Itai Panas

On the cluster convergence of chemisorption energies

A new rule is suggested for calculating chemisorption energies using the cluster model. This rule is built on the realization that relatively large clusters (50 atoms and more) often need to be prepared for bonding by making an excitation to a proper bonding state (such a state will always be easily accessible in an infinite cluster). For hydrogen chemisorption, this bonding state must have a singly occupied orbital of the same symmetry as the hydrogen 1 s orbital. The new rule is applied to hydrogen chemisorption in the hollow sites of Ni(100) and Ni(111). When a cluster is prepared in a bonding state, even quite small clusters (≈ 10 atoms) give chemisorption energies in reasonable agreement with experimental surface results.

Model studies of the chemisorption of hydrogen and oxygen on nickel surfaces. I. The design of a one-electron effective core potential which includes 3d relaxation effects

The present article is the first in a series describing investigations of the chemisorption and dissociation of hydrogen and oxygen on nickel surfaces. This paper deals with methodological questions, mainly concerning the form and properties of the crucial one-electron effective core potential (ECP) on nickel, which is used to describe chemisorption situations with sufficiently large nickel—adsorbate distances. Atomic chemisorption in the fourfold position on the Ni(100) surface is chosen as the model problem for testing and designing the nickel ECP. All-electron calculations on Ni4 and Ni5 with and without adsorbed atoms have been performed to produce reference results. Particularly noteworthy of the all-electron results is the large effect of 3d relaxation. To mimic cluster 3d relaxation effects a diffuse attractive 3d projection operator was added to the original standard form of the ECP.

A theoretical study of CHx chemisorption on the Ni(100) and Ni(111) surfaces

Cluster model calculations have been performed for CH(x), x = 0-3, chemisorbed on Ni(100) and Ni(111). The predicted chemisorption energies, at the present level of theory, based on bond-prepared clusters for Ni(100) are for carbon 150 kcal/mol, for CH 136 kcal/mol, for CH2 91 kcal/mol and for CH3 46 kcal/mol. The corresponding energies for Ni(111) are for CH 120 kcal/mol, for CH2 88 kcal/mol and for CH3 49 kcal/mol. These chemisorption energies lead to similar stabilities for all CH(x) fragments on both Ni(100) and Ni(111). Large basis sets multi-reference correlation treatments are found to be very important in particular for the multiply bonded species. The vibrational C-H stretching frequencies predicted for CH(x) on Ni(111) are for CH 3054 cm-1 (2980 cm-1), for CH2 3204 cm-1 and for CH3 2709 cm-1 (2680 cm-1), where the available experimental values are given in parenthesis. The predicted ionization spectra of adsorbed CH(x) are also in general agreement with experimental findings.

Model studies of the chemisorption of hydrogen and oxygen on nickel surfaces

Atomic chemisorption of hydrogen and oxygen on the Ni(100) surface has been studied using an Effective Core Potential (ECP) approach described in a previous paper. Clusters of up to 50 nickel atoms have been used to model the surface. The computed chemisorption energies are 62 kcal/mol (exp. 63 kcal/mol) for hydrogen and 106 kcal/mol (exp. 115–130 kcal/mol) for oxygen. Correlating the adsorbate and the cluster-adsorbate bonds is extremely important for obtaining accceptable results, particularly for oxygen. Reasonable convergence of chemisorption energies is obtained with 40–50 cluster atoms for both hydrogen and oxygen. For hydrogen the addition of a third cluster layer stabilizes the results considerably. Both hydrogen and oxygen are adsorbed at (or close to) the four-fold hollow site. The calculated barriers for surface migration are also in good agreement with the experimental estimates. The calculated equilibrium heights above the surface are on the other hand too high compared with experiments. This disagreement is believed to be due to core-valence correlation effects, which are not incorporated in the present ECP. The cluster convergence for the height above the surface is much slower than for the chemisorption energy.

N2O2, N2O2- AND N2O22- : STRUCTURES, ENERGETICS AND N-N BONDING

The density functional theory and the ab initio quantum chemistry methods, Becke3LYP and CASPT2, have been employed to determine structures and energetics of neutral and anionic N2O2 species. When going from a neutral NO dimer to an anionic species the N---N bond lengths decrease drastically and the corresponding N---N frequencies increase. The relative stabilities of the different N2O2− isomers suggest an ONNO structure in a Trans configuration to be the most stable mono valent anion. The formation energies of 1.4–1.7 eV relative to the NO + NO− asymptote are in agreement with experiments. Excitation energies are determined and specific formation channels for three N2O2− isomers are discussed. The hyponitrite ion, N2O22−, is also studied. Its total energy is 2.7–2.8 eV above the total energy of two NO−. In order to connect to an experimental study of NO adsorption on MgO the ability of two NO molecules to form a complex with Mg and Mg+ is investigated.

