Adsorbate-surface System Research Articles

Capturing Weak Interactions in Surface Adsorbate Systems at Coupled Cluster Accuracy: A Graph-Theoretic Molecular Fragmentation Approach Improved through Machine Learning.

The accurate and efficient study of the interactions of organic matter with the surface of water is critical to a wide range of applications. For example, environmental studies have found that acidic polyfluorinated alkyl substances, especially perfluorooctanoic acid (PFOA), have spread throughout the environment and bioaccumulate into human populations residing near contaminated watersheds, leading to many systemic maladies. Thus, the study of the interactions of PFOA with water surfaces became important for the mitigation of their activity as pollutants and threats to public health. However, theoretical study of the interactions of such organic adsorbates on the surface of water, and their bulk concerted properties, often necessitates the use of ab initio methods to properly incorporate the long-range electronic properties that govern these extended systems. Notable theoretical treatments of "on-water" reactions thus far have employed hybrid DFT and semilocal DFT, but the interactions involved are weak interactions that may be best described using post-Hartree-Fock theory. Here, we aim to demonstrate the utility of a graph-theoretic approach to molecular fragmentation that accurately captures the critical "weak" interactions while maintaining an efficient ab initio treatment of the long-range periodic interactions that underpin the physics of extended systems. We apply this graph-theoretical treatment to study PFOA on the surface of water as a model system for the study of weak interactions seen in the wide range of surface interactions and reactions. The approach divides a system into a set of vertices, that are then connected through edges, faces, and higher order graph theoretic objects known as simplexes, to represent a collection of locally interacting subsystems. These subsystems are then used to construct ab initio molecular dynamics simulations and for computing multidimensional potential energy surfaces. To further improve the computational efficiency of our graph theoretic fragmentation method, we use a recently developed transfer learning protocol to construct the full system potential energy from a family of neural networks each designed to accurately model the behavior of individual simplexes. We use a unique multidimensional clustering algorithm, based on the k-means clustering methodology, to define our training space for each separate simplex. These models are used to extrapolate the energies for molecular dynamics trajectories at PFOA water interfaces, at less than one-tenth the cost as compared to a regular molecular fragmentation-based dynamics calculation with excellent agreement with couple cluster level of full system potential energies.

EUV-induced carbon growth at contaminant pressures between 10-10 mbar and 10-6 mbar: Experiment and model.

Carbon contamination induced by ultraviolet (UV) radiation affects precision optics in applications as diverse as semiconductor lithography and satellite observations of the Sun. Our previous experiments have shown that low-intensity UV-induced surface contamination depends quasi-logarithmically on the partial pressure of the organic contaminant due to the poly-dispersive nature of the surface-adsorbate system. This complex dependence presents difficulties because, without a physically motivated model, it cannot be extrapolated to low pressures. We present measurements and a model of carbon growth induced by UV exposure in the presence of tetradecane vapor. The model, which includes a coverage-dependent adsorption energy, describes the measurements over four orders of magnitude in pressure, and we expect that it can be extrapolated to the lower pressures of interest to the extreme ultraviolet (EUV) lithography and solar astronomy communities. Our experience with other contaminants leads us to expect that other organic contaminants will behave similar to tetradecane. The results also provide insights into the kinetics governing coverage isotherms at extremely low partial pressures.

A hierarchical effective mode approach to phonon-driven multilevel vibrational relaxation dynamics at surfaces

We discuss an efficient Hierarchical Effective Mode (HEM) representation of a high-dimensional harmonic oscillator bath, which describes phonon-driven vibrational relaxation of an adsorbate-surface system, namely, deuterium adsorbed on Si(100). Starting from the original Hamiltonian of the adsorbate-surface system, the HEM representation is constructed via iterative orthogonal transformations, which are efficiently implemented with Householder matrices. The detailed description of the HEM representation and its construction are given in the second quantization representation. The hierarchical nature of this representation allows access to the exact quantum dynamics of the adsorbate-surface system over finite time intervals, controllable via the truncation order of the hierarchy. To study the convergence properties of the effective mode representation, we solve the time-dependent Schrödinger equation of the truncated system-bath HEM Hamiltonian, with the help of the multilayer extension of the Multiconfigurational Time-Dependent Hartree (ML-MCTDH) method. The results of the HEM representation are compared with those obtained with a quantum-mechanical tier-model. The convergence of the HEM representation with respect to the truncation order of the hierarchy is discussed for different initial conditions of the adsorbate-surface system. The combination of the HEM representation with the ML-MCTDH method provides information on the time evolution of the system (adsorbate) and multiple effective modes of the bath (surface). This permits insight into mechanisms of vibration-phonon coupling of the adsorbate-surface system, as well as inter-mode couplings of the effective bath.

