PRA degenerate d-band complex magnons in metallic ferromagnets at zero and finite temperatures: Inclusion of system specificity and d-electron(hole) induced charge and spin polarity fluctuations

The Barnea-Horwitz (BH) RPA nondegenerate Hubbard model (NDHM) of complex magnons in metallic ferromagnets at zero and finite temperatures is generalized to include ten-fold d-electron(hole) degeneracy using the Moriya-Kemeny-Siegel (MKS) ten-fold degenerate Hubbard model (TDHM). As such, in the RPA, Hartree-Fock (HF) one-electron(hole) regime, metallic ferromagnet specificity is automatically included via the inherent d-bandwidth (w^TB_d) and d-bandfilling (?_B) dependence of the TDHM electron(hole)-electron(hole) interaction. As such, local HF limit electron(hole) spin and charge polarity fluctuations, via the increased possibility of multiple electron(hole) site occupancy, even in the RPA-HF, are automatically included.     

Ten-fold spin-orbital degeneracy in the true d-electron Hubbard model of the Mott metal-insulator transition

The ten-fold spin-orbital degeneracy of true d-electrons is included in the non-spin-orbitally degenerate Hubbard model treatment theory of the Mott metal-insulator transition of Rice and Brinkman, and Gutzwiller. The Rice-Brinkman and Kohn phase diagrams of the Mott metal-insulator transition are shown to shift due to spin-orbital degeneracy of true d-electrons so as to increase the available phase space in the temperature-“interaction” Mott phase diagram for the formation of the antiferromagnetic insulator, superlattice metal or superlattice insulator states in these phase diagrams.     

Ten-fold degenerate d-electron correlation and the metal-nonmetal transition in a disordered binary system: Inclusion of Hund's rule coupling

The Yonezawa-Watabe (YW) study of the metal-nonmetal transitions in nondegenerate, s-electron disordered binary systems: doped semiconductors, metal-ammonia solutions, liquid metals and mixed crystals (alloys), is generalized to include ten-fold d-electron (hole) degeneracy. Such degeneracy automatically includes Hund's rule d-electron coupling and intra-site enhanced Coulomb and exchange interactions. Such a calculation is specifically relevant only to transition metal alloys, transition metal oxides and mixed transition metal oxides. It is seen that potential fluctuations exist in these systems and the possibility of Anderson localization in these disordered degenerate binary transition metal systems is explored. The YW CPA treatment of the effect of substitutional disorder (alloying) upon the mobility gap and quasiparticle states of the density of states at the extreme band edges and localization due to random spin configuration are generalized to these degenerate d-electron systems and it is shown that the disappearance of the mobility gap, not the density of states gap, causes the metal-nonmetal transition for degenerate d-electrons.     

Coordination of acetonitrile (CH3CN) to platinum (111): Evidence for an η2(C,N) species

Acetonitrile (CH₃CN) coordination to a Pt(111) surface has been studied with electron energy loss vibrational spectroscopy (EELS), XPS, thermal desorption and work function measurements. We compare data for the surface states with known acetonitrile coordination complexes. For CH₃CN adsorbed on Pt(111) at 100 K, the molecule is rehybridized and adsorbs with the CN bond parallel or slightly inclined to the surface plane in an η²(C, N) configuration. The ν(CN) frequency is 1615 cm^?1 and the C ls and N ls binding energies are 284.6 eV and 397.2 eV respectively. By contrast, weakly adsorbed multilayer acetonitrile exhibits a ν(CN) vibrational frequency of 2270 cm^?1, and C ls and N ls binding energies of 286.9 eV and 400.1 eV respectively. Both the EELS and XPS results are consistent with rehybridization of the CN triple bond to a double bond with both C and N atoms of the CN group attached to the surface. In addition to this majority η²(C, N) monolayer state, evidence is found for a second, more strongly bound minority molecular state in thermal desorption spectra. As a result of the low coverage of this state, EELS was unable to spectroscopically identify it and we tentatively assign it as an η⁴(C, N) species associated with accidental step sites. By contrast to the surface complexes, almost all of the known platinum-nitrile coordination complexes are end-bonded via the N lone-pair orbital. Several cases of side-on bonding are known, however, and we compare the results with the known complex Fe₃(η²-NCCH₃)(CO)₉. The difference in the coordinative properties of a Pt(111) surface versus a single Pt atom must be due to the increased ability of multi-atom arrays to back-donate electrons into the π¹ system of acetonitrile. Previously published EELS and XPS results for monolayer acetonitrile on Ni(111) and polycrystalline films are almost identical to the present results on Pt(111). We believe that the monolayer of $\text{CH}{}_3\text{CN}\text{Ni}\text{(111)}$ is also an η²(C, N) species, not an end-bonded species previously proposed by Friend, Muetterties and Gland.     

