales of energy flow and the partitioning of excess energy between different degrees of freedom within the reaction product. A response time of the desorption yield of only few hundred femtoseconds as manifested in two-pulse correlation measurements and a pronounced isotope effect in the H 2 versus D 2 yield unambiguously indicate a hot substrate electron-driven reaction mechanism. All experimental results can be well reproduced within the framework of electronic friction between the ruthenium substrate and the hydrogen adlayer. In addition, it is found that adsorbate–adsorbate interactions crucially influence the reaction as seen in promotion effects within isotopically substituted surroundings of the reactants. Finally, energy partitioning into various degrees of freedom is investigated by time-of-flight measurements and state-resolved detection of desorbing hydrogen molecules via resonance enhanced multiphoton ionization. The results show a pronounced preference for translational energy versus intramolecular excitations like vibrations and rotations. These studies provide an ideal benchmark for theoretical calculations of potential energy surfaces and photoinduced reaction dynamics of this adsorbate–substrate system.

Reactive scattering of H2 from Cu(100): Six-dimensional quantum dynamics results for reaction and scattering obtained with a new, accurately fitted potential-energy surface

Six-dimensional quantum dynamical calculations are reported for the dissociative chemisorption of (v=0, 1, j=0) H(2) on Cu(100), and for rovibrationally inelastic scattering of (v=1, j=1) H(2) from Cu(100). The dynamics results were obtained using a new potential-energy surface (PES5), which was based on density-functional calculations using a slab representation of the adsorbate-substrate system and a generalized gradient approximation to the exchange-correlation energy. A very accurate method (the corrugation reducing procedure) was used to represent the density-functional theory data in a global potential-energy surface. With the new, more accurately fitted PES5, the agreement between the dynamics results and experimental results for reaction and rovibrationally elastic scattering is not as good as was obtained with a previous potential-energy surface (PES4), which was based on a subset of the density-functional theory data not yet including the results for the low-symmetry Cu sites. Preliminary density-functional theory results suggest that the agreement between theory and experiment will improve over that obtained with PES5 if the density-functional calculations are repeated using a larger basis set and using more copper layers than employed in PES4 and PES5.

Unequal-sphere packing model for the structural arrangement of the well-ordered adsorbate-substrate system

In order to understand the well-ordered adsorbate-substrate systems at atomic level, a method is developed based on the simulation of packing arrangements for layers of unequal spheres, in three-dimensional space. The model, based on geometrical principles, is developed for fcc structure consisting of two hexagonal ordered layers. During simulation, adsorbate spheres were accommodated in different positions, forming a great variety of structures, in dependence of the intersphere distance of the upper layer spheres. Using the average height of the adsorbate layer on the flat substrate as a determinant parameter, several specific structures have been selected as the most probable: $(\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$, $(\ensuremath{\surd}7\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}7)\mathrm{R}19.1\ifmmode^\circ\else\textdegree\fi{}$, and $(3\ifmmode\times\else\texttimes\fi{}3)$. Indeed, they correspond to typical accommodations of the iodine adatoms on the Pt(111) surface, earlier found in experimental studies, which clearly supports the validity of our model. The model developed in our study could completely and satisfactorily describe the accommodation process of the iodine adlayer on the Pt(111) surface. This methodology could be of great help for interpretation of scanning tunneling microscopy images, better understanding of adlayer structures, and design of adsorbate-substrate systems with exciting properties.

Double photoionization of polyatomic molecules by arbitrarily polarized light: theory

This paper develops a theoretical framework suitable for studying simultaneous ejection of two electrons following the absorption of a single photon by a molecule belonging to one of the 32 point groups. The ionizing electromagnetic radiation is assumed to have any arbitrary polarization. Both photoelectrons are analysed in terms of their energies as well as directions of propagation. The transformation properties of the molecular point symmetry group are used to their full advantage to reduce the various expressions obtained herein to their simplest possible forms. The ionization amplitude is thus shown to decompose into a sum of transitions each involving a final state wavefunction belonging to the irreducible representations of the point group of the molecular target. Molecules with both a random or a fixed orientation in space have been considered. The results obtained herein can therefore be used to study double photoionization of any point symmetry molecule in either its gaseous phase or oriented on liquid and solid surfaces. The analysis is general in that it is independent of any particular dynamical models for the description of photoionization. The present results show that such double photoionization experiments are richer sources of information, specifically on interaction between adsorbed molecules and surfaces and on the geometry of the adsorbate–substrate system, in addition to the electron–electron correlation, than their counterpart experiments on single photoionization. Various possible cases and several observables are discussed.

