Ag Band Research Articles

Study of supported silver states by the method of electron spectroscopy of diffuse reflectance

Different states of silver supported on oxide carriers have been studied by the method of electron spectroscopy of diffuse reflectance (ESDR). The nature of the support, the modifying additions and redox treatments have a great effect on the position and intensity of the absorption bands of Ag 0, Ag + and Ag n δ+ in the EDR spectra of supported Ag-containing systems. The ESDR method can be used effectively for the investigation of model and commercial silver catalysts.

Electronic structure of Au and Ag overlayers on Ru(001): The behavior of the noble-metal d bands.

The metal d bands of Au and Ag individually adsorbed on Ru(001) have been observed with photoemission as have the core 4f levels of Au. The adlayer d bands display shifts in their centroids as well as band narrowing due to having fewer noble-metal nearest neighbors relative to bulk metals. The narrowing may be monitored by measuring the doublet nature of these valence-band densities of states or by sampling the overall d-band width with the former not susceptible to instrumental broadening problems. These widths receive contributions from spin-orbit splitting ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{s}.\mathrm{o}.}$ and intrinsic d-band effects ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{band}}$. These have been separated and the inferred ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{band}}$ for a Au monolayer suggests that the substrate is contributing measurably to the Au bandwidth while having little or no effect on Ag. Providing that the Au adlayer 4f-level shifts are measured with respect to the surface of Au rather than the bulk, a consistent picture of d-band and core-level chemical shifts emerges that, granted the lower number of unlike nearest-neighbor atoms, is compatible with what has been previously observed for bulk noble-metal alloys.

Influence of substrate steps on the properties of epitaxial noble-metal films

In this work we report on the influence of surface steps on the growth structure, the energetics and the electronic-properties of Ag on a stepped Ru(101̄17) surface as deduced from TDS, AES, LEED, ARUPS and PAX measurements. Increased desorption temperatures as well as PAX data suggest a selective “decoration” of the surface step sites at low Ag coverages. In particular no Ag island formation is detected neither with PAX, nor with LEED, nor with ARUPS for θ Ag < 0.2 ML, in contrast to the findings on flat Ru(0001). As an interesting consequence, a significant change in the Ag valence band spectra is observed. While already 0.05 ML Ag on a flat Ru(0001) surface exhibit a clear 4d-spin-orbit- and crystal-field-splitting of 1.35 eV no such splitting is detectable for up to 0.15 ML Ag on the stepped surface. Furthermore, a quasi-one-dimensional Ag band structure is suggested by ARUPS data for these low Ag coverages showing a dispersion of the Ag d-states only parallel but not perpendicular to the substrate steps.

Silver clusters in the cages of zeolites: A quantum chemical study

The electronic structure of zeolite A is developed in a step by step procedure from the simple OhH8Si8O12 molecule, to the ∞ 1 [(-O)2H4[Si8O12)] chain, to the ∞ 2 [(-O4)(Si8O12)] layer, and finally to the silica zeolite A framework ∞ 3 [Si24O48]. It is remarkable how well the calculated band structures of both, ∞ 2 [(-O)4(Si8O12)] and ∞ 3 [Si24O48] correspond to the experimentally determined band structure of α-quartz with a Fermi level of -10.55 eV. The HOMO region consists in each case of nonbonding 2p-oxygen bands which in a localized language can be denoted as oxygen lone pairs ( | O<). We observe in each case the typical behaviour of an insulator with saturated valencies whose electronic structure can be described as being localized and is already present in the starting Oh-H8Si8O12 molecule. The double-8-rings D8R of the ∞ 2 [(-O)4(Si8O12)] layer have a pore diameter of 4.1 A, the same as the pore opening of zeolite A. It is large enough to accept up to four Ag, forming ∞ 2 [(-O)4(Si8O12)Agn], n = 1, 2, 3, 4, layers, suitable for modelling the electronic interactions between the zeolite cavity embedded silver clusters and between the clusters and the zeolite framework. With one Ag per D8R the band structure is simply a superposition of the 4d, 5s and 5p levels of a layer of nearly noninteracting Ag and the silicon dioxide layer. The Ag-d band lies below the oxygen lone pairs, the Ag-s band lies about 3 eV above the oxygen lone pairs, and the Ag-5p bands are in the antibonding silicon dioxide region. The first electronic transition is of oxygen lone pair to Ag-5s LMCT type. Increasing silver content results in progressive splitting of the 5σ Ag bands and shifts the first (Agm+ n)• ← (| O<) charge transfer transition to lower energies. The filled Ag 4d-bands lie always significantly below the (| O<) HOCOs (highest occupied crystal orbitals) but their band width increases with increasing silver content. In all cases the zeolite environment separates the Ag clusters through antibonding Ag-(← O<) interactions so that the coupling remains weak and it makes sense to describe the Ag clusters in the D8R as quantum dots weakly interacting with each other.

