Dynamical steric effect in the decomposition of methyl chloride on a silicon surface

We report results of a study on the incident energy and the surface-temperature dependence of the steric effects in the dissociative adsorption of CH3Cl on a Si{100} surface. Data presented here show that the initial sticking probability for the Cl-end collision is larger at an incident energy of 120 meV than that in the CH3-end collision. Furthermore, this steric preference is quite sensitive to the kinetic energy and the rotational state of CH3Cl and the surface temperature. This study shows that the nonequilibrium surface trapping plays a key role in the initial step of the decomposition of CH3Cl on Si{100}.     

Pu(100)表面吸附CO2的密度泛函研究

采用广义梯度密度泛函理论的改进Perdew-Burke-Ernzerh方法结合周期性层晶模型,研究了CO₂分子在Pu（100）面上的吸附和解离.吸附能和几何构型的计算表明,CO₂以穴位C4O4构型吸附最为有利,吸附能为1.48 eV.布居分析和态密度分析表明,CO₂与Pu表面相互作用的本质主要是CO₂分子的杂化轨道2π_μ与Pu5f,Pu6d,Pu7s轨道通过强电子转移和弱重叠杂化的方式相互作用而生成了新的化学键.计算的CO₂→CO+O解离能垒为0.66 eV,解离吸附能为2.65 eV, 表明在一定热激活条件下CO₂分子倾向于发生解离性吸附.O₂,H₂,CO和CO₂在Pu （100）面吸附的比较分析表明,较低温度下的吸附强度顺序依次为O₂,CO,CO₂,H₂;较高温度下的吸附强度顺序依次为O₂,CO₂,CO,H₂. 关键词： 密度泛函理论 Pu （100） 2')" href="#">CO₂ 吸附和解离     

Effects of surface corrugation on the molecular rotational dependence of H2 dissociative adsorption dynamics on Cu(100)

We investigate and discuss how surface corrugation affects the molecular rotational dependence of H₂ dissociative adsorption dynamics on Cu(100) by performing six-dimensional (6D) quantum dynamics calculations. We calculate the dissociative adsorption probability as a function of the initial rotational state J and the normal energy E_norm of incident molecules, and compare with the dissociative adsorption results obtained by four-dimensional (4D) quantum dynamics calculations where the surface is treated as flat. In our calculation, for the case of normal incidence, the increase in dissociative adsorption probability with increasing E_norm and the non-monotonic behavior of dissociative adsorption probability with respect to J are suppressed on a corrugated surface as compared to that on a flat surface.     

Effect of mutual configuration between molecular orientation and atomic orientation in the oriented Ar (3P2)+oriented CF3H reaction

The steric effect in the dissociative energy transfer reaction of Ar (3P2,MJ=2)+CF3H has been observed as a function of the mutual configurations between the molecular orientation and the atomic orientation in the collision frame. The reactivity is significantly changed depending on the mutual configuration between the molecular orientation and the atomic orientation. Especially, the molecular alignment dependence is dramatically changed depending on the configuration of the atomic alignment.     

H_2在Ni,Pd与Cu表面的解离吸附

用EAM方法(embeded-atommethod)研究H_2在Ni,Pd与Cu的(100),(110)与(111)面上的解离吸附.首先通过拟合单个H原子在Ni,Pd与Cu不同表面上的吸附能和吸附键长,得到H与这些金属表面相互作用的EAM势,然后计算H_2在这些表面上以不同方式进行解离吸附时的活化势垒E_a,吸附热q_(ad)与吸附键长R.并给出H_2在(110)面上解离吸附的势能曲线.计算结果表明H_2的解离吸附与衬底种类、衬底表面取向及解离方式有关.H_2在Ni表面上解离时活化势垒很低,而在Cu表面解 关键词：     

Partially dissociative adsorption of water vapor on the Si(111) (7×7) surface

High-resolution vibrational electron energy-loss spectra have been measured of the Si(111) (7×7) surface exposed to water vapor at 300 K. Direct experimental evidence is given that water adsorption on the Si(111) (7×7) surface is partially dissociative.     

