Wilson Agerico Diño

Many body effects in elementary processes at metal surfaces

This review consists of a brief survey of some elementary surface processes that exhibit many body effects. We begin by discussing the electron ground state of dilute magnetic alloys and the observation of this state in real space at surfaces. We provide a short history of the Kondo effect and extend the analysis of this effect in the bulk to dilute magnetic impurities at metal surfaces. Next, we discuss how these metallic electrons behave when they are perturbed by external fields on femtosecond time scales for both bulk and surface-state electrons. We follow this with a brief discussion of how the dynamics of tunneling electrons affects the translational and rotational motion of molecular adsorbates on metal surfaces.

Potential Energy of H2 Dissociation and Adsorption on Pt(111) Surface: First-Principles Calculation

We investigate the interaction of molecular hydrogen with the Pt surface. In particular, we calculate the potential energy surfaces (PESs) corresponding to the dissociative adsorption of H2 at different symmetric sites on the Pt(111) surface and for parallel and perpendicular orientations of H2 with respect to the surface, on the basis of the density functional theory. The PES results show the dependence of the activation barriers on the H2 adsorption site and orientation relative to the surface. For parallel orientations, the barrier is lowest (almost zero) when the H2 center of mass (CM) is directly above the top site while the H atoms are directed towards the hcp and fcc hollow sites. The activation barriers for the perpendicular orientation are always much higher than those for the parallel orientation, which indicates that the parallel orientation is favored for H2 adsorption. This result also suggests that an incident H2 initially in a nonparallel (e.g., perpendicular) orientation tends to reorient itself towards a parallel orientation where it is easier for H2 to be adsorbed.

Surface science-based reaction design: increasing the ortho-para hydrogen conversion yield via molecular orientation, a case study

Abstract One of the ultimate goals of surface science is to be able to design and control reactions as they progress on surfaces. This entails an atomic-level understanding of the fundamental principles (elementary processes) underlying the bond-making and bond-breaking at surfaces. Our current understanding has gained significantly from systematic experimental and theoretical studies on such benchmark systems as the interaction of hydrogen with metal surfaces. Yet, fundamental, examples of surface science-based reaction design are extremely few. Here, we consider the ortho – para ( o – p ) H 2 conversion as a case study. We invoke two processes derived from fundamental, surface science insights, based on the effect of molecular orientation on the hydrogen-solid surface reaction, viz., dynamical quantum filtering (DQF) and Steering , and apply them to enhance the o – p H 2 conversion yield/rate. The orientation dependence of the o – p H 2 conversion ( steric effect , SE) dictates that cartwheel-like rotating (CLR) H 2 will have a higher rate of conversion than helicopter-like rotating (HLR) H 2 . Applying DQF, we can then prepare rotationally aligned H 2 , doing either HLR or CLR. This enables us to increase the o – p H 2 conversion yield.

Spatial and spectroscopic profiles of the Kondo resonance for magnetic atoms on metal surfaces

Manifestations of the Kondo effect on an atomic length scale on and around magnetic atoms adsorbed on a nonmagnetic metal surface differ depending on which spectroscopic mode the scanning tunneling microscope is operated. In a previous report [T. Kawasaka, H. Kasai, and A. Okiji, Phys. Lett. A 250, 403 (1998)], we suggested the possibility that the Kondo effect can be directly observed as protrusions (peaks) in the spatial distribution of the tunneling current. By comparing the spatial distributions of the conduction electron and localized electron wave function, we show that the protrusions (peaks) observed in the spatial distribution of the tunneling current correspond to the resonances observed in the differential conductance spectra. Furthermore, we investigate the temperature dependence of the spatial distribution of the tunneling current and of the differential conductance to study the electronic properties of various metal–adatom systems.