Isotope effects in the Raman spectra of 13C enriched C 60

Recent studies carried out on an organic superconductor, ß-(BEDT-TTF) 2I 3, have shown the important role played by the 13C isotope on the T c transition, when substitution is made on the central CC bond of the molecule. Simultaneously, Raman studies determined the frequency shifts observed in the CC stretching vibrations in the substituted samples and consequently, put in evidence the role of the intramolecular Ag vibrations. We have extended these studies to compounds of the new carbon family, the fullerenes, in which 82% of the carbon atoms were substituted with 13C in C 60. The superconducting transition measured on the same sample doped with potassium is lowered by 1.5 to 2K as compared to the standard material doped in similar conditions. In the Raman spectrum, the main Ag band measured at 1467 cm −1 in 12C 60 is shifted to 1420 cm −1 in enriched 13C 60. Taking into account the ratio between the molecular weight of the enriched 13C 60 and of the standard material, one predicts a theoretical value for this mode at 1418 cm −1 in the enriched sample. A preliminary analysis of the Raman spectrum will be presented.

Quantum-well states and magnetic coupling between ferromagnets through a noble-metal layer.

Using inverse photoemission and photoemission we find that the bulk bands become discretized in highly perfect layer structures, such as Cu on fcc Co(100), Cu on fcc Fe(100), Ag on bcc Fe(100), Au on bcc Fe (100), fcc Co on Cu(100), and bcc Fe on Au(100). The electronic structure is analyzed in the framework of quantum-well states consisting of bulk Bloch functions modulated by an envelope function. The wavelength of the envelope function is determined from the \ensuremath{\lambda}/2 interferometer fringes produced by the periodic appearance of quantum-well states with increasing film thickness. Using k conservation, one obtains an absolute measurement of the band dispersion for the s,p bands of Fe, Cu, Ag, and Au. Quantum-well states at the Fermi level are found to be closely connected with oscillatory magnetic coupling in superlattices. They are spin polarized, even in noble metals, due to the spin-dependent band structure of the confining ferromagnet. The oscillation period is half the wavelength of the envelope function. The corresponding wave vector is given by the Fermi wave vector and by the wave vector of the nearest s,p band edge via 2(${\mathit{k}}_{\mathrm{edge}}$-${\mathit{k}}_{\mathit{F}}$). This turns out to be equivalent to Ruderman-Kittel-Kasuya-Yosida theory.

Vibrational spectroscopy of excited electronic states in carotenoids in vivo. Picosecond time-resolved resonance Raman scattering

The vibrational spectroscopy and population dynamics of excited singlet (2(1)Ag), excited triplet (3B u), and the ground (1Ag) electronic states of carotenoids in chromatophores of Chromatium vinosum (mainly spirilloxanthin and rhodopin) and of the same carotenoids in benzene solutions are examined by picosecond time-resolved resonance Raman scattering. Coherent Stokes Raman scattering from the ground states of carotenoids in chromatophores also is observed. Resonance Raman spectra of in vitro rhodopin and spirilloxanthin when compared with in vivo data demonstrate that scattering from spirilloxanthin dominates the in vivo spectrum. Comparisons of the time-dependent intensities of 2(1)Ag and 1Ag resonance Raman bands from both in vitro and in vivo carotenoids suggest that vibrationally excited levels in 1Ag are populated directly by the decay of the 2(1)Ag state and that these levels relax into a thermalized distribution in less than 50 ps. The appearance of asymmetrically broadened, ground-state resonance Raman bands supports this conclusion. Formation of the 3Bu state is observed for carotenoids in chromatophores, but not for in vitro spirilloxanthin indicating that the 3Bu state is formed by fission processes originating from the spatial organization of pigments within chromatophores. The rate at which the intensities of 2(1)Ag resonance Raman bands decay is faster for the carotenoids in vivo than for those in vitro thereby indicating that additional relaxation channels (e.g., energy transfer to bacteriochlorophylls) are present in the chromatophore. The similarity of the in vivo and in vitro 2(1)Ag resonance Raman spectra shows that no significant modifications in the vibronic coupling has been caused by the chromatophore environment.