Dispersion-mediated steering of organic adsorbates on a precovered silicon surface.

The chemistry of organic adsorbates on surfaces is often discussed in terms of Pauli repulsion as limiting factor regarding the packing of molecules. Here we show that the attractive part of the van der Waals potential can be similarly decisive. For the semiconductor surface Si(001), an already covalently bonded molecule of cyclooctyne steers a second incoming molecule via dispersion interactions onto the neighbouring adsorption site. This helps in understanding the nonstatistical pattern formation for this surface–adsorbate system and hints toward an inclusion of dispersion attraction as another determining factor for surface adsorption.

Isotopic effects in vibrational relaxation dynamics of H on a Si(100) surface

In a recent paper [U. Lorenz and P. Saalfrank, Chem. Phys. 482, 69 (2017)], we proposed a robust scheme to set up a system-bath model Hamiltonian, describing the coupling of adsorbate vibrations (system) to surface phonons (bath), from first principles. The method is based on an embedded cluster approach, using orthogonal coordinates for system and bath modes, and an anharmonic phononic expansion of the system-bath interaction up to second order. In this contribution, we use this model Hamiltonian to calculate vibrational relaxation rates of H-Si and D-Si bending modes, coupled to a fully H(D)-covered Si(100)-(2×1) surface, at zero temperature. The D-Si bending mode has an anharmonic frequency lying inside the bath frequency spectrum, whereas the H-Si bending mode frequency is outside the bath Debye band. Therefore, in the present calculations, we only take into account one-phonon system-bath couplings for the D-Si system and both one- and two-phonon interaction terms in the case of H-Si. The computation of vibrational lifetimes is performed with two different approaches, namely, Fermi's golden rule, and a generalized Bixon-Jortner model built in a restricted vibrational space of the adsorbate-surface zeroth-order Hamiltonian. For D-Si, the Bixon-Jortner Hamiltonian can be solved by exact diagonalization, serving as a benchmark, whereas for H-Si, an iterative scheme based on the recursive residue generation method is applied, with excellent convergence properties. We found that the lifetimes obtained with perturbation theory, albeit having almost the same order of magnitude-a few hundred fs for D-Si and a couple of ps for H-Si-, are strongly dependent on the discretized numerical representation of the bath spectral density. On the other hand, the Bixon-Jortner model is free of such numerical deficiencies, therefore providing better estimates of vibrational relaxation rates, at a very low computational cost. The results obtained with this model clearly show a net exponential decay of the time-dependent survival probability for the H-Si initial vibrational state, allowing an easy extraction of the bending mode "lifetime." This is in contrast with the D-Si system, whose survival probability exhibits a non-monotonic decay, making it difficult to define such a lifetime. This different behavior of the vibrational decay is rationalized in terms of the power spectrum of the adsorbate-surface system. In the case of D-Si, it consists of several, non-uniformly distributed peaks around the bending mode frequency, whereas the H-Si spectrum exhibits a single Lorentzian lineshape, whose width corresponds to the calculated lifetime. The present work gives some insight into mechanisms of vibration-phonon coupling at surfaces. It also serves as a benchmark for multidimensional system-bath quantum dynamics, for comparison with approximate schemes such as reduced, open-system density matrix theory (where the bath is traced out and a Liouville-von Neumann equation is solved) or approximate wavefunction methods to solve the combined system-bath Schrödinger equation.

Photodesorption of NO from Au(100) using 3D surface-velocity map imaging.

We measured the fully resolved 3-dimensional velocity distributions of nitric oxide photodesorbed from a gold single crystal. These experiments combine time-of-flight measurements and the velocity map imaging technique to yield velocity distributions resolved in three dimensions for a prototypical surface-adsorbate system. Nitric oxide adsorbed on Au(100) was photodesorbed using a 355 nm laser beam. The desorbed NO molecules were ionised in the gas-phase by resonance-enhanced multi-photon ionisation within a set of velocity map imaging optics. The NO molecules preferentially leave the surface along the surface normal with a very narrow angular distribution, indicating a non-thermal desorption process.