A THEORETICAL-STUDY OF THE PEROXO AND SUPEROXO FORMS OF MOLECULAR-OXYGEN ON METAL-SURFACES

Simple cluster model calculations have been performed for the peroxo and superoxo forms of O2 on metal surfaces. The ionization spectra of superoxo-O2 on Ag(110) and Pt(111) and for peroxo-O2 on Ag(110) are predicted in good agreement with the most reliable experimental measurements. It is shown that the large qualitative difference between these spectra can be used to decide experimentally whether O2 is chemisorbed in the peroxo or superoxo form. The O---O bond distance is calculated to be 1.32 A for superoxo-O2 and 1.43 A for peroxo-O2 in good agreement with the most recent NEXAFS measurements on Pt(111) and Ag(110), respectively. A problem in a recent analysis of a UPS spectrum for obtaining the O---O bond distance is pointed out.

The mechanism for the O2 dissociation on Ni(100)

The dissociation of O2 on Ni(100) has been studied using a cluster model approach. The three principally different reaction pathways, over an on‐top position, over a bridge position, and over a fourfold hollow position, were considered. The dissociation mechanisms were found to be very similar for these pathways. In the entrance channel a chemisorbed, peroxo‐form, of molecular O2 is first formed, which is strongly bound to the Ni(100) surface by two polar covalent bonds. The binding energy at the fourfold hollow site is found to be 78 kcal/mol, which is about 20 kcal/mol larger than for the other two sites, and much larger than the chemisorption energies for the experimentally observed O2 on Pt(111) and Ag(110). The reason for this difference is discussed. In a simplified valence‐bond picture the wave function of this molecularly bound O2 has a large component of a πu to πg excited state of O2. The dissociation of O2 then proceeds by two stepwise electron transfers from the surface over to the O2 3σu orbital, which completes the breaking of the O–O bond. In this latter process the energy passes over a local barrier, which is still far below the long distance asymptote, however. The local barrier height is much higher for the fourfold hollow dissociation, 35 kcal/mol over the local molecular minimum, than for the other two pathways, where the barrier height is only 6–8 kcal/mol. The 3d orbitals on nickel remain passive for all the three dissociation pathways, which is in line with the fact that also nontransition metals dissociate O2. This behavior is in contrast to the dissociation of H2 on Ni(100), where the 3d orbitals play a key role for the on‐top dissociation.

Vibrational motion and geometrical structure in adsorbed CO studied by core level photoelectron spectroscopy

High resolution core level spectra from CO adsorbed on clean and hydrogen precovered Ni(100) and CO adsorbed on Cu(100) are presented. The core level binding energy is shown to be sensitive to the adsorption site. Cluster calculations reproduce the general trend of the binding energy shifts between the on top and hollow sites of CO/Ni(100). In the coadsorption system CO/H/Ni(100) three different adsorption sites have been observed with a maximum binding energy shift of 2.6 eV for the Ols level. The temperature dependence of the Cls and Ols line profiles in CO/Ni(100) has been carefully investigated. The temperature dependent broadening is due to thermal excitations of frustrated translations parallel to the surface. The spectra from CO on Cu(100) show no temperature dependence below 200K.

Aspects of density functional theory in ab initio quantum chemistry: external correlation for free

The restricted Hartree-Fock method is subject to a first-order perturbation theory regularization of the electron repulsion integrals representation, to mimic the electron-electron correlation hole effect. The efficiency of this regularization is demonstrated for electron affinities and ionization energies of atoms, and on atomization energies of molecules. The accuracy of the method is comparable to that of modern density functional approaches as it provides significant improvement on standard Hartree-Fock results. Conceptual similarities with density functional theory are discussed, as are the implications for ab initio calculations on large systems.

Infrared spectra of cis and trans-(NO)2− anions in solid argon

Laser-ablation of over 20 different metal targets with concurrent 10 K codeposition of Ar/NO mixtures produces metal independent infrared bands at 1589.3 cm−1 due to (NO)2+, a new absorption at 1221.0 cm−1, and a band set at 1300.3, 1222.7, 884.4 cm−1. The latter bands decrease more on annealing than the 1221.0 cm−1 band. Isotopic substitution (14NO,15NO, 15N18O, and mixtures) shows that these new vibrations involve two equivalent N–O oscillators, which identifies two new (NO)2 species. The excellent agreement with frequencies, intensities, and isotopic frequency ratios from density functional theory calculations substantiates assignment of the 1221.0 cm−1 band to trans-(NO)2− and the three band set to cis-(NO)2−. The observation of a weak combination band at 2492.0 cm−1 further substantiates assignment of the two N–O stretching modes in cis-(NO)2−