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Interactions of N2 at oxide surfaces are important for understanding electrocatalytic nitrogen reduction reaction (NRR) mechanisms. Interactions of N2 at the polycrystalline vanadium oxide/vapor interface were monitored at room temperature and total pressures up to 10−1 Torr using Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). The oxide film was predominantly V(IV), with V(III) and V(V) components. XPS spectra were acquired in environments of both pure N2 and equal pressures of N2 and H2O vapor. In pure N2, broad, partially resolved N1s features were observed at binding energies of 401.0 and 398.7 eV, with a relative intensity of ∼3:1, respectively. These features remained upon subsequent pumpdown to 10−9 Torr. The observed maximum N surface coverage was ∼1.5 × 1013 cm−2—a fraction of a monolayer. In the presence of equal pressures of H2O, the adsorbed N intensity at 10−1 Torr is ∼25% of that observed in the absence of H2O. The formation of molecularly adsorbed H2O was also observed. Density functional theory-based calculations suggest favorable absorption energies for N2 bonding to both V(IV) and V(III) cation sites but less so for V(V) sites. Hartree–Fock-based cluster calculations for N2–V end-on adsorption show that experimental XPS doublet features are consistent with the calculated shake-up and normal, final ionic configurations for N2 end-on bonding to V(III) sites but not V(IV) sites. The XPS spectra of vanadium oxide transferred in situ between electrochemical and UHV environments indicate that the oxide surfaces studied here are stable upon exposure to the electrolyte under NRR-relevant conditions.

Ab initio cluster model wavefunctions for Al and Pt surfaces have been used to investigate the response of metal surfaces to the presence of point charges above the surface. The dipole moment curves, μ(z), as a function of the distance of the point charge above the surface have been used to characterize the metallic response. There are three regions where the behavior of μ(z) is different. In the region where z>∼15–20 bohr, the asymptotic behavior given by image charge theory holds. In the region where z

The common interpretation of the shifts of the core level binding energies, BEs, of atoms in molecules is based on considerations of the effective charges of the atoms in the molecule. An isolated atomic ion will have a different core level BE from a neutral atom and core level BE shifts have been interpreted as reflecting the charge of a core ionized atom as though it were a free ion. Sometimes, the effect of the presence of electric fields due to the effective or nominal charge of the surrounding atoms is also taken into account. It is shown that, for organic compounds, the 2s involvement in the bonding has an important effect on the shift of core level binding energies. Following standard chemical interpretation, we refer to this contribution as 2s hybridization. Specifically, we consider the shift of the N(1s) binding energy in a series of four compounds. However, we expect this hybridization mechanism to be general for the proper interpretation of X-ray photoelectron spectroscopy.

A new method, termed CDA, for charge distribution analysis, to estimate net charges of atoms in molecules or clusters is proposed. The method is based on well defined physical quantities and avoids many of the shortcomings of the Mulliken population analysis while keeping the idea that charge or electronic density on atoms is defined through their basis functions. Several examples, including the CH4 molecule, bulk MnO and chemisorption of O and CN on Cu(100), are presented.

The ab initio Hartree-Fock cluster model method has been used to investigate the existence of excitons in bulk MgO. Excitations either from the oxygen 2p valence band orbitals or from the oxygen 1s core orbitals to unoccupied cluster orbitals always lead to delocalized orbitals representative of the MgO conduction band. However, when the electron is excited from a 1s or 2p core level of Mg, the occupied level is found to be localized around the atom with the core hole. These results, for the first time, give strong direct evidence for excitonic states in oxide crystals.

An ab initio molecular cluster model approach has been used to investigate the adsorption geometry and the nature of the interaction of the NO molecule with the Cu2O(111) surface. The two possible NO orientations, N‐ and O‐down, have been studied for adsorption on both onefold and threefold surface positions. We show that, in all cases, the most important contribution to the bonding is the electrostatic interaction, with negligible or small (depending on the orientation and position) contributions from chemical effects. In the monocoordinated position it is found that the two NO orientations exhibit opposite vibrational frequency shift with respect to the free NO molecule. For the three‐coordinated site the vibrational shift is found to be nearly zero. In the first case, when NO is bound through the O atom, its shift is negative, as observed experimentally on Cu2O surfaces. Therefore, the present model calculations suggest that the species which is experimentally observed is O‐down chemisorbed NO on a one...