Nonequilibrium Green's functions and atom-surface dynamics: Simple views from a simple model system

We employ Non-equilibrium Green's functions (NEGF) to describe the real-time dynamics of an adsorbate-surface model system exposed to ultrafast laser pulses. For a finite number of electronic orbitals, the system is solved exactly and within different levels of approximation. Specifically i) the full exact quantum mechanical solution for electron and nuclear degrees of freedom is used to benchmark ii) the Ehrenfest approximation (EA) for the nuclei, with the electron dynamics still treated exactly. Then, using the EA, electronic correlations are treated with NEGF within iii) 2nd Born and with iv) a recently introduced hybrid scheme, which mixes 2nd Born self-energies with non-perturbative, local exchange- correlation potentials of Density Functional Theory (DFT). Finally, the effect of a semi-infinite substrate is considered: we observe that a macroscopic number of de-excitation channels can hinder desorption. While very preliminary in character and based on a simple and rather specific model system, our results clearly illustrate the large potential of NEGF to investigate atomic desorption, and more generally, the non equilibrium dynamics of material surfaces subject to ultrafast laser fields.

CHx adsorption (x = 1–4) and thermodynamic stability on the CeO2(111) surface: a first-principles investigation

We present an ab initio investigation of the interaction between methane, its dehydrogenated forms and the cerium oxide surface. In particular, the stoichiometric CeO2(111) surface and the one with oxygen vacancies are considered. We study the geometries, energetics and electronic structures of various configurations of these molecules adsorbed on the surface in vacuum, and we extend the analysis to realistic environmental conditions. A phase diagram of the adsorbate–surface system is constructed and relevant transition phases are analyzed in detail, showing the conditions where partial oxidation of methane can occur.

Effect of Conformational Symmetry upon the Formation of Cysteine Clusters on the Au(110)-(1 × 1) Surface: A First-Principles Study

The physical properties of cysteine clusters formed on the Au(110)-(1 × 1) surface are investigated by means of density-functional theory. We take into account the clusters formed from three types of rotational conformers of cysteine; for each structure we investigate the protonated and unprotonated forms. Our investigations are based on three different geometrical models for the surface–adsorbate system, allowing us to describe the formation of new nucleation centers on the surface as well as the properties of long chains of cysteine molecules adsorbed on the Au(110)-(1 × 1) surface. We point out significant differences between the relaxed structures formed by each conformer as well as the specific physicochemical properties leading to formation of new nucleation centers compared with those of already formed large cysteine structures. In particular, we emphasize the role of the NH2 group of cysteine in these processes, by correlating our data with its relative position with respect to the gold surface.

Open-boundary cluster model implemented in first-principles calculations for electronic excited states of an adsorbate-surface system

Our recently developed open-boundary cluster model (OCM), which allows us to calculate electronic states of a semi-infinite adsorbate-surface system with a finite-small cluster, has been implemented in first-principles calculations to investigate excited states of a real system of a low-coverage Cs/Cu(111). The first-principles calculations are based on a real-space density functional theory (DFT) approach, and the Cs/Cu(111) system is reasonably represented in terms of a cluster of CsCu13 within the OCM approach. An absorption spectrum and the lifetime of excited states of the system are calculated successfully within the linear-response approximation, and the computed results qualitatively agree with experimental observations. Such excited properties are difficult to calculate by using a conventional cluster model (CCM) approach. Despite these advantages, the OCM-DFT approach requires a computational cost almost identical to the cost of CCM.

Surface adsorbate vibrations explored by infrared spectroscopy and DFT cluster calculations at the anharmonic level: CO on Cu(100)

The vibrational properties of the CO/Cu(100) surface adsorbate system have been explored by infrared spectroscopy and DFT cluster calculations. We show that all four fundamental, FT(x,y), FR(x,y), FT(z) and ν(C-O), vibrational modes are very well reproduced with respect to experiments by the present calculations and they are at the highest level reported to date. Our work demonstrates that it is essential to include both anharmonicity and cluster relaxation when modeling the CO/Cu(100) system. The absence and presence of binary modes: 2 ×ν(C-O) and FT(z) + ν(C-O) in our experimental data are discussed as well.

Ultrafast studies of electronic processes at surfaces using the laser-assisted photoelectric effect with long-wavelength dressing light

We show that ultrafast surface science studies using the laser-assisted photoelectric effect can benefit from longer-wavelength infrared dressing beams. We compare soft-x-ray photoemission from a Pt(111) surface dressed by 1300 and 780 nm light. Using 1300 nm light, the amplitude of the laser-assisted photoelectric signal is enhanced sevenfold compared with 780 nm. This allows lower dressing laser intensity to be used, which dramatically suppresses undesirable processes such as above-threshold photoemission, desorption, and distortion of the photoemission spectrum due to space charge. This work enables ultrafast studies of surface-adsorbate systems and attosecond electron dynamics over a wider energy range.