Studies of the Relationship of the 230-kD and 180-kD Bullous Pemphigoid Antigens

In bullous pemphigoid (BP), autoantibodies from most patients recognize a high molecular weight 230-kD epidermal antigen (Ag) by immunoprecipitation. By Western immunoblotting, 50-70% of sera recognize the high molecular weight Ag, but 30-50% recognize a low molecular weight, 180-kD epidermal Ag. We examined the specificities of affinity-purified antibodies against these Ag. Antibodies specific for the 230- and 180-kD Ag were prepared by immunoaffinity against Ag immobilized on nitrocellulose and released by acid glycine. IgG eluted from the 230-kD Ag band retained its specific binding to the 230-kD Ag by immunoblotting, and bound to the epidermal basement membrane zone (BMZ) by indirect immunofluorescence (IF) and to hemidesmosomes by indirect immunoelectron microscopy (EM). IgG affinity purified by the 180-kD Ag band bound only the 180-kD Ag in immunoblotting, with no cross reaction to the 230-kD Ag, bound the epidermal BMZ by indirect IF, and also bound to hemidesmosomes in immuno-EM. IgG specific for the 230-kD Ag in immunoblotting immunoprecipitated only the 230-kD Ag, with no apparent precipitation of the 180-kD Ag. Surprisingly, IgG specific for the 180-kD Ag precipitated both the 180- and the 230-kD Ag in immunoprecipitation, and the 230-kD Ag band was much more intense than the 180-kD Ag band. This study shows that apparent cross-reactivity between these Ag by BP autoantibodies can only be detected in native conditions by immunoprecipitation, and cannot be demonstrated using denatured Ag in immunoblotting. The two proteins appear to be distinct Ag, closely associated in the epidermal hemidesmosome, but the exact relationship of these Ag to each other may not be clarified until complete structural data become available.

Studies on the pathogenic roles of antigens isolated from renal tubular cells in the classical Heymann nephritis

In order to analyze the antigens (Ags) which cause Heymann nephritis (HN) and mechanisms of IC formation in the subepithelial site of GBM, we isolated two Ags from renal tubular epithelium (RTE) from Wistar rats and prepared rabbit antiserum against these Ags. Classical HN was induced by immunization with crude renal tissue suspension by the method of Heymann et al. Immunofluorescence method in normal rat kidney revealed that the Ag with MW of 65 KD (= beta) existed not only in RTE, but also in the glomerular capillary wall. On the other hand, the Ag with MW of 35 KD (= gamma) only existed in RTE. Circulating Abs and glomerular eluted Abs from the kidney of classical HN, reacted to RTE only by indirect immunofluorescence. Soluble Fx1A, beta and gamma were electrophoresed on SDS-PAGE gels and transblotted onto nitrocellulose membrane, anti beta Abs reacted to several bands including 330 KD band of Fx1A and beta Ag. However anti gamma Abs reacted to only 330 KD band of Fx1A and gamma Ags. On the other hand, Glomerular eluted Abs from the kidney of classical HN reacted to several bands of Fx1A and gamma Ag, but not to beta Ag. These facts suggest that beta and gamma are components of 330 KD protein of Fx1A. The gamma Ag is thought to be one of the major pathogenic Ag in HN.