Signatures of site-specific reaction of H2 on Cu(100)

Six-dimensional quantum dynamical calculations are presented for the reaction of (v,j) H2 on Cu(100), at normal incidence, for v=0–1 and j=0–5. The dynamical calculations employed a potential energy surface computed with density functional theory, using the generalized gradient approximation and a slab representation for the adsorbate-substrate system. The aim of the calculations was to establish signatures from which experiments could determine the dominant reaction site of H2 on the surface and the dependence of the reaction site on the initial rovibrational state of H2. Two types of signatures were found. First, we predict that, at energies near threshold, the reaction of (v=1) H2 is rotationally enhanced, because it takes place at the top site, which has an especially late barrier and a reaction path with a high curvature. On the other hand, we predict the reaction to be almost independent of j for (v=0) H2, which reacts at the bridge site. Second, we predict that, at collision energies slightly above threshold for which the reaction probabilities of the (v=0) and (v=1) states are comparable, the rotational quadrupole alignment of (v=1) reacting molecules should be larger than that of (v=0) reacting molecules, for j=1, 4, and 5. For (j=2) H2, the opposite should be true, and for (j=3) H2, the rotational quadrupole alignment should be approximately equal for (v=1) and (v=0) H2. These differences can all be explained by the difference in the predicted reaction site for (v=1) and (v=0) H2 (top and bridge) and by the differences in the anisotropy of the potential at the reaction barrier geometries associated with these sites. Our predictions can be tested in associative desorption experiments, using currently available experimental techniques.

Rovibrational preexcitation in the photodesorption of CO from Cr 2O 3

The influence of rovibrational preexcitation in the CO/Cr 2O 3(0 0 0 1)-system is studied theoretically using a three-dimensional quantum wave packet approach. We find that selectively excited hindered rotational levels of the adsorbate-substrate system influence the experimentally observed rotational alignment of the desorbing CO-molecule significantly. Pure vibrational preexcitation has less effect on the observables of interest. We predict only low surface temperature dependence of the rotational alignment and of the desorption yield. Our study is based on ab initio potential energy surfaces (PESs) for the electronic states involved and on a stochastic wave packet method.

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.

Photodissociation of methyl nitrite on Ag(111): Nitric oxide ejection dynamics

The thermal and photochemical properties of CH3ONO adsorbed on Ag(111), with and without a thick spacer layer of hexane, C6H14, are described. Angle-resolved time-of-flight measurements of NO ejected during photolysis at 248 and 351 nm exhibit trimodal velocity distributions. Two of the components are wavelength dependent and nonthermal, and are modeled as NO ejection with and without partially thermalizing collisions with surrounding species. The third component of the distribution is wavelength independent and characterized by the bulk temperature of the adsorbate–substrate system. These three components are present for all coverages of CH3ONO, submonolayer to multilayer, and persist even when there is a spacer of C6H14. The photochemistry is dominated by direct excitation of CH3ONO; there is no evidence for NO ejection as the result of substrate excitation. Comparing NO time-of-flight distributions for 1 monolayer (ML) of CH3ONO on clean Ag(111) to those for 1 ML on a thick layer of C6H14, evidences substrate involvement in the dynamics of nascent NO.

Desorption of CO from Ru(001) induced by near-infrared femtosecond laser pulses

Irradiation of a Ru(001) surface covered with CO using intense femtosecond laser pulses (800 nm, 130 fs) leads to desorption of CO with a nonlinear dependence of the yield on the absorbed fluence (100–380 J/m2). Two-pulse correlation measurements reveal a response time of 20 ps (FWHM). The lack of an isotope effect together with the strong rise of the phonon temperature (2500 K) and the specific electronic structure of the adsorbate–substrate system strongly indicate that coupling to phonons is dominant. The experimental findings can be well reproduced within a friction-coupled heat bath model. Yet, pronounced dynamical cooling in desorption, found in the fluence-dependence of the translational energy, and in a non-Arrhenius behavior of the desorption probability reflect pronounced deviations from thermal equilibrium during desorption taking place on such a short time scale.