Direct Measurement of Core-Level Relaxation Dynamics on a Surface-Adsorbate System

The coupling between electronic states in a surface-adsorbate system is fundamental to the understanding of many surface interactions. In this Letter, we present the first direct time-resolved observations of the lifetime of core-excited states of an atom adsorbed onto a surface. By comparing laser-assisted photoemission from a substrate with a delayed Auger decay process from an adsorbate, we measure the lifetime of the 4d(-1) core level of xenon on Pt(111) to be 7.1+/-1.1 fs. This result opens up time-domain measurements of surface dynamics where energy-resolved measurements may provide incomplete information.

Laser-assisted photoemission from surfaces

We investigate the laser-assisted photoelectric effect from a solid surface. By illuminating a $\mathrm{Pt}(111)$ sample simultaneously with ultrashort 1.6 and $42\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ pulses, we observe sidebands in the extreme ultraviolet photoemission spectrum, and accurately extract their amplitudes over a wide range of laser intensities. Our results agree with a simple model, in which soft x-ray photoemission is accompanied by the interaction of the photoemitted electron with the laser field. This strong effect can definitively be distinguished from other laser surface interaction phenomena, such as hot electron excitation, above-threshold photoemission, and space-charge acceleration. Thus, laser-assisted photoemission from surfaces promises to extend pulse duration measurements to higher photon energies, as well as opening up measurements of femtosecond-to-attosecond electron dynamics in solid and surface-adsorbate systems.

Interplay between steps and nonequilibrium effects in surface diffusion for a lattice-gas model of O∕W(110)

The authors consider the influence of steps and nonequilibrium conditions on surface diffusion in a strongly interacting surface adsorbate system. This problem is addressed through Monte Carlo simulations of a lattice-gas model of OW(110), where steps are described by an additional binding energy EB at the lower step edge positions. Both equilibrium fluctuation and Boltzmann-Matano spreading studies indicate that the role of steps for diffusion across the steps is prominent in the ordered phases at intermediate coverages. The strongest effects are found in the p(2x1) phase, whose periodicity Lp is 2. The collective diffusion then depends on two competing factors: domain growth within the ordered phase, which on a flat surface has two degenerate orientations [p(2x1) and p(1x2)], and the step-induced ordering due to the enhanced binding at the lower step edge position. The latter case favors the p(2x1) phase, in which all adsorption sites right below the step edge are occupied. When these two factors compete, two possible scenarios emerge. First, when the terrace width L does not match the periodicity of the ordered adatom layer (LLp is noninteger), the mismatch gives rise to frustration, which eliminates the effect of steps provided that EB is not exceptionally large. Under these circumstances, the collective diffusion coefficient behaves largely as on a flat surface. Second, however, if the terrace width does match the periodicity of the ordered adatom layer (LLp is an integer), collective diffusion is strongly affected by steps. In this case, the influence of steps is manifested as the disappearance of the major peak associated with the ordered p(2x1) and p(1x2) structures on a flat surface. This effect is particularly strong for narrow terraces, yet it persists up to about L approximately 25Lp for small EB and up to about L approximately 500Lp for EB, which is of the same magnitude as the bare potential of the surface. On real surfaces, similar competition is expected, although the effects are likely to be smaller due to fluctuations in terrace widths. Finally, Boltzmann-Matano spreading simulations indicate that even slight deviations from equilibrium conditions may give rise to transient peaks in the collective diffusion coefficient. These transient structures are due to the interplay between steps and nonequilibrium conditions and emerge at coverages, which do not correspond to the ideal ordered phases.

Laser-Assisted Photoelectric Effect from Surfaces

We report the first observation of the laser-assisted photoelectric effect from a solid surface. By illuminating a Pt(111) sample simultaneously with ultrashort 1.6 eV and 42 eV pulses, we observe sidebands in the extreme ultraviolet photoemission spectrum. The magnitude of these sidebands as a function of time delay between the laser and extreme ultraviolet pulses represents a cross-correlation measurement of the extreme ultraviolet pulse. This effect promises to be useful to extend extreme ultraviolet pulse duration measurements to higher photon energies, as well as opening up femtosecond-to-attosecond time-scale electron dynamics in solid and surface-adsorbate systems.

