Effects of adsorbates on the formation of doubly excited He atoms during impact of He2+ ions on Fe(110) and Ni(110) surfaces

The formation of doubly excited states of He atoms during collisions of He²⁺ ions with projectile energies between 74 eV and 124 eV with an Fe(110) and a Ni(110) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionization of doubly excited states of 2s², 2s2p, 2p² configurations show a pronounced dependence on the coverage with oxygen for both surfaces. For a controlled O₂ adsorption on the Fe(110) and Ni(110) surfaces we can explain the resulting changes in the electron spectra by the modification of the measured work functions of the target surfaces. In terms of thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures, we present an alternative interpretation of similar previous studies by another group, where the local electron spin polarization of Fe(110) and Ni(110) surfaces was deduced from changes in the electron spectra as function of target temperature.     

The configuration and electronic state of SO3 adsorbed on Au surface

We evaluated the adsorption of SO₃ molecule on Au (1 1 1) surface using first principles calculation by a slab model with a periodic boundary condition. We find that there are six stable adsorption configurations on an Au surface, where the SO₃ molecule is adsorbed above the three-fold fcc and hcp hollow sites and on the atop site. In two of these configurations, S and two O atoms are bound to the Au atoms, the next two configurations have all the three O atoms bound to the Au surface atoms, and the last two configurations have the S atom bound to an Au surface atom on the atop site and O atoms situated above the hollow sites. In these configurations, the electronic structures of SO₃ on the Au surface show that molecular orbitals of SO₃ and those of the Au surface are hybridized in the active metal d-band region, that the localized molecular orbitals in SO₃ are stabilized, and that charge is transferred from Au to S 3p by SO₃ adsorption on the Au surface though there is little other interaction of the S and O (bound to Au) component with Au. Moreover, the bond between the S and O atoms bound to Au is weakened due to SO₃ adsorption on the Au surface due to the charge polarization of the O-Au bond. This interaction is likely to encourage the S-O bond to break.     

Molecular orbital study of the chemisorption of carbon monoxide on a tungsten (100) surface

The adsorption energies of carbon monoxide chemisorbed at various sites on a tungsten (100) surface have been calculated by extended Hückel molecular orbital theory (EHMO). The concept of a “surface molecule” in which CO is bonded to an array of tungsten atoms W_n has been employed. Dissociative adsorption in which C occupies a four-fold, five-coordination site and O occupies either a four- or two-fold site has been found to be the most stable form for CO on a W surface. Stable one-fold and two-fold sites of molecularly adsorbed CO have also been found in which the CO group is normal to the surface plane and the C atom is nearest the surface. Adsorption energies and molecular orbitals for the stable molecularly and dissociatively adsobred CO sites are compared with the experimental data on various types of adsorbed CO, i.e., virgin-, α-, and β-CO. Models are suggested for each of these adsorption types. The strongest bonding interactions occur between the CO 5σ orbital and the totally symmetric 5d and 6s orbitals of the W_n cluster. Possible mechanisms for conversion of molecularly adsorbed CO to dissociatively adsorbed CO are proposed and the corresponding activation energies are estimated.     

Interaction of neutral hydrogen atoms with KCl(OO1)

Neutral hydrogen atoms with velocity selected thermal energies, have been scattered from KCl(001) cleavage planes in UHV. For cold crystal surfaces (T_SF ? 170 K) even a small residual pressure of water resulted in an ordered monolayer coverage of the (001) plane. From this surface, diffraction has been observed with diffracted beams up to third order and characteristically varying intensities were observed and analysed in terms of a quantum mechanical rainbow scattering theory. Resonant transitions of atoms to bound states at the surface (“selective adsorption”) were observed and used to determine the interaction potential of hydrogen atoms with this water covered surface: first in terms of a Morse potential, second in terms of a $\text{C}{}_3\text{z}{}^3$-dependent attractive potential. The surface Debye-Waller factor has been measured for clean KCl(001), for the water covered (001) plane as well as for partially covered surfaces. The results are interpreted within a simple model.     