ELS study on chemisorbed state of CO on Cr(110)

The chemisorbed state of CO on a Cr(110) surface has been investigated at 300 K by electron energy loss spectroscopy (ELS) with the in-situ combined supplementary techniques. The ELS spectrum of the Cr(110) surface after CO adsorption is characterized by the peaks at 2, 4.4, 6–7, 9, 11, 14.5, 19 and 23 eV, and is found to be practically the same as that of the oxygen covered surface. The C-KLL Auger spectra obtained in the range 0.1–900 L CO agree with those of metal carbides. These results are considered to indicate that CO is dissociatively chemisorbed on the Cr(110) surface throughout the whole exposure region examined. The average sticking probability of CO on Cr(110) is 0.7 at below 0.5 L, and the maximum work function increase at 1 L is ~0.1 eV. The adsorbed state of O atoms produced from dissociative adsorption of CO is also discussed.     

Hydrogen dissociation dynamics on precovered Pd surfaces: Langmuir is still right

A recent scanning tunneling microscopy study by Mitsui et al. [Nature (London) 422, 705 (2003)] challenged the well-accepted picture based on early studies of Langmuir that an ensemble of at least two empty, catalytically active sites is required for the dissociative adsorption; instead, aggregates of three or more vacancies should be necessary. We have performed the first ab initio molecular dynamics study of the adsorption dynamics on a precovered surface providing detailed insights into the coverage dependence of the adsorption probability. The simulations show that there is no need to refine the Langmuirian picture: A dimer vacancy is still sufficient to dissociate hydrogen provided the kinetic energy of the molecules is large enough to overcome the relatively small adsorption barrier. In addition, we elucidate further aspects of the dissociation dynamics at precovered surfaces.     

Mechanism of H2S molecule adsorption on the GaAs(100) surface: Ab initio quantum-chemical analysis

Ab initio quantum-chemical cluster calculations within the density-functional theory were carried out to study the mechanism of H₂S molecule adsorption on the gallium-rich surface of GaAs(100). It was shown that adsorption can occur in four stages: molecular adsorption; dissociative adsorption, during which an HS radical is adsorbed on a gallium atom comprising a dimer while the detached hydrogen atom is adsorbed on another surface atom of the semiconductor; hydrogen adatom migration between neighboring surface atoms of the semiconductor; and the formation of a Ga-S-Ga bridge bond and of a hydrogen molecule. The stationary-state energies and energy barriers to transitions between these states were determined. The conclusions drawn based on an analysis of calculated diagrams of the potential energy of the processes that occur are in good agreement with the experimental data available in the literature.     

Room-temperature-adsorption behavior of acetic anhydride on a TiO2(1 1 0) surface

We have studied the adsorption structure of acetic anhydride on a TiO₂(1 1 0) surface using XPS (X-ray photoelectron spectroscopy), LEED (low energy electron diffraction) and HREELS (high resolution electron energy loss spectroscopy) to determine the origins of the unique adsorption properties of carboxylic acids on a TiO₂(1 1 0) surface. The C 1s XPS data indicated that the saturation carbon amount of adsorbed acetic anhydride was 12 ± 3% larger than that of the adsorbed acetic acid. LEED showed p(2 × 1) weak spots for the acetic anhydride adsorbed surface. The HREELS spectra revealed the dissociative adsorption of acetic anhydride. Based on these findings, we concluded that the neutralization of the bridging oxygen atoms associated with the dissociative adsorption is necessary for the stable adsorption of carboxylates on the 5-fold Ti sites.     

CO在Pu(100)表面吸附的研究

采用密度泛函理论(DFT)研究了CO分子在Pu (100)面上的吸附. 计算结果表明：CO在Pu (100)表面的C端吸附比O端吸附更为有利，属于强化学吸附. CO吸附态的稳定性为穴位倾斜>穴位垂直>桥位>顶位. CO分子与表面Pu原子的相互作用主要源于CO分子的杂化轨道和Pu原子的杂化轨道的贡献. 穴位倾斜吸附的CO分子的离解能垒较小(0.280eV)，表明在较低温度下，CO分子在Pu (100)表面会发生离解吸附，离解的C，O原子将占据能量最低的穴位.     