Ab initio cluster model wave functions of increasing complexity have been obtained for alkaline‐earth oxides MgO to BaO. Using a wave function corresponding to the superposition of the electronic densities of the cations and anions obtained in a Madelung field, an ab initio version of the ionic model is obtained. This simple ionic model is improved with self‐consistent field (SCF) and large multireference configuration interaction (CI) wave functions. Analysis of these different types of wave functions shows that the ground state of these oxides is strongly ionic with the ideally ionic configuration having a weight of ≊95% in the total CI wave function. With all the criteria that we have used, the degree of charge transfer from O2− to M2+ is always very small. Furthermore, the instantaneous electron–electron interactions (correlation effects) treated in the CI wave function have been found to be mainly intra‐atomic and especially important for the 2p electrons of O2−. Point charges were used to represent the contribution to the Madelung field made by the atoms not explicitly included in the cluster; they were chosen to reproduce the Madelung field arising when a fully ionic crystal is assumed. Sets of scaled point charges which correspond to a smaller Madelung field were also used. The cluster model results were not significantly changed when the point charges were reduced by as much as a factor of 2 from the values for the fully ionic crystal. This is strong evidence that the ionicity of the crystals results from chemical forces and is not due to the use of an assumed Madelung field external to the cluster.

We investigated the 3d5/2 core-level binding energy of Ru in Ru nanoislands spontaneously deposited on a Pt(111) electrode [Pt(111)/Ru], and the binding energies of 3d5/2 iodine and 1s CO adsorbed on Pt(111)/Ru by the use of X-ray photoelectron spectroscopy. Both iodine and CO were used as surface probes of the electronic properties of Pt(111)/Ru. Little difference was found in the binding energy of Ru in Pt(111)/Ru and in Ru(0001). However, the addition of Ru to Pt(111) induces major changes in the core-level binding energies of chemisorbed iodine and CO as referenced to those adsorbed on Ru(0001). We conclude that the iodine 3d5/2 and CO 1s C core levels experience higher electronic charge on Pt(111)/Ru than on Ru(0001), suggesting a charge transfer from Pt to Ru, or to a Ru-I “surface molecule” within the deposit. The charge transfer from Pt to Ru is in agreement with the result of previous in situ electrochemical NMR investigations [P.K. Babu, H.S. Kim, A. Wieckowski, E. Oldfield (2003) J. Phys. Chem. B 107:7595] and confirms the general trend of reduction in the density of states of Pt due to alloying with Ru [J. McBreen, S. Mukerjee (1995) J. Electrochem. Soc. 142:3399]. Theoretical calculations are in progress to further interpret the origin of the binding-energy shifts observed in this study.

The interaction of ultrathin Pb films with Ru(0001) and Pd(111) has been studied with x-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). XPS and TDS show that at room temperature Pb grows onRu(0001) via a Stranski-Krastanov mechanism, i.e., after completion of the first adlayer of Pb on Ru(0001), further Pb deposition leads to the formation of three-dimensional islands. Upon annealing to 500 K, the overlayer Pb undergoes significant clustering with the extent dictated by the initial coverages. XPS results show that the Pb 4f 7 / 2 core level binding energy (BE) increases slightly (+0.3 eV) as the Pb coverage increases from 0.07 ML to multilayers. When the first monolayer of Pb/Ru(0001) has been completed, the Pb 4f 7 / 2 BE has nearly reached the BE value for bulk Pb; it is only 0.1 eV lower. At ∼ 2 ML, the Pb 4f 7 / 2 BE is equivalent to that of bulk Pb. In contrast, at room temperature, Pb alloys with the Pd(111) surface, the Pd 3d 5 / 2 BE increasing ∼ 0.8 eV with coverage from 0.07- to 4.3-ML Pb. The Pb 4f 7 / 2 BEs also increase with increasing coverage reaching the bulk value of Pb at a coverage of ∼ 2 ML. The Pb 4f 7 / 2 BE at 0.07 ML is 0.55 eV smaller than that of bulk Pb. The lineshape and position of the x-ray induced Pd Auger features (M 4 , 5 VV) also display significant changes upon alloying. For Pb/Pd alloys, annealing to 500 K does not induce further changes in the Pd (M 4 , 5 VV) Auger features. The core level BE shifts are discussed in terms of possible initial and final state contributions. In particular, the initial state BE shift mechanisms of hybridization and environmental conduction band charge density are explained in the context of prior theoretical studies. It is shown that 6s→6p hybridization in Pb can be expected to be sufficiently large to contribute to the observed BE shifts.