H2O adsorption on Ni(100): Evidence for oriented water dimers

H₂O adsorption on clean Ni(110) surfaces at T ≦ 150 K leads at coverages below $\text{θ ? 0.5}$ to the formation of chemisorbed water dimers, bound to the Ni substrate via both oxygen atoms. The linear hydrogen bond axis is oriented parallel to the [001] surface directions. With increasing H₂O coverage $\text{(θ ≧ 0.5)}$, the accumulation of further hydrogen bonded water molecules induces some modification of the dimer configuration, producing at $\text{θ ? 1}$ a two-dimensional hydrogen bonded network with a slightly distorted ice lattice structure and long range order.     

Water adsorption on the Be(0001) surface: from monomer to trimer adsorption

In this paper, the density functional theory has been used to perform a comparative theoretical study of water monomer, dimer, trimer, and bilayer adsorptions on the Be(0001) surface. In our calculations, the adsorbed water molecules are energetically favoured adsorbed on the atop sites, and the dimer adsorption is found to be the most stable with a peak adsorption energy of ～437 meV. Further analyses have revealed that the essential bonding interaction between the water monomer and the metal substrate is the hybridization of the water 3a₁-like molecular orbital with the (s, p_z) orbitals of the surface beryllium atoms. While in the case of the water dimer adsorption, the 1b₁-like orbital of the H₂O molecule plays a dominant role.     

Adsorption of water on TiN (1 0 0), (1 1 0) and (1 1 1) surfaces: A first-principles study

We use first-principles density functional theory-based calculations in the analysis of the interaction of H₂O with (1 0 0), (1 1 0) and (1 1 1) surfaces of TiN, and develop understanding in terms of surface energies, polarity of the surface and chemistry of the cation, through comparison with H₂O adsorption on ZrN. While water molecule physisorbs preferentially at Ti site of (1 0 0) and (1 1 1) surfaces, it adsorbs dissociatively on (1 1 0) surface of TiN with binding stronger than almost 1.32 eV/molecule. Our analysis reveals the following general trends: (a) surfaces with higher energies typically lead to stronger adsorption, (b) dissociative adsorption of H₂O necessarily occurs on a charge neutral high energy surface and (c) lower symmetry of the (1 1 0) plane results in many configurations of comparable stability, as opposed to the higher symmetry (1 0 0) and (1 1 1) surfaces, which also consistently explain the results of H₂O adsorption on MgO available in literature. Finally, weaker adsorption of H₂O on TiN than on ZrN can be rationalized in terms of greater chemical stability of Ti arising from its ability to be in mixed valence.     

Evaluation for the configurational and electronic state of SO3 adsorbed on Pt surface

We evaluate the adsorption of SO₃ molecule on the Pt (1 1 1) surface using the first-principles calculations by a slab model with a periodic boundary condition. We find that there are four stable adsorption configurations on the Pt surface, where SO₃ molecules are adsorbed above the three-fold fcc and hcp sites. In two of these configurations, S and two O atoms are bound to the Pt atoms, and in two other of them, all the three O atoms are bound to Pt surface atoms. Besides, it is found that molecular orbitals of SO₃ and those of Pt surface are hybridized in the active metal d-bands region, that the localized molecular orbitals in SO₃ are stabilized, and that the charge is transferred from Pt to S 3p by SO₃ adsorption on Pt surface though the other interaction of S and O (bound to Pt) component with Pt is little. In addition, the bond between S and O bound to Pt become weak by SO₃ adsorption on Pt surface because the charge polarization to O-Pt bond weakens the bond between S and O bound to Pt. This interaction is assumed to encourage the breakage of S-O bond.     

UPS spectra of H2O,CH3OH and C5H9OH adsorbed onto Cu(111)/Na and Na(cp)

The present communication presents ultraviolet photoemission spectra (UPS) of three different “alcohols”; water (H₂O), methanol (CH₃OH), and cyclopentanol (C₅H₉OH), chemisorbed onto a Cu(111) surface partially covered by sodium atoms as well as onto closely packed sodium films, a free electron adsorbent. Whereas all three alcohols ROH bind reversibly and associatively to Cu(111) they react with adsorbed sodium atoms to metal bound alcoxides RO. The chemisorption bond, characterized by the interaction between O 2pπ orbitals and metal atoms as an electron donor, the alcoxide being the acceptor, is similar for all groups R. The O 2pπ orbitals shift to higher UPS binding energies with increasing electron density, i.e. decreasing r_s/a_o of the sodium overlayer. Only for HONa, the sterically smallest group R, does the alcoxide growth continue in three dimensions. Although, possibly failing to reproduce the electron density profile of a free electron surface, Hartree-Fock-Slater cluster calculations of small models ROH and RONa₃ enable correlations to be made between UPS intensity peaks and one electron orbitals.     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