Effects of Rotational State Excitations on the Dissociative Adsorption Dynamics of D2/Cu(111)

Recent time-of-flight experiments show a non-monotonous dependence of the dissociative adsorption/sticking * probability of D 2 (H 2 )/Cu(111) on the initial rotational state. There is an observed decrease in the sticking probability with a slight increase in the initial rotational state quantum number j. As j is increased further, the sticking probability eventually increases. We show that, to account for this interesting behaviour, there are two opposing factors working for and against the adsorption process: STEERING EFFECT and ENERGY TRANSFER EFFECT. The first is due to a dynamical reorientation of the molecule, and the latter is due to the coupling of the rotational motion to the translational motion along the reaction path. To illustrate these two effects, we performed quantum mechanical model calculations using a variation of the coupled-channel method, a physically realistic model potential based on available potential energy (hyper-) surface plots for D 2 (H 2 )/Cu for two different orientations-...

Ab Initio Study of H2 Desorption from Magnesium Hydride MgH2 Cluster

In the present study, we investigate H 2 desorption from a magnesium hydride MgH 2 cluster by performing total energy calculations based on the density functional theory (DFT). We calculate the potential energy surface (PES) for the H 2 desorption from a (MgH 2 ) 5 cluster. According to the total atomic charges of the Mg and H atoms at the initial (MgH 2 ) 5 cluster, the Mg–H bond is ionic-like. Because of this, the activation barrier for the H 2 desorption is considerably high (3.30 eV). Moreover, the structural relaxation for the system, allowing for only the two Mg atoms closest to the two desorbing H atoms, does not largely affect the potential energy of the system. The results indicate that the ionic-like Mg–H interaction is related to the thermodynamical stability of MgH 2 .

Rotational and vibrational coupling effects on the dissociative adsorption and associative desorption dynamics of D2/Cu(111)

We investigate and discuss the effects of the coupling between molecular rotation and vibration on the dissociative adsorption and associative desorption dynamics of D 2 /Cu(111), with the aid of a model Hamiltonian and the concept of a reaction path. In our model Hamiltonian, we take into account the molecular orientation dependence of the reaction path curvature in the kinetic energy term. We calculate for the dissociative adsorption probability of D 2 as a function of its initial translational energy, vibrational and rotational states, using the coupled channel method. By invoking the principle of microscopic reversibility, we obtain the corresponding associative desorption probability results for D 2 as a function of its final translational energy, vibrational and rotational states from the dissociative adsorption probability results. Furthermore, we show that the calculation results qualitatively agree with experimental results.

Rotational Alignment in the Associative Desorption Dynamics of Hydrogen Molecules from Metal Surfaces

We report that the alignment factors of hydrogen molecules desorbing in the vibrational ground state from Cu(111) and Pd(111) vary nonmonotonically with the final rotational state j depending on the translational energy. Upon averaging over the Boltzmann distribution of total kinetic energies at surface temperatures relevant to existing experiments, the resulting alignment factor value corresponds to a small preference for helicopter-like rotation.

Dynamical quantum filtering in hydrogen-surface reactions

Abstract We report on how surfaces that adsorb hydrogen could act as rotational quantum state filters and cause, for example, D2 molecules desorbing in the vibrational ground state from Cu(111) to exhibit strong rotational alignment. For low final translational energies, we found that desorbing D2 molecules have rotational alignment factor values corresponding to cartwheel-type rotational preference. As the final translational energy increases, the corresponding alignment factor increases initially to values corresponding to helicopter-type rotational preference and then, eventually, decreases to values almost compatible with a spatially isotropic distribution, as the translational energy increases further.

Combined effect of molecular rotational and surface vibrational excitations on the dissociative adsorption dynamics of D2Cu(111)

Abstract To study the surface temperature dependence of the dynamics of dissociative adsorption processes (e.g. D 2 Cu (111) ), we invoked the Born-Oppenheimer approximation and did quantum mechanical calculations using the coupled-channel method. Based on physical arguments and our numerical results, we show that in order to explain experimental observations, it is necessary to consider the coupling between the rotational degree of freedom of the impinging molecule and the vibrational degree of freedom of the surface.