Coverage dependence of the spin-resolved photoemission from epitaxially grown Ag layers on Pt(111)

Ag/Pt(111) has been studied by spin-, angle-, and energy-resolved photoemission with circularly polarized synchrotron radiation of BESSY for different Ag coverages. The prepared layers were characterized by AES and LEED and turned out to grow epitaxially. An independent layer thickness determination and some additional structure information were obtained from Rutherford backscattering studies in combination with channeling. The spin-resolved photoemission experiments were performed for normal incidence of circularly polarized light and normal emission of the photoelectrons. The spin information allowed a separation of spin-orbit split peaks of the Ag adsorbate layer and partly also a separation of Ag adsorbate peaks from the Pt substrate background intensity. This “method-induced improved resolution” was used to deduce information about the development of the Ag band structure in the Λ-direction from the coverage dependence of spin-resolved photoemission spectra. Dispersion of occupied bands was found to be already almost completely developed for a three-layer system. Information about the development of the band structure in its unoccupied part was obtained from the resonant behavior of peaks at about 6 eV below E F. The resonant behavior is already observed for three Ag layers and increases with layer thickness. We observe a peak in the photoemission spectra contributing to the resonances which is not predicted by existing theoretical models.

Angle-resolved photoemission study of the Ag band structure along the ΓΛL line

Angle-resolved photoemission experiments have been carried out on Ag(111) in the photon energy range from 13 to 65 eV with both s- and sp-polarised synchrotron radiation. The valence band structure of Ag has been determined along the high-symmetry Lambda line by normal emission measurements. For photon energies smaller than 24 eV, the photoemission peaks could be interpreted as results of direct transitions between initial and final energy bands in the relativistic band structure calculated by Christensen (1972), and by Eckardt et al. (1984). For photon energies larger than 24 eV, the final band was obtained by fitting a free-electron-like final band (m*e=1.10, V0=-5.0 eV) to experimentally verified calculated symmetry points. The experimental valence bands deviate by about -0.3 eV from theory. Measured critical-point energies (in eV, band indices in parentheses) are: E( Gamma 8+(2,3))=-6.15+or-0.10, E( Gamma 7+(4))=-5.70+or-0.10, E( Gamma 8+(5,6))=-4.95+or-0.10, E(L6+(1))=-7.10+or-0.10, E(L4,5+(2))=-6.25+or-0.10, E(L6+(3))=-5.80+or-0.10, E(L6+(4))=-4.55+or-0.10 and E(L4,5+(5))=-4.30+or-0.10. A surface resonance was detected 4.2 eV below the Fermi level at the centre of the surface Brillouin zone Gamma , which has an origin similar to that of the surface resonances that have been observed on Cu(001) and Ag (001). A possible surface state is also reported at about 7.4 eV below the Fermi level at the Gamma point. The nature of this impurity-sensitive structure is not understood.

Angle-resolved photoemission study of the Ag band structure along the ΓΔX line

Angle-resolved photoemission experiments have been carried out on Ag(001) in the photon-energy range from 13 to 105 eV with both s- and s-p-polarised synchrotron radiation. The valence-band structure has been determined along the high-symmetry Delta line. For photon energies smaller than 30 eV the photoemission peaks could be interpreted as results of direct transitions between initial and final energy bands in the relativistic self-consistent band structure calculated by Eckardt, Fritsche and Noffke (1984). For photon energies larger than 30 eV the final band was obtained by extrapolation from the calculated Delta 6 band to the 2 Gamma X point of a free-electron band with inner potential -5.2 eV. Measured critical points for bands numbered with decreasing energy are: Gamma 8+=-4.8 eV, Gamma 7+=-5.50 eV and Gamma 8+=-5.95 eV, in good agreement, within 0.13 eV, with the theoretical values. A surface resonance is also reported 4.1 eV below the Fermi level at the centre of the surface Brillouin zone Gamma which has the same origin as the surface resonances that have been observed on Cu(001) and Pd(001).

Growth and thermal stability of Ag or Au films on Nb(110).