Surface atomic distribution and water adsorption on Pt-Co alloys

Density functional theory is used to study surface atomic distributions on slabs of PtCo and Pt₃Co overall compositions, as well as water molecule adsorption on PtCo(1 1 1) and Pt-skin structures. Pt-rich surfaces are energetically favored under vacuum in the PtCo and Pt₃Co alloys. The adsorption trend on the studied structures agrees with the d-band model, with stronger adsorption at higher surface Co composition. The most stable adsorption site for a water molecule on PtCo surfaces is on top of Co atoms, with the dipole vector parallel to the surface. This water/surface interaction is as strong as that of water molecule on Pt(1 1 1), whereas bonding to Pt-skin monolayers is found much weaker than that on Pt(1 1 1). It is found that water interacts mainly through its 1b₁ and 3a₁ orbitals with d orbitals of the Pt(1 1 1), PtCo(1 1 1) and Pt-skin surface atoms. Compared to the sum of the electron densities of the separated systems, the electron density of the water/surface gets depleted along O-Pt on Pt-skin surfaces while it becomes richer in the O-Co bonding region of PtCo.     

Density functional and dynamics study of the dissociative adsorption of hydrogen on Mg (0 0 0 1) surface

A first principles study is performed to investigate the adsorption characteristics of hydrogen on magnesium surface. Substitutional and on-surface adsorption energies are calculated for Mg (0 0 0 1) surface alloyed with the selected elements. To further analyze the hydrogen-magnesium interaction, first principles molecular dynamics method is used which simulates the behavior of H₂ at the surface. Also, charge density differences of substitutionally doped surface configurations were illustrated. Accordingly, Mo and Ni are among the elements yielding lower adsorption energies, which are found to be −9.2626 and −5.2995 eV for substitutionally alloyed surfaces, respectively. In light of the dynamic calculations, Co as an alloying element is found to have a splitting effect on H₂ in 50 fs, where the first hydrogen atom is taken inside the Mg substrate right after the decomposition and the other after 1300 fs. An interesting remark is that, elements which acquire higher chances of adsorption are also seen to be competent at dissociating the hydrogen molecule. Furthermore, charge density distributions support the results of molecular dynamics simulations, by verifying the distinguished effects of most of the 3d and 4d transition metals.     

SrTiO3(001)表面上Au和N原子相互作用的第一性原理研究

采用基于密度泛函理论的第一性原理平面波赝势方法研究了SrTiO₃(001)表面上Au和N原子间相互作用的微观机理.通过比较分析N置换表面层O原子前后SrTiO₃(001)表面吸附Au原子体系的相关能量和电子结构,发现SrTiO₃(001)表面吸附Au原子和N替代表面层O原子的置换过程二者之间存在明显的"协同效应",即N原子置换SrTiO₃(001)表面层O原子的过程增强了相应表面吸附Au原子的稳定性,而SrTiO关键词： 表面结构 相互作用 第一性原理     

A first principle study on the magnetic properties of Cu2O surfaces

Spin-polarized density functional theory calculations were performed to investigate the magnetism of bulk and Cu₂O surfaces. It is found that bulk Cu₂O, Cu/O-terminated Cu₂O(111) and (110) surfaces have no magnetic moment, while, the O-terminated Cu₂O(100) and polar O-terminated Cu₂O(111) surfaces have magnetism. For low index surfaces with cation and anion vacancy, we only found that the Cu vacancy on the Cu₂O(110) Cu/O-terminated surface can induce magnetism. For atomic and molecular oxygen adsorption on the low index surfaces, we found that atomic and molecular oxygen adsorption on the Cu-terminated Cu₂O(110) surface is much stronger than on the Cu/O-terminated Cu₂O(111) and Cu-terminated Cu₂O(100) surfaces. More interesting, O and O₂ adsorption on the surface of Cu/O terminated Cu₂O(111) and O₂ adsorption on the Cu-terminated Cu₂O(110) surface can induce weak ferromagnetism. In addition, we analysis origin of Cu₂O surfaces with magnetism from density of state, the surface ferromagnetism possibly due to the increased 2p–3d hybridization of surface Cu and O atoms. This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.     