Interaction of S, SH and H 2S with Ag(1 0 0)

A theoretical study of the interaction of S, SH, and H 2S with Ag(1 0 0) has been carried out using ab initio configuration interaction (CI) theory. The adsorbate–surface system is described by an embedded cluster model. It is found that S adsorption at the fourfold hollow site is the most stable with an adsorption energy of 77.2 kcal mol −1. For SH adsorption, atop, bridge and hollow sites have nearly identical adsorption energies, 43.1 kcal mol −1, 42.9 kcal mol −1 and 42.7 kcal mol −1, respectively, but very different equilibrium geometries. Adsorption energies of H 2S on Ag(1 0 0) range from 3.2 kcal mol −1 to 4.8 kcal mol −1, with the atop site favored by 1.4 kcal mol −1. Dissociation of H 2S to desorbed H 2 and adsorbed S is predicted to be exothermic by 32.6 kcal mol −1.

X-ray standing waves imaging

Modern disciplines in materials science such as coating technology, microelectronics, photovoltaics, sensors, catalysis, and nano-technology, and the increasing complexity of the systems studied have created an insatiable need for more and more powerful analytical tools in surface and interface analysis. An excellent review about current problems and solutions in surface science is presented in the special edition of the 500th issue of Surface Science [1]. The prime information demanded from any system under study is its chemical composition and atomic structure. Without this knowledge, all other properties cannot be fully understood. The paper by Zhang et al. [2] in this issue shows how this information can be obtained from crystalline surface adsorbate systems with the help of the X-ray standing wave (XSW) technique, in a new and very direct way that completely removes the guesswork. Equally exciting is the fact that this has been done to reveal the structure of adsorbates on a solid surface under aqueous solution. Indeed, the XSW technique is one of the few that can be applied to probe such ‘‘buried interfaces’’. The last decades have seen the advent of very powerful techniques, such as scanning tunneling microscopy (STM) [3], transmission electron microscopy (TEM) [4], electron spectroscopy for chemical analysis (ESCA) [5], grazing incidence X-ray diffraction (GID) [6], and more, which all quickly had a strong impact on surface/interface analysis. Hardly noticed went the birth of another method already 40 years ago: the X-ray standing wave technique. The dynamical theory of X-ray diffraction [7] predicts that during Bragg reflection from a crystal, due to the interference of the incoming and reflected X-ray wave, an X-ray standing wave is created, which can be shifted through the unit cell of the diffracting crystal by scanning through the Bragg reflection condition in angle (or energy). Boris Batterman reported in a landmark paper that the X-ray fluorescence of the atoms of the germanium crystal he used, if recorded as a function of the angle of the incident radiation while passing the range of strong reflection, bears the signature of the movement of the exciting X-ray standing wave [8]. In a subsequent paper, he demonstrated five years later that on the other hand the characteristic X-ray fluorescence radiation of foreign, in this case As dopant atoms in silicon, monitored as a function of Bragg-angle while traversing the range of strong Bragg reflection bears the signature of the position of the As atoms within the unit cell of the host crystal [9].

Theoretical study of the UV-induced desorption of molecular oxygen from the reduced TiO2 (110) surface

We investigate the UV-induced desorption of O2 from a reduced TiO2 rutile surface. The desorption mechanism considered is a direct, optical excitation of the adsorbate-substrate complex. The low-lying excited electronic states of the adsorbate-surface system and the transition dipole moments are calculated with a cluster model. We also estimate the effects of nonadiabatic coupling on the desorption yield. As a consequence of the localized nature of the adsorption process, ground state properties calculated with the cluster model agree well with previous results obtained with a slab model. To reduce the size of the calculation, we use optimal orbitals for each of the interacting electronic states, followed by transformation to biorthonormal orbital sets. Comparison with experimental photodesorption cross-section data shows excellent agreement for photon energies close to the vertical transition energy. At higher energies, we speculate that substrate-mediated electronic excitation mechanisms must be included in the model.

First-principle calculations of the experimental vibrational spectrum of a surface adsorbate: anharmonic resonance coupling between fundamental and binary modes.

By including relevant physical properties in our modeling of a surface adsorbate system we can, from first principles, correctly calculate all experimentally observed features in the vibrational spectrum of CH3CH2O- and CD3CH2O- adsorbed on Cu(100). That is, we reproduce the number of observed modes, the vibrational frequencies, and intensities of the modes including the presence/absence of binary overtone and combination modes. No scaling was performed. Our calculations show that the anharmonic terms of the potential energy surface of free ethanol are transferrable to the corresponding surface adsorbate, ethoxy, while the harmonic terms are not.