The growth and thermal stability of ultrathin (<50-A-thick) Ag and Au films on Nb(110) was investigated with use of photoemission spectroscopy and low-energy electron-diffraction (LEED) patterns. The LEED results show that Ag or Au forms a monolayer which is commensurate with the substrate, but also show that the structure of this overlayer reverts to a more close-packed structure after three monolayers (ML) have been deposited. Photoemission spectra show trends that suggest the evolution of the Ag or Au band structure with coverage and also show that a surface deposit of at least 2 ML of Ag or Au is required to observe valence-band states characteristic of the bulk metal. Further examination of the photoemission spectra suggests that the Ag or Au d-band states play only a small role in the chemical bonding that occurs between the noble metals and Nb(110), and this can be contrasted with the extensive d-band involvement in chemisorption bonds formed between Pd or Pt and Nb(110). Heating does not cause substantial change in the electric structure of 12.8-ML-thick Ag films and they thermally desorb from Nb when heated above 830 K. Similar Au films intermix with Nb to form a disordered surface phase when heated beyondmore » 750 K. Desorption or complete in-diffusion of Au occurs when the substrate is heated beyond 1100 K. Implications for the formation of strained surface layers and surface chemical properties of noble metals on Nb(110) are discussed.« less

1,4-Dihydropyrrolo[3,2-b]pyrrole: The Electronic Structure Elucidated by Photoelectron Spectroscopy

Abstract The electronic structures of 1,4-dihydropyrrolo[3,2-b]pyrrole and some of its alkyl derivatives were studied by means of He(I) photoelectron spectroscopy combined with the MNDO calculation and a first-order perturbation theoretical method. The first five bands of the parent pyrrolopyrrole are assigned, from the top, to the π5(au), π4(au), π3(bg), π2(bg), and σ(ag) bands respectively. The MO model, which explains well the spectral data, indicates that the nitrogen lone-pair electrons are mainly delocalized in the π4, π2, and π1 orbitals. The ionization potentials of the HOMO levels of these compounds, which are characterized by the large pz contributions from the α-carbons, are the lowest among the hitherto known π-electron-excessive heteroaromatic compounds. 1,4-Dihydropyrrolo[3,2-b]pyrrole is concluded to be the system with the most efficient π-electron-donating ability among 10π-electron systems.

Charge-transfer band and sers mechanism for the pyridine-Ag system

Transmission spectra in the ultraviolet and visible region as well as SERS spectra were measured for chemisorbed pyridine on vacuum evaporated (island) Ag films and on annealed (heated) Ag films. Strong and clear SERS spectra were observed with an enhancement factor of ~10 6 and were definitely assigned to the N-bonded pyridine. On adsorption of pyridine a new peak was detected at ~600 nm as the charge-transfer (CT) band for the pyridine-Ag interaction, which could neither be ascribed to any impurity nor to aggregated Ag particles. The surface morphologies of the Ag films observed by a SEM (highest resolution 1.5 nm) did not show any appreciable changes on adsorption of pyridine, when the Ag substrates were properly prepared. The molar extinction coefficient of the CT band was 10 3–10 4 ℓ mol −1 cm −1. On the annealed Ag films the CT band, which was well isolated from the Ag band, appeared on adsorption of pyridine and disappeared on desorption of pyridine, reversibly. The stronger the CT band, the stronger the SERS spectrum. The excitation profile of the Raman 1010 cm −1 band overlapped well with the CT band. The number of adsorbed pyridine molecules was estimated, from the π — π ∗ band of pyridine at 250 nm, to be (6–15) × 10 15 molecules cm 2 corresponding to a coverage factor of 20–60. The contributions of the electromagnetic (EM) effect and the CT effect to the apparent SERS enhancement factor were estimated to be approximately 10–10 2 and 10 3, respectively. The reason why the CT band had not been detected as an optical absorption band was discussed. The CT band observed is compared with those reported by EELS or reflection measurements.

Electronic states at clean and adsorbate-covered metal surfaces

Angle-resolved electron spectroscopies, in particular ultraviolet photoemission and ultraviolet inverse photoemission, supply rather detailed information about the energy eigenvalues E( k) and the corresponding electron momenta hk of occupied and/or empty electron states at solid surfaces. For metals like Ni, Cu, Ag and Au bulk bands E( k) may be mapped with considerable accuracy using methods of absolute k-determination. The systematics of surface bands E( k ∥) is well established now and the shape of the surface barrier potential can be derived from these data with high precision. We summarize what is known experimentally, what is believed to be understood and we point out some open questions. Finally the potential of inverse photoemission for the study of molecular adsorbates is discussed in short.