H2O interaction with clean and oxygen precovered Ni(110)

The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H₂O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E_F (6.8, 9.4 and 12.7 eV below E_F). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H₂O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (ΔE_max = 1.5 eVat 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H₂O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E_F (He(II)), being interpreted as due to hydroxyl species (OH)^δ? on the surface. A kinetic model for the H₂O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H₂O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH)^δ? recombines and desorbs as H₂O at ? 320 K, leaving behind an oxygen covered Ni surface.     

An Ab Initio Study of the Structural and Electronic Properties of the Low-Defect TiC(110) Surface Simulating Oxygen Adsorption after Exposure to Laser Plasma

An ab initio simulation of the adsorption of atomic oxygen on the low-defect titanium carbide (110) surface reconstructed by laser radiation was performed. The relaxed atomic structures of the (110) surface of the O/Ti _x C _y system with Ti and C vacancies observed during the thermal treatment were studied in terms of the density functional theory. DFT calculations of their structural, thermodynamic, and electronic properties were performed. The bond lengths and adsorption energies were determined for various reconstructions of the atomic structure of the O/Ti _x C _y (110) surface. The effects of the oxygen adatom on the band and electronic spectra of the O/Ti _x C _y (110) surface were studied. The effective charges on the titanium and carbon atoms surrounding the oxygen atom in various reconstructions were determined. The charge transfer from titanium to oxygen and carbon atoms was found, which is determined by the reconstruction of the local atomic and electronic structures and correlates with chemisorption processes. The potential mechanisms of laser nanostructuring of the titanium carbide surface were suggested.     

The adsorption of water on clean and oxygen covered Cu(110)

The water adsorption on clean and oxygen precovered Cu(110) surfaces is studied by means of UPS, LEED, work function measurements and ELS. At 90 K on the clean surface molecular water adsorption is indicated by UPS. The H₂O molecules are bonded at the oxygen end and the H-O-H angle is increased as compared with the free molecule. In the temperature range between 90 and 300 K distorted H₂O molecules and adsorbed hydroxyl species (OH) are detected, which are desorbed at room temperature. On an oxygen covered surface hydroxyl groups are formed by dissociation of adsorbed water molecules at a lower temperature than on the clean surface. Multilayers of condensed water are found below 140 K in both cases.     

TiO2分子在GaN(0001)表面吸附的理论研究

采用基于密度泛函理论的平面波超软赝势法,计算了TiO₂分子在GaN(0001)表面的吸附成键过程、吸附能量和吸附位置. 计算结果表明不同初始位置的TiO₂分子吸附后,Ti在fcc或hcp位置,两个O原子分别与表面两个Ga原子成键,Ga-O化学键表现出共价键特征,化学结合能达到7.932-7.943eV,O-O连线与GaN[1120]方向平行,与实验观测(100)[001] TiO₂//(0001)[1120]GaN一致. 通过动力学过程计算分析,TiO₂分子吸附过程经历了物理吸附、化学吸附与稳定态形成的过程,稳定吸附结构和优化结果一致. 关键词： GaN(0001)表面 2分子')" href="#">TiO₂分子 密度泛函理论 吸附     

First-principles studies on the adsorption of molecular oxygen on Ba(110) surface

The adsorption of O₂ on Ba(110) surface is studied with first-principles calculations based on density functional theory. Our calculations predict that O₂ may prefer to dissociative adsorption on Ba(110) surface without obvious barrier. Also our results do not support the model of charge transfer from the surface to the molecule as a bond breaking mechanism. Instead, the increasing hybridization between O₂ orbitals and the d states of Ba(110) surface may play an important role in the dissociation adsorption.