Ab initio theoretical results are reported to determine the role of interatomic screening of the metal core hole in Mn 3s x-ray photoelectron spectra (XPS) of MnO. We present information that finally resolves the controversy about the relative importance of intra-atomic and interatomic effects to the XPS spectra of ionic manganese compounds. We focus on the transitions to high-spin 3s core hole states, using a nonorthogonal configuration interaction method. This method allows for a balanced treatment of configurations that involve different degrees of screening of the core hole. The differences between MnO and NiO are analyzed. Although at first glance the two XPS spectra seem to have similar features, the 3s hole final states have completely different characteristics. In MnO, interatomic screening of the core hole is found to play a minor role. This is in contrast with NiO, where the inclusion of interatomic screening of the metal core hole has been shown to be crucial for a proper explanation of the Ni 3s XPS. The main reason for the difference is an essentially atomic effect, namely the smaller electron affinity of Mn as compared to Ni. We have assigned the weak feature observed in the Mn 3s XPS spectrum of MnO at around 10 eV to be a charge-transfer-type high spin satellite.

Coadsorption of K and CO at the Cu(100) surface and adsorption and reaction of CO and CO2 at surface and defect sites of MgO are reviewed. The cluster technique is used to determine optimal adsorption geometries, the physical properties of the adsorbed species, in particular the vibrational frequencies, and the nature of the surface chemical bond. It is shown that the large red-shift of the CO ωe when K is coadsorbed on a metal surface can be explained only with very short K-CO distances. Chemisorption at oxide surfaces is also discussed and the important role of the surface defects in determining the reactivity of the MgO surface is shown.

Ab initio cluster model calculations on the interaction of CO and CO2 with NiO and MgO surface and defect sites are reviewed. NiO behaves very similarly as the non-transition metal oxide MgO towards CO chemisorption because the bonding of CO to these surfaces is dominated by electrostatic terms and involves a very modest σ donation or, in the NiO case, π back donation. The role of the surface defects in determining the reactivity of oxides toward CO and CO2 chemisorption is shown.

We present the result of polar angle resolved x¿ray photoemission spectroscopy on Al(111)/O and cluster calculations of the O(1s) binding energy (BE) for various model situations. In the experimental data two O(1s) peaks are observed, separated by 1.3 eV. The angular behavior (depth¿resolution) could indicate that the lower BE peak is associated with an O atom under the surface, and the higher BE peak with an O atom above the surface. Equally, it could indicate oxygen islands on the surface where the perimeter atoms have a higher O(1s) BE than the interior atoms. The cluster calculations show that the former interpretation cannot be correct, since an O ads below the surface has a higher calculated O(1s) BE than one above. Cluster calculations simulating oxygen islands are, however, consistent with the experimental data.

We report the results of ab initio molecular orbital self‐consistent field calculations on the ground state and low lying excited states of the PMDA molecule. Emphasis is placed on the consequences of localization of the molecular orbitals and on the relative positions of the n→π* and π→π* excited states. It is shown that reduction of the symmetry constraint from D2h to C2v or Cs has a large effect on the n→π* excitation but only a small one on π→π*. A study of the effect of using basis sets of different size is also presented.