过渡金属羰基化合物Mn(CO)5Br和Re(CO)5Br结构与动力学的超快光谱

利用超快泵浦探测红外光谱、稳态线性红外光谱和计算化学方法,对过渡金属羰基化合物Mn(CO)₅Br和Re(CO)₅Br的振动和结构动力学进行了研究. 借助羰基的两个伸缩振动峰(处于低频的A₁模式和处于高频的简并E模式)进行了观测. 结果表明,在两个配合物中,A₁和E模式振动峰的振动频率位置及频率差都与中心金属原子对羰基的键级和振动力常数的影响相关. 而A₁模式比E模式的线宽宽一些,部分由于振动寿命的影响. 此外,从瞬态光谱中获得了振动模式依赖的对角非谐性常数,发现在两个羰基化合物中E模式的非谐性总是较小.     

On the importance of sp-electrons in the surface relaxation of d-band metals

We present arguments to the effect that the self-consistent smoothing of the mobile sp-electron distribution gives rise to electrostatic forces which are a major contribution to the initial relaxing forces on the outermost ions at a d-band metal surface. Numerical estimates for the (001) face of α-Fe are given, showing that for the surface ions the surface Madelung force from the sp-electrons is inward and about $\text{3}\text{1}\text{2}$ times stronger than an outward short-range force simulating the effect of d-electrons.     

XPS,UPS and thermal desorption studies of alcohol adsorption on Cu(110): I. Methanol

The adsorption of methanol on clean and oxygen dosed Cu(110) surfaces has been studied using temperature programmed reaction spectroscopy (TPRS), ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Methanol was adsorbed on the clean surface at 140 K in monolayer quantities and subsequently desorbed over a broad range of temperature from 140 to 400 K. The UPS He (II) spectra showed the 5 highest lying emissions seen in the gas phase spectrum of methanol with a chemisorption bonding shift of the two highest lying orbitais due to bonding to the surface via the oxygen atom with which these orbitals are primarily associated. A species of quite a different nature was produced by heating this layer to 270 K. Most noticeably the UPS spectrum showed only 3 emissions and the maximum coverage of this state was approximately $\text{1}\text{2}$ monolayer. The data are indicative of the formation of a methoxy species, thus showing that methanol is dissociated on the clean Cu(110) surface at 270 K. The same dissociated species was observed on the oxygen dosed surface, the main difference in this ease being the production of large amounts of H₂CO observed in TPRS at 370 K.     

Magnetovolume effect in transition metal-metalloid amorphous alloys

The thermal expansion, spontaneous volume magnetostriction ω_s, forced volume magnetostriction $\text{(}\text{?ω}\text{?H}\text{)}$ and Young's modulus of amorphous Fe-B, Fe-P, Co-B and (Fe-M)₇₇Si₁₀B₁₃ (M = Cr, Mn, Co, Ni) alloys have been measured to make clear the magnetovolume effect in transition metal-metalloid amorphous alloys. The thermal expansion coefficient α, ω_s and $\text{(}\text{?ω}\text{?H}\text{)}$ are dependent on the number of d-electrons per transition metal atom n_eff calculated based on the charge transfer model. The n_eff vs. α, ω_s and $\text{(}\text{?ω}\text{?H}\text{)}$ curves are quite similar to the corresponding curves in fcc alloys. The maxima in those curves are, however, found at n_eff ≈ 8.2 for the amorphous alloys in contrast with n_eff ≈ 8.7 for the fcc Fe-Ni alloys. On the other hand, Young's modulus measured under the saturation of magnetization is governed by the molar volume, irrespective of n_eff. The magnetovolume effect in transition metal-metalloid amorphous alloys is discussed in connection with the instability of ferromagnetism of amorphous Fe.     

