Structural characterization of a (krypton-methane) film adsorbed on (0001) graphite

Krypton-methane films adsorbed on (0001) graphite have been studied by neutron diffraction, between 45 and 98 K. A two-dimensional solid solution has been observed. Its melting temperature has been determined for two compositions of the layer. The melting appears to be a first-order phase transition.     

Thermodynamic characterization of a dichloromethane-xenon film adsorbed on (0001) graphite

Xenon adsorption between 95 and 115 K on graphite pre-plated with a dichloromethane monolayer results in a partial displacement of the pre-adsorbed layer and in the formation of a two-dimensional (dichloromethane-xenon) solution. When the dichloromethane pre-plating is in the sub-monolayer range, a xenon monolayer is condensed on the bare part of the surface before the composite layer organization. For dichloromethane coverages greater than a monolayer, kinetic problems arise for the formation of xenon bilayer patches, which implies in this case its separation from the two-dimensional solution.     

Vibrational properties of the graphite (0001) surface

The temperature dependence of low-energy electron diffraction intensity-energy spectra from the graphite (0001) surface has been measured from 100 to 600 K. The perpendicular and parallel effective surface Debye temperatures and the effective mean-square vibration amplitudes as a function of electron energy have been determined from these data. The values show surface enhancement and approach the bulk values with increasing electron energy. At energies below ≈ 100 eV, however, there is little anisotropy observed between the effective Debye temperatures parallel and perpendicular to the graphite surface.     

Two-dimensional phase transition in xenon submonolayer films adsorbed on (0001) graphite

Ultra high vacuum molecular beam techniques coupled with LEED and Auger electron spectroscopy are particularly well suited to the study of surface chemical reactions because of the ability to assess the effect of the surface conditions on the reaction probability. Investigation of the hydrogen-deuterium exchange reaction on a series of low and high Miller index platinum single crystals has indicated that the steps present on the high index surfaces are necessary for the dissociation and subsequent recombination of hydrogen. We have undertaken a systematic study of a series of small molecule reactions on these stepped surfaces to determine the reaction probability on stepped platinum surfaces. Reactions involving dissociation of Ha₂, D₂, O₂, OH, NH, and CH bonds proceed on the stepped surfaces with much higher reaction probabilities than reactions requiring dissociation of N₂, or CO bonds. All of the reactions studied resulted in cosine product angular distributions except for the formation of CO₂, which exhibited a distribution more peaked at the normal to the surface.     

Infrared reflection absorption spectra of Fe(CO)5 adsorbed on Au surfaces

Infrared reflection absorption spectroscopy (IRAS) shows that the CO stretching bands of Fe(CO)₅ adsorbed on Au surfaces are significantly different in band shape as well as in frequency from the bands observed in a transmission mode. This difference has been observed for other metal substrates and explained in terms of the anomalous dispersion of the refractive index in the region of the observed bands. The refractive indices of Fe(CO)₅ are calculated using the Kramers-Kronig relation from the transmission spectra of Fe(CO)₅ adsorbed on a sapphire plate, an SiO₂-coated sapphire plate, and an Au film evaporated on a sapphire plate, and the IRA spectra of Fe(CO)₅ adsorbed on Au are calculated using Fresnel's formula. The results show that the ν₁₀ band of Fe(CO)₅ becomes very sharp and shifts to higher frequencies by more than 10 cm⁻¹, while the ν₆ band becomes a shoulder of the v₁₀ band, in good agreement with the observed IRA spectra. The IRAS calculation also shows that the weak band observed at 2114 cm⁻¹ for the Au film remains unchanged in position, in agreement with the observed IRAS.     