This paper will review the use of ab initio Hartree–Fock wave functions for small metal/ligand clusters as a basis for the interpretation of processes related to molecular chemisorption. The primary concern will be with absorbate derived photoemission spectra (PES). It will be shown that quite small clusters can, indeed, give considerable insight into the structure, shifts, and character of this spectra. Ab initio molecular orbital theory is a particularly valuable approach since it permits one to relate these features of the PES to the nature of the adsorbate bonding in a direct way. The satellite structure originating from screening of the absorbate hole by metal electrons will be analysed for both core and valence levels. The decomposition of adsorbate shifts into initial and final state contributions will be considered (in order to understand the relationship of these shifts to the binding).

Restricted-Hartree-Fock, self-consistent-field calculations have been performed for the ground and ionic states of clusters of lithium and fluorine atoms. These clusters are appropriately charged to represent an ${\mathrm{Li}}^{+}$${\mathrm{F}}^{\ensuremath{-}}$ ionic crystal and point charges are used to represent the Madelung field due to ions not explicitly included in the cluster. Factors giving rise to core- and valence-level binding-energy shifts have been examined and separated into compressional, relaxational, and effective Madelung contributions. Many tests of ionicity are shown to be well satisfied, although this is often caused by fortuitous cancellations. Extra-ionic relaxation energies on both the cation and anion are shown to be small, and shakeup probabilities are calculated.

The molecular orbital cluster model has been used to study the vibrations of CO interacting with metal surfaces. In all cases the equilibrium CO geometry is taken normal to the surface with C directly above a metal atom; SCF and correlated CI wave functions have been obtained. For CO/Cu(100), we have studied the C–O and Cu–C stretch modes. The C–O frequency ωe and effective charge e* have been analyzed in terms of σ and π contributions; the adsorption site metal π, 3dπ, and 4pπ, donation to CO 2π* contributes in a major way to e*. The ωe and e* for the C–O stretch have been compared for one metal atom–CO clusters of Cu, Ni, and Fe. The ωe decrease in the order Cu, Ni, and Fe; the e* increase in the same order. This is to be expected from the order of the metal dπ to CO 2π* donation and bonding which is Fe>Ni>Cu.

Recent progress in preparing organic solids with ferromagnetic interactions between spins is reviewed. The three principal models are discussed together with relevant materials that have been prepared in Japan, at Columbia University, in Moscow, and at IBM, Almaden. These are compared with the molecular ferromagnets recently developed by Miller and Epstein, which are not strictly organic.

The stability of RbHF and RbH2O complexes has been investigated by performing Hartree–Fock calculations on Rb ⋅ HF, H ⋅ RbF, and Rb ⋅ H2O species in various orientations. Heats of interaction of 2–5 kcal/mol are typically obtained. In each system, the free radical prefers to approach the dipolar molecule at the negative end. This preference cannot be explained by a simple point‐dipole induced‐dipole model; the finite size of the dipole is important and produces the directional sensitivity. These results indicated that such species should be stable in no‐collision environments; such as the ultrahigh vacuum conditions in which RbH2O has been observed.

Local models on the basis of the surface cluster approach have been applied extensively in the study of surface and adsorbate systems. The energetic details of the adsorbate-substrate interaction can be characterized in these models by the constrained space orbital variation (CSOV) method. This method allows a detailed study of adsorbate binding in different surface sites. Further, consequences of the chemisorptive interaction for adsorbate valence photoemission and vibrational properties of adsorbed molecules can be studied.