Local moment and ferromagnetic state formation in dilute,degenerate d-electron alloys: Ferromagnetic versus antiferromagnetic interactions

In this note the Kim [1] non-degenerate Anderson model (NDAM) of random dilute alloys treatment of local moment and ferromagnetic state formation is generalized to the ten-fold degenerate Anderson model (TDAM) of Siegel and Kemeny [2], Siegel [3,4] and Moriya [12]. We first determine how an impurity state is modified by neighboring impurities. For a simple binary alloy the local electron state at each impurity site depends upon the local distribution of other impurities. Second we derive a TDAM general relationship for the occurrence of a local moment on one impurity and the ferromagnetic ordering of the total impurity spins. Lastly we derive the impurity-impurity TDAM magnetic interaction; for the direct transfer interaction the impurity-impurity magnetic interaction can be ferromagnetic or antiferromagnetic depending upon the fractional occupation of impurity states. At each stage we compare our results with those of Kim's NDAM treatment.     

Adsorption-desorption kinetics of CO from clean and sulfur covered Ru(001)

The total uptake of CO, its adsorption kinetics and its desorption kinetics from clean and partially sulfur covered surfaces of the basal plane of ruthenium have been investigated. The method of desorption rate isotherms applied to the CO flash desorption spectra from these surfaces was used to evaluate the coverage dependence of the binding energy of CO as well as the effect of various levels of sulfur on this binding energy. Below a total surface concentration of 1 adsorbate atom per 3 surface Ru atoms, the binding energy and sticking probability of CO on the clean and sulfur covered surfaces are the same. Above this concentration of total adsorbates, the adsorption kinetics is the same on all surfaces studied, the binding energy decreases linearly with CO coverage while the magnitude of the decrease increases with sulfur coverage. The total uptake of CO depends on the amount of preadsorbed sulfur. At low coverages of sulfur, total CO uptake is effected by the excluded volume of sulfur. At higher coverages of sulfur (approaching $\text{1}\text{2}$ the maximum sulfur concentration on the clean surface) the site requirements of sulfur limits the amount of CO that can adsorb on the remaining surface, to the quantity of 1 adsorbate atom per 2 Ru atoms.     

The adsorption and decomposition of methanol on the Rh(100) surface

The adsorption and decomposition of methanol on the Rh(100) surface have been studied using high-resolution electron energy loss spectroscopy and thermal desorption mass spectrometry. Below 200 K, methanol is molecularly adsorbed and bonds to the surface via the oxygen atom. At 200–220 K, a saturated methanol layer undergoes two competing reactions: desorption and OH bond cleavage to form an O-bonded methoxy species. The methoxy species is stable to approximately 250 K. Between 250 and 320 K, a fraction of the methoxy species decomposes to form coadsorbed CO and hydrogen adatoms while the remainder recombines with hydrogen adatoms to desorb as molecular methanol. The hydrogen adatoms remaining on the surface desorb as H₂ between 270 and 400 K, and the CO desorbs between 450 and 550 K. Following a saturation exposure, approximately 0.2 monolayers of methanol decompose to eventually yield CO and H₂ as desorption products. These results are compared to the chemistry of methanol on other metal surfaces.     