High-resolution angle-resolved photoemission study of the Ag band structure along Lambda.

High-resolution angle-resolved photoelectron spectra have been obtained from the (111) face of an Ag crystal for normal electron emission. These spectra made it possible to determine experimentally the valence-band structure of Ag along the high-symmetry line ..lambda.. with better accuracy than previously (P. S. Wehner et al., Phys. Rev. B 19, 6164 (1979)), and also revealed a second flat band near point GAMMA in the conduction bands 23 eV above the Fermi level. The new binding energies at points GAMMA and L were used along with the data of other workers to construct an experimental band structure of Ag with the mixed-basis interpolation scheme of Smith (Phys. Rev. B 9, 1365 (1974)). The augmented-plane-wave calculations of Eckhardt, Fritsche, and Noffke (J. Phys. F 14, 97 (1984)) agree very well with the experimentally determined bands.

Indirect Magnetoelastic Interaction and Magnetostriction of Dilute Rare‐Earth Alloys

AbstractThe theory of indirect magnetoelastic interaction in dilute rare‐earth alloys is rederived in more general form. Expressions are obtained for conduction electron contribution to the crystal field parameters and magnetoelastic coupling constants. They appear to depend on Slater integrals, electron‐phonon interaction, and band structure of the alloy. The particular case of an alloy of the cubic symmetry is discussed as an example. Magnetoelastic coupling constants of some AgRE alloys are calculated within the nearly free electron approximation for the Ag band states.

Infrared spectroscopic study of electrically conducting tetracyanoquinodimethane salts of copper chelates with ethylenediamine

The infrared spectra of Cu(en)(TCNQ)2 and Cu(en)2(TCNQ)2(en: ethylenediamine, TCNQ: 7,7,8,8-tetracyanoquinodimethane) have been shown to exhibit totally symmetric TCNQ Ag bands activated by a vibronic process in addition to inherently i.r.-active Bu bands. In the spectrum of Cu(en)2(TCNQ)3 some Ag modes were developed as sharp indentations in a very broad electronic absorption envelope which had a steep edge at 1100 cm–1. The assignment of the i.r. spectra was made by comparing them with those of Cu(TCNQ) and Cu(TCNQ)2. The results indicate the following formal charge distributions of the compounds in accordance with magnetic data: Cu+(TCNQ–), Cu+(TCNQ–)(TCNQ0), Cu+(en)(TCNQ–)(TCNQ0), Cu2+(en)2(TCNQ–)2 and Cu2+(en)2[(TCNQ)3]2–. The electrical properties, especially the thermoelectric power, of each compound were explained consistently by taking into account a solid-state equilibrium on the basis of the corresponding formula described above.

Raman spectroscopic studies on the molecular reorientation and vibrational relaxation in liquid ethylene

We report detailed reorientation (ø2R ) and vibration correlation functions derived from the Raman v 2(Ag ) and v 3(Ag ) bands of liquid ethylene between the freezing (92 K) and boiling (169 K) points. Rotation is essentially ‘free’ for ∼0·2 ps (even though the band profiles are essentially lorentzian) and the ø2R functions are in poor agreement with those predicted by the J-diffusion model since τ J values fitted to the long time part of the correlation functions are much lower than 0·2 ps. The reorientation correlation times follow the liquid viscosity with a κ factor of 0·17—reflecting the relative weakness of the intermolecular torques in this liquid. The vibrational part of v 3 shows the effects of ‘motional narrowing’ observed when phase relaxation occurs via relatively long ranged potentials. The Kubo environmental relaxation times τe are least an order of magnitude longer than that expected if phase relaxation occurred via collisional interaction. Their temperature and viscosity dependence are also consistent with the Kubo model of dephasing by homogeneous broadening due to long ranged potentials but not with the modulation of such potentials by reorientational motions.