To evaluate the importance of correlation effects in metallic and insulating polymers self‐consistent field (SCF) and configuration interaction (CI) calculations were performed for equidistant (metallic) and alternant (insulating) hydrogen rings. Two main effects–increasing the number of electrons and changing the distance between the hydrogen atoms–were studied. Minimal basis set calculations were performed for 6‐, 10‐, 14‐, 22‐, and 30‐membered rings, potential curves for the metallic and insulating H14 system have been computed with a double‐zeta basis supplemented by polarization functions. The correlation energy of one‐dimensional metallic hydrogen is estimated to be about 0.84 eV/atom compared with 0.54 eV in H2. For molecular hydrogen a van der Waals minimum was found for separation of two H2 molecules in the ring of about 5.5 a.u. The calculated value for the energy minimum suggests nonpairwise‐additive contributions to the van der Waals interaction energy of seven H2 molecules. Finally, the pressure to induce a transition from the molecular to the metallic phase was calculated to be about 1.8 Mbar both on the SCF and CI level.To evaluate the importance of correlation effects in metallic and insulating polymers self‐consistent field (SCF) and configuration interaction (CI) calculations were performed for equidistant (metallic) and alternant (insulating) hydrogen rings. Two main effects–increasing the number of electrons and changing the distance between the hydrogen atoms–were studied. Minimal basis set calculations were performed for 6‐, 10‐, 14‐, 22‐, and 30‐membered rings, potential curves for the metallic and insulating H14 system have been computed with a double‐zeta basis supplemented by polarization functions. The correlation energy of one‐dimensional metallic hydrogen is estimated to be about 0.84 eV/atom compared with 0.54 eV in H2. For molecular hydrogen a van der Waals minimum was found for separation of two H2 molecules in the ring of about 5.5 a.u. The calculated value for the energy minimum suggests nonpairwise‐additive contributions to the van der Waals interaction energy of seven H2 molecules. Finally, the press...

The cluster model approach has been used to compare the chemisorption properties, including vibrational frequencies (ωe) and dynamic dipole moments (e*), of C‐ and O‐bonded CO and N2 at an on‐top site of Cu(100). The binding energies and metal–adsorbate stretch ωe and e* are found to decrease in the order CO>N2>OC, with N2 expected to be bound and OC almost unbound; the metal–adsorbate bond distances and intra‐adsorbate stretch e* (including the sign) are in the opposite order, OC>N2>CO. While the C–O stretch frequency for C‐bonded CO is strongly reduced with respect to the free molecule, the frequencies for N2 and O‐bonded CO are found to be only slightly changed. The sequence in the strength of the interaction is explained by the different amount of metal‐to‐adsorbate charge transfer which is identified as the driving force behind the chemisorption process. Our theoretical results are compared with experimental data and the differences in the bonding of CO and N2 to transition metals are addressed.

For pt. III see abstr. A14127 of 1970. Multiconfiguration Hartree-Fock calculations are given to study term-splittings, electron-affinities and ionization energies for the 2p3 isoelectronic series (Z=6.0 to 30.0) and 2p4 series (Z=7.0 to 30.0). The contribution to the correlation energy of the configurations used has also been analysed. The predicted multiconfiguration Hartree-Fock term splitting ratios for the 2p3 configuration are in good agreement with experiment and in much better agreement than the Hartree-Fock values. This is also true for the individual 4S-2D and 4S-2P transition energies. For the 2p4 series, the predicted term splittings are not as good as for the 2p3 series. Ionization potentials for the 2p3 and 2p4 series, are given up to Z=30. The computed ionization potentials and the electron affinities of C and N are compared with experiment. The Z dependence of the computed ionization potentials is discussed. The results are not in as good agreement, relatively, for the electron affinities of C and N as for the ionization potentials but the absolute errors are roughly the same.

Multi-configuration Hartree-Fock calculations are reported to study the term splitting in the 1s22s22p2 configuration from Z = 70 to Z = 150. The wave functions include the Hartree-Fock configuration 1s22s22p2, the `quasi-degenerate' configuration 1s22p4, the single replacement configurations 1s22s3d2p2 and 1s22s22p4f, the double replacement configurations from the 2p shell 1s22s23l3lp (l, lp = 0, 1, 2) and, finally, the intershell configuration 1s22s3s2p3p. The separations of the 3P, 1D and 1S levels predicted from these functions are in very good agreement with experiment not only for the ratio (3P-1D)/(1D-1S) but also for the individual transitions. These functions have also been studied from Z = 60 to Z = 300 to compute the contribution to the correlation energy which arises from each configuration. For the 3P and 1D states the 1s22s3d2p2 configurations make a larger contribution to the correlation energy than does the 1s22p4 configuration at low Z. At higher Z the reverse is true. For the 1S state all the configurations except the 1s22p4 configuration make a small contribution to the correlation energy throughout the isoelectronic series.