Sodium adsorption on aluminum (100) and (111) surfaces: ELEED,Auger, and contact potential measurements

The adsorption of Na on sputter-cleaned Al(100) and Al(111) single-crystal surfaces has been investigated under ultrahigh vacuum conditions. Sodium was deposited with an apertured ion source permitting quantitative dosage determination. Low energy electron diffraction patterns show well-ordered coherent structures corresponding to $\text{1}\text{2}$ monolayer on both surfaces, and $\text{1}\text{3}$ monolayer on Al(111). A hexagonal pattern corresponding to $\text{7}\text{8}$ monolayer occurs at a nonconforming dosage on Al(100), indicating a reduction in effective sticking coefficient. This result is verified by Auger intensity measurements, which show saturation beginning at $\text{1}\text{2}$ monolayer on both surfaces. Changes in contact potential with exposure were inferred from shifts in the low-energy cutoff of secondary electrons. The initial dipole moment is in good agreement with that on transition metal substrates, while the saturation value of the contact potential corresponds closely to the difference between reported work functions for bulk Al and bulk Na. A true minimum in the work function was not observed. Present results are compared with results obtained from transition metal substrates. Essential differences are attributed to the more metallic character of the bonding in the Al:Na system.     

Quantum size effect in electron transmission through Cu and Ag films on W(110)

The transmission coefficient of very low energy electrons ( ? 10 eV) normally incident on (111) epitaxial films of Cu and Ag on W(110) is modulated by interference between scattering from the vacuum/metal and metal/metal interfaces. Comparison with calculations of free-electron scattering from a one-dimensional potential model, in which grading of the metal/metal interface is represented by a smoothing of the potential step, indicates that this interface is abrupt within approximately one layer spacing. We obtain a value of 11.0 (8.0) ± 1.0 eV for the inner potential of Cu (Ag) and mean free path lengths of $\text{39 ± 8}\text{A}\text{?}$ at an energy of 7.0 eV relative to the Fermi energy and $\text{29 ± 11}\text{A}\text{?}$ at 9.0 eV for Cu, and $\text{25 ± 10}\text{A}\text{?}$ at 7.5 eV for Ag. Work function values are obtained by the field emission retarding potential technique. We investigate the effects of the surface potential barrier, inelastic scattering and surface roughness, and evaluate the validity of the one-dimensional model presented.     

Characterization of oxygen,carbon, and sulfur adlayers on W(211)

Adlayers of oxygen, carbon, and sulfur on W(211) have been characterized by LEED, AES, TPD, and CO adsorption. Oxygen initially adsorbs on the W(211) surface forming p(2 × 1)O and p(1 × 1)O structures. Atomic oxygen is the only desorption product from these surfaces. This initial adsorption selectively inhibits CO dissociation in the CO(β₁) state. Increased oxidation leads to a p(1 × 1)O structure which totally inhibits CO dissociation. Volatile metal oxides desorb from the p(1 × 1)O surface at 1850 K. Oxidation of W(211) at 1200 K leads to reconstruction of the surface and formation of p(1 × n)O LEED patterns, 3 ? n ? 7. The reconstructed surface also inhibits CO dissociation and volatile metal oxides are observed to desorb at 1700 K, as well as at 1850 K. Carburization of the W(211) surface below 1000 K produced no ordered structures. Above 1000 K carburization produces a c(6 × 4)C which is suggested to result from a hexagonal tungsten carbide overlayer. CO dissociation is inhibited on the W(211)?c(6×4)C surface. Sulfur initially orders into a c(2 × 2)S structure on W(211). Increased coverage leads to a c(2×6)S structure and then a complex structure. Adsorbed sulfur reduces CO dissociation on W(211), but even at the highest sulfur coverages CO dissociation was observed. Sulfur was found to desorb as atomic S at 1850 K for sulfur coverages less than $\text{7}\text{6}$ monolayers. At higher sulfur coverages the dimer, S₂, was observed to desorb at 1700 K in addition to atomic sulfur desorption.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