Potassium adsorption on graphite(0001)

Potassium adsorption on graphite has been studied with emphasis on the two-dimensional K adlayer below one monolayer. Data are presented for the work function versus coverage, high-resolution electron energy loss spectroscopy (HREELS) vibrational spectra of K-adlayers, low energy electron diffraction and ultraviolet photoemission spectroscopy (UPS) spectra at different coverages. The data provide information regarding the vibrational properties of the K-adlayer, the metallization of the adlayer at submonolayer coverages, and the charge transfer from the K adatoms to the graphite substrate. Analysis of the work function, HREELS, and UPS data provides a qualitatively consistent picture of the charge state of the K adatoms, where at low coverages, below a critical coverage θ_c (θ_c=0.2–0.3), the K adatoms are dispersed and (partially) ionized, whereas at θ>θ_c islands of a metallic 2×2 K phase develops that coexist with the dispersed a K adatoms up to θ=1. We show that it is possible to understand the variation of the work function data based on a two-phase model without invoking a depolarization mechanism of adjacent dipoles, as is normally done for alkali-metal adsorption on metal surfaces. Similarly, the intensity variation as a function of coverage of the energy loss peak at 17 meV observed in HREELS, and the photoemission peak at E_b=0.5 eV seen in UPS can be understood from a two-phase model. A tentative explanation is presented that connects apparent discrepancies in the literature concerning the electronic structure of the K adlayer. In particular, a new assignment of the K-induced states near the Fermi level is proposed.     

Unusually low stretching frequency for CO adsorbed on Fe(100)

For CO adsorption on Fe(100) different adsorption species are detected with high resolution EELS (electron energy loss spectroscopy) which sequentially fill in with increasing coverage. Up to ～ 350 K and low CO exposure (≦1 L), a predominant molecular species with an unusually low stretching frequency, 1180–1245 cm^?1, is detected. This unusual CO bond weakening is consistent with a “lying down” binding configuration of CO. For higher CO coverages at 110 K, further CO adsorption states with vibrational frequencies of 1900–2055 cm^?1 are populated which are due to CO bound with the molecular axis perpendicular to the surface.     

Structures,electronic properties and reaction paths from Fe(CO)5 molecule to small Fe clusters

The geometries, electrical characters and reaction paths from Fe(CO)₅ molecule to small Fe clusters were investigated by using all-electron density functional theory. The results show that in the decomposition process of pentacarbonyl-iron, Fe(CO)₅ molecule prefers to remove a carbon monoxide and adsorb another Fe(CO)₅ molecule to produce nonacarbonyldiiron Fe₂(CO)₉ then Fe₂(CO)₉ gradually removes carbon monoxide to produce small Fe clusters. As It can be seen from the highest occupied molecule orbital–lowest unoccupied molecule orbital gap curves, the Fe(CO)_n=3,?and?5 and Fe₂(CO)_n=3,?7?and?9 intermediates have higher chemical stability than their neighbors. The local magnetic moment of the carbon monoxide is aligning anti-ferromagnetic. The effect of external magnetic field to the initial decomposition products of Fe(CO)₅ can be ignored.     

Dynamical anomalies in physisorbed films of Sn(CH3) and Fe(CO)5 on graphite basal plane

Several dynamical properties of Sn(CH₃)₄ and Fe(CO)₅ molecules physisorbed on graphite, are influenced by the form of two-dimensional solid growth. As inferred from Mössbauer results, at temperatures where the molecules form solid superstructures and below monolayer coverages, abnormal changes are observed in the mean squared displacement of the molecules. It is suggested that these dynamical anomalies are associated with substrate-adsorbate registry.     

Structural and electronic properties of carbon adsorbed on Fe(100)

Density functional theory within general gradient approximation (GGA) has been used to investigate sub-monolayer carbon atom adsorbed on Fe(100) as a function of coverage. The carbon atoms prefer to adsorb in the fourfold hollow site and bind strongly with the Fe surfaces. There is a substantial and strong coverage dependence of the carbon-induced expansion of the first interlayer spacing, reflecting a weakening of Fe–Fe bonds between the two outermost substrate layers. Some charge is found to transfer from substrate Fe to the adsorbate C atoms, which is responsible for the increase of work function. The density of states (DOS) analysis indicates the bonding of carbon with the first surface layer Fe atoms is primarily due to the interaction between Fe 3d_{x2-y2, xy} and C 2p_{x, y} orbitals, and the bonding of carbon with the second surface layer Fe atom that sits directly below the carbon atom is mainly from interaction between the minority spin Fe 3d_z2 and C 2p_z orbitals.     

