Ag 5s Research Articles

Evidence for a double charge transfer contribution to SERS in the Ag/Ag (CN) 2− system

Using a square wave potential perturbation and monitoring the Raman intensity in parallel, the potential dependence of Ag(CN) 2 − spectra was studied with different exciting wavelengths. A double peak structure was found for the C-N stretch frequency both peaks having different excitation characteristics. In addition, the quenching rate of ad-complexes was measured at negative potentials together with the activation energy. From E A = 75 kJ mol −, the energy of an acceptor level can be estimated located about 3 eV above E F. This level presumably corresponds to the Ag 5s orbital which is shifted upwards due to the interaction with the ligands of the ad-complex. Since the CN − 5σ or 1π orbitals form a donor level, the possibility of a double charge transfer contribution to the SERS enhancement exists.

Electronic structure and Schottky-barrier formation of Ag on n-type GaAs(110).

The electronic structure and Schottky-barrier formation of the Ag/[cleaved n-type GaAs(110)] interface are studied at room temperature by using ultraviolet and x-ray photoemission spectroscopy. It is found that more metal coverage is needed to pin the surface Fermi level with Ag than with most other metals. The Schottky barrier is resolved as 0.83\ifmmode\pm\else\textpm\fi{}0.1 eV high (compared to 0.90\ifmmode\pm\else\textpm\fi{}0.1 eV for Cu/[n-type GaAs(110)] and 0.92\ifmmode\pm\else\textpm\fi{}0.1 eV for Au/[n-type GaAs(110)]). At the early stage of Ag deposition, the so-called atomiclike Ag 5s states and Ag 3d core levels are observed, which may imply that the interaction between Ag and GaAs is weak and raises the possibility that the Ag atoms may form clusters during the initial stages of metal deposition. It is found that the Ag valence-band photoyield has a maximum at a coverage of 10\ensuremath{\sim}30 A\r{}. This photoyield enhancement is explained in terms of Ag clustering and the Stranski-Kristanov growth pattern. In the framework of the unified defect model, it is proposed that the heat released due to the Ag-Ag interaction is responsible for the creation of the defect levels which pin the Fermi level in the Ag/[n-type GaAs(110)] interface. Thus, defects are produced under the clusters but not on the open GaAs surfaces not covered by Ag; this leads to the increased amount of metal needed to complete the surface Fermi-level pinning.

On the electronic structure of Ag chalcogenides

The electronic structure of Ag chalcogenides in the α phase, which exhibit an interesting, electronic semiconducting behaviour as well as the fast ion transport, is discussed on the basis of an energy band structure calculation. As a simplest way of simulating the effect of the Ag ions on the electronic states, some hypothetical crystalline compounds are constructed such as the perovskite, the sodium chloride and the flourite structures. The absolute magnitude of the calculated conduction electron effective mass is quite small irrespectively of the structures, about 10% of the free-electron mass, in semiquantitative agreement with experiments. A deviation from an effective mass approximation near the conduction band bottom is found to be appreciable, and to explain reasonably well experimental results. An origin of these features of the conduction band is a rather strong hybridization of the Ag 5s band and the chalcogen s band. The calculation also shows that the hybridization of the Ag 4d band and the chalcogen p band can affect the absolute magnitude of the hole effective mass appreciably, and that the energy band gap depends sensitively on these s-s and d-p hybridization effects.

The e.s.r. spectrum of Ag(CO)3, a 2A1? ground state molecule

Ag(CO)3 has been prepared by the reaction of carbon monoxide with silver atoms in a rotating cryostat at 77 K and its isotropic e.s.r. spectrum observed; in the case of 107Ag(13CO)3 in adamantane it consists of a doublet of quartets with a107(1)=-566.5, a13(3)= 19.2 G (1 G = 10–4T), and g= 2.00086, which suggests a D3th molecule with a 2A1′ ground state and the unpaired electron located largely in the Ag 5s orbital.

Charge transfer effects in surface enhanced Raman scattering

Surface enhanced Raman scattering (SERS) due to charge transfer interactions between the adsorbed molecule and the metal surface is analyzed using the semiempirical Wolfsberg-Helmholz method 1 to relate the molecule-surface interactions and the resulting charge transfer states to the overlap integrals between the metal conduction-band orbitals and an acceptor or donor molecular orbital of the molecule. Calculations for the model system of ethylene adsorbed on silver (approximated as a simple cubic metal with tight binding wave functions constructed from Ag 5s valence orbitals), with charge-transfer excitation of an electron from the metal to the antibonding ethylene π orbital, show that charge-transfer Raman enhancements of the order of 10 to 1000 are possible if the charge-transfer band is partially resonant with the exciting radiation. The net enhancement is the product of the charge-transfer gain and the electrodynamic enhancement due to plasmon resonances at surface roughness elements. Symmetric vibrations usually will be enhanced substantially more than nonsymmetric ones by charge-transfer because, in contrast to non-resonant Raman scattering, the vibrational coupling is primarily Franck- Condon (due to differences in the equilibrium nuclear configurations of the ground and excited charge transfer states and the resulting nonorthogonality of different vibrational sublevels of these states) rather than Herzberg-Teller (due to vibrationally induced changes in the electronic wave functions). The charge-transfer mechanism is selective with the most enhanced vibrations involving those atoms which experience the greatest change in electron density between the ground and excited charge-transfer state. A recent report of SERS for benzene on platinum, 2 strongly suggests charge-transfer enhancement because the electromagnetic-field-enhancing plasmon resonances are strongly damped in this metal. The complete paper will be published in the December 1, 1982 issue of the Journal of Chemical Physics.

Non-empirical valence-electron calculations on ethylene complexes of silver(I) and palladium(0)

Non-empirical valence-electron calculations have been made on the complexes [Ag(C2H4)]+ and [Pd(C2H4)n](n= 1, 3, or 4). The conclusion of earlier ab-initio calculations on [Ag(C2H4)]+ that donation of electrons from ethylene π to Ag 5s and 5p orbitals is the dominant bonding feature is confirmed. For the palladium(0) complexes, electron transfer occurs mainly from Pd to ethylene with both σ* and π* orbitals accepting electrons. A Mulliken-population analysis shows that the net charge residing on the ligands has a greater component in σ than in π orbitals. The important role of σ* orbitals in the bonding provides an explanation of why C2F4 complexes are more stable than C2H4.

ESCA and molecular orbital studies of small silver particles

The results of x-ray photoelectron spectroscopy studies of evaporated silver clusters on carbon are reported. The samples were prepared and the particle-size distribution was determined. Spectra have been recorded at coverages from 2.7×1013 to 4×1016 atoms cm−2. The very low coverages yield particle distributions dominated by single-atom centers, whereas the high coverages represent bulk metallic silver. Low-coverage spectra show low binding energy states that are thought to result either from interactions with the carbon substrate or reaction with sulfur contamination. CNDO calculations show low binding energy states to exist for Ag4 on a C16 model substrate. At higher coverages the Ag 4d states broaden and split, and the threshold moves 2.5 eV to lower binding energy. The splitting of the Ag 4d band is attributed to spin–orbit and crystal field effects. The change in threshold can be attributed to the filling and spreading of the Ag 5s states to form the s–p conduction band of the bulk metal. The measured threshold shift of 2.5 eV (Fermi-level reference) is to be compared with 1.5 eV (vacuum-level reference) predicted by extended Hückel calculations. Small shifts in the Ag 3d core levels are also observed and follow the trends predicted by CNDO charge calculations.