Quantum dynamics of the dissociation of H<Subscript>2</Subscript> on Rh(111)

The dissociative adsorption of H₂ on Rh(111) has been studied by high-dimensional quantum calculations using a coupled channel scheme. The potential energy surface was derived from ab initio total energy calculations using density functional theory together with the generalized gradient approximation to describe exchange-correlation effects. Experimentally, at high kinetic energy a step in the dissociative adsorption probability as a function of kinetic energy has been observed [M. Beutl et al., Surf. Sci. 429, 71 (1999)] which has been attributed to the opening up of a new adsorption channel. This feature in the dissociation probability is reproduced in the calculations for H₂ molecules initially in the ro-vibrational ground state but it is not related to the opening up of an additional dissociation channel. Instead, it is caused by purely dynamical effects. In addition, rotational effects in the H₂ dissociation are addressed.     

DFT study of NH3 dissociation on Si(1 1 1)-7 × 7. The role of intermolecular interactions

The adsorption of NH₃ molecule on the Si(1 1 1)-7 × 7 surface modelled with a cluster has been studied using density functional theory (DFT). The results indicate the existence of a precursor state for the non-dissociative chemisorption. The active site for the molecular chemisorption is the adatom; while the NH₃ molecule adsorbs on the Si restatom via this preadsorbed state, the adsorption on the Si adatom is produced practically without an energy barrier. The ammonia adsorption on the adatom induces an electron transfer from the dangling bond of this atom to the dangling bond of the adjacent Si restatom, hindering this site for the adsorption of a second NH₃ incoming molecule. However, this second molecule links strongly by means of two H-bonds. The dissociative chemisorption process was studied considering one and two ammonia molecules. For the dissociation of a lonely NH₃ molecule an energy barrier of ∼0.3 eV was calculated, yielding NH₂ on the adatom and H on the restatom. When two molecules are adsorbed, the NH₃-NH₃ interaction yields the weakening of a N-H bond of the ammonia molecule adsorbed closer the Si surface. As a consequence, the dissociation barrier practically disappears. Thus, the presence of a second NH₃ molecule at the adatom-restatom pair of the Si(1 1 1)-7 × 7 surface makes the dissociative reaction self-assisted, the total adsorption process elapsing with a negligible activation barrier (less than 0.01 eV).     

Crystal field surface orbital-bond energy bond order (CFSO-BEBO) calculations of adsorption: II. Carbon monoxide,oxygen and carbon dioxide on platinum (111) and oxygen on nickel (111)

The Crystal Field Surface Orbital-Bond Energy Bond Order (CFSO-BEBO) model of chemisorption is applied to the interaction of carbon monoxide, oxygen and carbon dioxide with a (111) platinum surface; and the interaction of oxygen with a (111) nickel surface. No activation energy for molecular adsorption of carbon monoxide on platinum is predicted; however a large activation energy for dissociative chemisorption is calculated. The molecular state has a binding energy of approximately 28 kcal/mole, and vibrational stretching frequencies of 1935 and 1975 cm^?1 are calculated by combining the CFSO-BEBO model with Badger's Rule. The adsorption of oxygen on (111) platinum and (111) nickel are predicted to be different in the following respects: (1) There is an activation energy of 2.1–4.5 kcal/mole on platinum, whereas adsorption on nickel is unactivated; and (2) The dissociative heat of chemisorption on platinum is 58–68 kcal/mole, whereas on nickel it is substantially larger, 112–116 kcal/mole. The adsorption of carbon dioxide on (111) platinum is predicted to be not only highly activated but also endothermic. All of the calculated results are essentially in quantitative agreement with available experimental data.     

CO在Pu(100)表面吸附的研究

采用密度泛函理论(DFT)研究了CO分子在Pu (100)面上的吸附. 计算结果表明：CO在Pu (100)表面的C端吸附比O端吸附更为有利,属于强化学吸附. CO吸附态的稳定性为穴位倾斜>穴位垂直>桥位>顶位. CO分子与表面Pu原子的相互作用主要源于CO分子的杂化轨道和Pu原子的杂化轨道的贡献. 穴位倾斜吸附的CO分子的离解能垒较小(0.280eV),表明在较低温度下,CO分子在Pu (100)表面会发生离解吸附,离解的C,O原子将占据能量最低的穴位. 关键词： 密度泛函理论 Pu (100) CO 分子和离解吸附     