Oscillator strengths for the lowest 1P transitions in Mg, Ca, Sr and Ba calculated from modest multi-configuration Hartree-Fock wavefunctions are reported. The authors values agree with experimental data within 10%, and show that the correlation between the valence electrons is far more important than the correlation due to the core polarization recently reported by Hameed (see abstr. A31662 of 1972).

The cluster model approach involves choosing a finite, and not exceptionally large, number of atoms to represent an extended, condensed matter, system of interest. Typically, the number of atoms included in the cluster is ~20, see for example Refs. 1 and 2, but larger numbers have been used.3 The electronic wave function for this cluster is obtained using the methods of ab initio molecular orbital, MO, theory; methods which were developed for Quantum Chemistry. 1,4 When electron correlation effects are not included, a self-consistent field, SCF, wave function is used; when correlation effects are included, either multi-configuration SCF, MC SCF, or general configuration interaction, CI, wave functions are used. One particularly attractive feature of this approach is that approximations are included in a controlled fashion. Parameters which are adjusted to make the computed results fit experimental data are not used. In principle, the exact cluster wavefunction may be obtained by using a sufficiently large CI. The approach is truly ab initio.

The s core‐electron binding energy splittings observed by X‐ray photo‐electron spectroscopy (XPS) in transition metal and rare‐earth ions are discussed in terms of Multiplet Hole Theory (MHT), atomic electron correlation effects and covalency. MHT calculations alone give good agreement with experiment for the 2s splittings in the 3d ions and the 5s electrons in the rare‐earths. For these splittings the electron correlation is small. By contrast, the observed reduction by a factor of two in the 3s splittings (in the 3d series) and the 4s splittings (in the 4f ions) is found to be caused mainly by the “internal” (same n shell configuration interaction) correlation; covalency influences the splittings by about 2eV for the 3s shell and about 1/2eV for the 4s electrons in the rare‐earths.

Several states of septet and quintet spin of Fe0 are studied. It is concluded that the ground state of Fe0 is not a septet state. It is possible that a quintet state with molecular structure similar to that of the states studied may be identified with the c state of Fe0.

The character of low‐lying excited states of MgO has been examined using cluster models of the MgO crystal. Several different clusters were used; in general, the clusters contained a central Mg atom, one or more shells of nearest‐neighbor Mg and O atoms, and a large number of point charges to represent the Madelung potential in the space of the atoms explicitly included in the cluster. The excited orbital was occupied in two ways. First, an electron was added to a low lying excited level while the Mg2+ and O2− anion levels were kept fully occupied. In the second way, an electron was excited from the oxygen 2p band into a low lying excited level. The excited orbitals, for both of these cases are very similar. Although the energetics of the excited states depended on the cluster model, in particular whether it was Mg or O terminated, the general character of the excited orbital remained the same. In all clusters, the excited state orbital was diffuse and delocalized. The excited orbital has no simple or direct relationship to the 3s orbital of Mg2+. It is best described as a ‘‘conduction band’’ orbital. It extends up to and beyond the edge of the atoms explicitly included in the cluster. In our work, we did not find any evidence for localized excitonic states.

The adsorption of styrene on (111)-oriented thin layers of FeO and Fe3O4 epitaxially grown on Pt(111) single-crystal surfaces has been investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). High-resolution gas-phase measurements were recorded, and precise ab initio electronic structure calculations were performed in order to aid the assignment of the NEXAFS resonances. Whereas on Fe3O4 (magnetite), styrene forms a chemisorbed monolayer that is stable up to room temperature, on FeO (wustite), a significantly more weakly bound (physisorbed) species is observed that is stable only for temperatures below 200 K. A planar adsorption geometry is observed in the case of styrene on Fe3O4 in the low-coverage regime (ϑ < 0.5 ML); for higher coverages on the same surface and on FeO, the average tilt angle between the phenyl plane and the surface plane amounts to about 45°.