Phase transitions in physisorbed films on cadmium (0001): comparison with graphite (0001)

Physical adsorption of simple molecules has been studied experimentally on cadmium (0001). Adsorption isotherms of Kr and CF₄ at 77.3 K show a first-order phase transition in the first layer, characteristic of adsorption on uniform substrates.

The nature of the phase transition on cadmium (0001) in the first layer is investigated for Kr and CF₄ by a thermodynamic approach in comparison with results obtained earlier on graphite (0001).

From the comparison between the two uniform substrates, we show that (1) cadmium (0001) has a less attractive force field than graphite (0001); and (2) like graphite (0001), cadmium (0001) is a very uniform surface suitable for studying two-dimensional phases according to the Gibbsian formulation.     

Comments on transitions in bulk tetramethyltin Sn(CH3)4 and as adsorbate on a (0001) graphite plane

In the case of tetramethyltin, whether in bulk form or in the form of an adsorbate on (0001) graphite basal plane, it was found that anomalies in the Mössbauer or X-ray diffraction parameters occur at similar temperatures, viz: 83 K, 95 K, and 125 K. It is therefore suggested that these anomalies can be associated in some way with intramolecular rearrangements and be affected by intramolecular motions.Sponsored by the U.S.-Israel Binational Science Foundation     

Analysis of the production and clusterization of iron atoms under pulsed laser photolysis of Fe(CO)5

Atomic-resonance absorption spectroscopy is used to study the production and loss of iron atoms under dissociation of the Fe(CO)₅ vapor in a quartz reactor that is induced by the pulses of the KrF excimer laser. Iron atoms populate the ground state owing to the quenching of the excited states generated in the course of the laser photolysis and are detected using the resonance absorption at a wavelength of 385.99 nm. The effective quenching rates are in good agreement with the known rates of the quenching of metastable iron atoms by the Fe(CO)₅ molecules. It is demonstrated that a loss of iron atoms is related to the recombination with dimer and trimer formation and the secondary atomic reactions with the Fe(CO)₅, CO, and FeCO molecules. The rates of the main elementary reactions responsible for the loss of iron atoms are determined using the comparison of the experimental results and kinetic simulation data.     

CO分子在Co(0001)表面吸附的第一性原理研究

应用第一性原理对CO分子在Co(0001)表面( squar3× squar3)R30°−CO吸附结构进行不同形式的密度泛函计算研究. 结果表明：吸附能修正前,仅RPBE泛函预测CO顶位吸附;而修正后PW91、PBE和PKZB泛函结果也表明CO分子Top顶位吸附最稳定,与实验结果一致.对于吸附几何结构、吸附前后体系功函、C-O伸缩振动频率和CO分子态密度分布,所有泛函给出一致的结果,且与已有实验结果符合.     

CO分子在Co(0001)表面吸附的第一性原理研究

应用第一性原理对CO分子在Co(0001)表面( squar3× squar3)R30°−CO吸附结构进行不同形式的密度泛函计算研究. 结果表明：吸附能修正前,仅RPBE泛函预测CO顶位吸附;而修正后PW91、PBE和PKZB泛函结果也表明CO分子Top顶位吸附最稳定,与实验结果一致.对于吸附几何结构、吸附前后体系功函、C-O伸缩振动频率和CO分子态密度分布,所有泛函给出一致的结果,且与已有实验结果符合.     

Methane adsorption on graphite（0001） films： A first-principles study

Using first-principles methods, we have systematically investigated the electronic density of states, work function, and adsorption energy of the methane molecule adsorbed on graphite(0001) films. The surface energy and the interlayer relaxation of the clean graphite(0001) as a function of the thickness of the film were also studied. The results show that the interlayer relaxation is small due to the weak interaction between the neighboring layers. The one-fold top site is found most favourable on substrate for methane with the adsorption energy of 133 meV. For the adsorption with different adsorption heights above the graphite film with four layers, the methane is found to prefer to appear at about 3.21 A above the graphite. We also noted that the adsorption energy does not dependent much on the thickness of the graphite films. The work function is enhanced slightly by adsorption of methane due to the slight charge transfer from the graphite surface to the methane molecule.