Rotation of hydrogen molecules during the dissociative adsorption on the Mg(0001) surface: a first-principles study

Using first-principles calculations,we systematically study the potential energy surfaces and dissociation processes of the hydrogen molecule on the Mg(0001) surface.It is found that during the dissociative adsorption process with the minimum energy barrier,the hydrogen molecule first orients perpendicularly,and then rotates tobecome parallel to the surface.It is also found that the orientation of the hydrogen molecule in the transition state is neither perpendicular nor parallel to the surface.Most importantly,we find that the rotation causes a reduction of the calculated dissociation energy barrier for the hydrogen molecule.The underlying electronic mechanism for the rotation of the hydrogen molecule is also discussed in the paper.     

Cluster studies of CO adsorption: I. CO on small Li clusters

The various contributions to the adsorption energy of CO such as steric repulsion, σ bonding and π backbonding, are studied as a function of CO-cluster distance, C-O distance, adsorption site and cluster geometry. At the hollow site a double minimum is found in the adsorption energy as a function of CO-cluster distance. The effect of adsorption on the electronic structure of CO is large, in particular at the equilibrium distance close to the surface. Consequences for the C-O stretch frequency, and for the possibility of CO dissociation at the surface, are investigated.     

Dynamics of interaction of H2 and D2 with Ni(110) and Ni(110) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H₂ and D₂ on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s₀, irrespective of surface temperature T_s and mean kinetic energy normal to the surface 〈 E_⊥ 〉. The desorbing molecules exhibit a cos θ_e angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s₀ is only about 0.05 if both the gas and the surface are at room temperature. s₀ is again independent of T_s, but increases continuously with 〈 E⊥ 〉 up to a value of ～0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos⁵θ_e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.     

A NEXAFS and UPS study of the adsorption of tetrachloroethylene, trichloroethylene, iso-, cis-and trans-dichloroethylene on platinum surfaces at 95 K: multilayers and monolayers

UPS spectra of multilayers and monolayers of chlorinated ethylene molecules condensed or adsorbed on Pt(111) and Pt(110)(1 × 2) surfaces at 95 K are in close agreement. It shows that adsorption of these molecules is not dissociative and that they are weakly bonded to the surfaces in the monolayer range. NEXAFS has also been used in the case of Pt(111) samples. The multilayer spectra compare well with the analogous spectra of the fluorinated molecules, and the spacings between the various C 1s levels agree with the corresponding values for the π^* transitions. This shows that these resonances involve C 1s core level transitions to unoccupied π^* levels, which are located at almost the same energy in the excited state. The polarization dependence of the synchrotron light indicates that for monolayers the C=C axis and molecular planes arc parallel to the surface. The π^* resonance widths confirm that the adsorption of all chlorinated molecules is not dissociative at low temperatures, as suggested from the catalytic destruction of unsaturated chlorinated C2 compounds which occurs only at high temperatures. Above 120–130 K and under vacuum, desorption depletes the surface without conversion to a more firmly bonded species, a process occurring under catalytic conditions.     

LCAO studies of hydrogen chemisorption on nickel: II. Cluster models for adsorption sites

The adsorption of single hydrogen atoms, investigated by means of cluster calculations, has been compared with the adsorption of hydrogen monolayers on periodic crystals (paper I). From the similarity of the adsorption energy curves we conclude that the (direct and indirect) interactions between adsorbed hydrogen atoms are relatively small up to monolayer coverage. For adsorption on different sites of ideal low index surfaces the stability decreases in the order Atop > Bridge > Centred. For Atop adsorption it increases with a decreasing number of nearest neighbours to the nickel atom in the NiH “surface molecule”, thus leading to especially strong adsorption sites at the edges of a stepped surface and to low stability in the notches. In general, we find that the Ni_nH “surface molecule” with n = 1, 2, 3 or 4 determines the equilibrium positions for H adsorption; the inclusion of one shell of neighbours to the nickel atoms is sufficient to explain the differences in adsorption energy. The Extended Hückel method is not well suited to study dissociative chemisorption of H₂, although some qualitative trends are correct.