Binding of an adatom to a simple metal surface

The density functional formalism of Hohenberg and Kohn is used to investigate the energies, charge densities and forces which hold an adatom on the surface of a simple metal. The valence wavefunction of the adatom is fitted to the Herman-Skillman solutions at large distance and is simplified somewhat in the core region. The field of the ion is represented by the Ashcroft pseudopotential. For the metal the jellium model is used. Detailed calculations are carried out for a sodium adatom on a sodium surface. Simply juxtaposing adatom and surface gives a binding energy of about $\text{1}\text{3}\text{eV}$. This value is approximately twice the surface energy per atom in the close-packed plane. Charge redistributions as determined variationally increase the binding energy by about 10%. The redistribution is primarily a dipole induced on the adatom at close distances, but at somewhat larger distances a prolate quadrupole also appears on the atom. A small amount of charge is also drawn from the metal toward the atom. The equilibrium distance for the adatom turns out to be 1.66 Å from the surface, as compared with 1.52 Å, the observed value for one-half the distance between the close-packed planes. Contour plots of the piling-up of electronic charge between the adatom and the metal are presented.     

Co desorption and adsorption on Pt(111)

The kinetics of the desorption of CO from a Pt(111) crystal between 419 and 505 K is reported using a Low-Energy Molecular-Beam-Scattering (LEMS) technique with a helium probe beam and a CO dosing beam. The resulting first-order Arrhenius rate constant is $\text{k = 2.7 × 10}{}^{13}\text{exp(?31.1}\text{kcal}\text{mole}\text{· RT) s}{}^{\mathrm{?1}}$. We also report a study of the equilibriumadsorbed CO between 400 and 600 K using LEMS. These results, fitted to a Temkin isotherm model, indicate that the adsorption energy decreases linearly with surface coverage with the average value equal to $\text{31.1 + 1.2}\text{kcal}\text{mole}$ over the coverage range 0 < θ ? 0.5. The average harmonic oscillator frequency of the adsorbed CO molecules is 191 ± 76 cm^?1.     

A new model for CO ordering at high coverages on low index metal surfaces: A correlation between LEED,HREELS and IRS: I. CO adsorbed on fcc (100) surfaces

In this paper, we reexamine the surface structures of CO on (100) surfaces of copper, palladium, nickel and platinum. We use the types of site determined by High Resolution Energy Electron Loss Spectroscopy (HREELS), or Infra Red Spectroscopy (IRS), to propose new models for the arrangement of CO molecules at coverages exceeding $\text{1}\text{2}$, i.e. at coverages higher than those corresponding to simple structures c(2 × 2) and $\text{p}\text{(2}\text{2}\text{×}\text{2}\text{)}\text{R}\text{45°}$. Laser simulations allow us to decide the validity of the proposed models. The consequences of these models are the existence of at most two adsorption sites at all coverages, and the existence of antiphase domains separated by walls to form the complex structures. The transition between two consecutive structures due to an increase of coverage is a unidirectional compression, generating more wall regions.     

The adsorption and dehydrogenation of cyclopentane on Ru(001)

The adsorption of cyclopentane on Ru(001) has been studied using Electron Energy Loss Spectroscopy (EELS) and Thermal Desorption Mass Spectroscopy (TDMS). Thermal desorption shows with increasing coverage a chemisorbed first layer desorbing at 180 K with subsequent multilayer formation. The vibrational spectrum of the first chemisorbed layer is characterized by a C-H soft mode at 2610 cm^?1. This mode is ascribed to a C-H-metal interaction, which is also responsible for the dehydrogenation to cyclopentene upon annealing to 200 K. It appears that a $\text{close}$ geometrical fit between the $\text{entire}$ molecule and the metal substrate is not necessary for this type of interaction. Coadsorbed oxygen suppresses the C-H-metal interaction. This is believed to be due to site-blocking or ligand effects of oxygen on the three-fold hollow sites of Ru(001).     

Ab-initio calculation of the heat of formation in the metallic transition metal monoxides

In order to understand the cohesive properties of transition metal monoxides the ground state energies of the transition metal T, the monoxide TO and oxygen atom 0 involved in the reaction $\text{M}\text{+}\text{1}\text{2}\text{(}\text{0}{}_2\text{) ?}\text{M O}\text{+ Δ}\text{H}$ have been calculated for metallic TiO and VO following the density functional method in the augmented spherical wave approximation. The evaluated values of ΔH compare very well with those obtained experimentally.