On the Structure of the Helmholtz Layer and its Implications on Electrode Kinetics

Concepts and selected experiments on the structure of the Helmholtz double layer at the metal- and semiconductor - electrolyte phase boundary are reviewed. The widely used microcapacitor approach of the double layer and its limitations are assessed. Observations on the influence of the electrode potential on the energetic position of surface states at the Ag-electrolyte contact are compared to the predictions of classical charge transfer models that are based on transition state theory where adiabatic tunneling is assumed. Distance tunneling spectroscopy on Au(111) surfaces shows pronounced variations in tunneling barrier heights that are connected to the inner structure of the Helmholtz layer and implications on electrode kinetics are presented. At the semiconductor-electrolyte contact, the influence of the electrode potential on a charge injecting species that results in photocurrent doubling is reviewed for low- and higher doped Si(111) electrodes, showing that the complex that injects electrons into the conduction band is located outside the semiconductor surface. The observations are correlated with the search for low overpotential earth abundant electrocatalysts for solar fuel generation of solar fuels

Oscillations at the Si/electrolyte contact: Relation to Quantum Mechanics

The basic process at the surface of the Si electrode is characterized by a cyclic oxidation of a thin silicon layer and the subsequent removal of the oxide by etching. Here, the oxide thickness evolves not uniformly due to cracks and nanopores. The mathematical model used to describe the phenomenon is based on a sequence of time dependent (oxide thickness) oscillator density functions that describes the passing of the (infinitesimal) oscillators through their minimum at each cycle. Two consecutive oscillator density functions are connected by a second order linear integral equation representing a Markov process. The kernel of the integral equation is a normalized Greens Function and represents the probability distribution for the periods of the oscillators during a cycle. Both, the oscillator density function and the two-dimensional probability density for the periods of the oscillators, define a random walk. A relation between the oscillator density functions and solutions of the Fokker-Planck equation can be constructed. This allows a connection of the oscillations, originally considered only for the description of a photo-electrochemical observation, to the Schrodinger equation. In addition, if the trajectory of a virtual particle, located at the silicon oxide electrode surface, is considered during one oscillatory cycle, then it can be shown that the displacement of the particle measured at the electrode surface performs a Brownian motion

Quantum phases of a chain of strongly interacting anyons

We study a strongly interacting chain of anyons with fusion rules determined by SO(5)2. The phase portrait is identified with a combination of numerical and analytical techniques. Several critical phases with different central charges and their corresponding transitions identified.Comment: 5 pages, 4 figure

Core-Level X-Ray Photoemission Satellites in Ruthenates: A New Mechanism Revealing the Mott Transition

Ru 3d core-level x-ray photoemission spectra of various ruthenates are examined. They show in general two-peak structures, which can be assigned as the screened and unscreened peaks. The screened peak is absent in a Mott insulator, but develops into a main peak in the metallic regime. This spectral behavior is well explained by the dynamical mean-field theory calculation for the single-band Hubbard model with on-site core-hole potential using the exact diagonalization method. The new mechanism of the core-level photoemission satellite can be utilized to reveal the Mott transition phenomenon in various strongly correlated electron systems, especially in nano-scale devices and phase-separated materials.Comment: 4 pages, 5 figures, submitted to PR

Density of Superfluid Helium Droplets

The classical integral cross sections of large superfluid 4He_N droplets and the number of atoms in the droplets (N=10^3-10^4) have been measured in molecular beam scattering experiments. These measurements are found to be in good agreement with the cross sections predicted from density functional calculations of the radial density distributions with a 10-90 % surface thickness of 5.7\AA. By using a simple model for the density profile of the droplets a thickness of about 6-8\AA is extracted directly from the data.Comment: 27 pages, REVTeX, 5 postscript figure

Photoelectrochemical Conditioning of MOVPE p-InP Films for Light-Induced Hydrogen Evolution: Chemical, Electronic and Optical Properties

Homoepitaxial p-InP(100) thin films prepared by MOVPE (metallorganic vapor phase epitaxy) were transformed into an InP/oxide-phosphate/Rh heterostructure by photoelectrochemical conditioning. Surface sensitive synchrotron radiation photoelectron spectroscopy indicates the formation of a mixed oxide constituted by In(PO_3)_3, InPO_4 and In_(2)O_3 as nominal components during photo-electrochemical activation. The operation of these films as hydrogen evolving photocathode proved a light-to-chemical energy conversion efficiency of 14.5%. Surface activation arises from a shift of the semiconductor electron affinity by 0.44 eV by formation of In-Cl interfacial dipoles with a density of about 10^(12) cm^(−2). Predominant local In2O3-like structures in the oxide introduce resonance states near the semiconductor conduction band edge imparting electron conductivity to the phosphate matrix. Surface reflectance investigations indicate an enhanced light-coupling in the layered architecture

Probing the structure and dynamics of molecular clusters using rotational wavepackets

The chemical and physical properties of molecular clusters can heavily depend on their size, which makes them very attractive for the design of new materials with tailored properties. Deriving the structure and dynamics of clusters is therefore of major interest in science. Weakly bound clusters can be studied using conventional spectroscopic techniques, but the number of lines observed is often too small for a comprehensive structural analysis. Impulsive alignment generates rotational wavepackets, which provides simultaneous information on structure and dynamics, as has been demonstrated successfully for isolated molecules. Here, we apply this technique for the firsttime to clusters comprising of a molecule and a single helium atom. By forcing the population of high rotational levels in intense laser fields we demonstrate the generation of rich rotational line spectra for this system, establishing the highly delocalised structure and the coherence of rotational wavepacket propagation. Our findings enable studies of clusters of different sizes and complexity as well as incipient superfluidity effects using wavepacket methods.Comment: 5 pages, 6 figure

Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency

Recent rapid progress in efficiencies for solar water splitting by photoelectrochemical devices has enhanced its prospects to enable storable renewable energy. Efficient solar fuel generators all use tandem photoelectrode structures, and advanced integrated devices incorporate corrosion protection layers as well as heterogeneous catalysts. Realization of near thermodynamic limiting performance requires tailoring the energy band structure of the photoelectrode and also the optical and electronic properties of the surface layers exposed to the electrolyte. Here, we report a monolithic device architecture that exhibits reduced surface reflectivity in conjunction with metallic Rh nanoparticle catalyst layers that minimize parasitic light absorption. Additionally, the anatase TiO2 protection layer on the photocathode creates a favorable internal band alignment for hydrogen evolution. An initial solar-to-hydrogen efficiency of 19.3 % is obtained in acidic electrolyte and an efficiency of 18.5 % is achieved at neutral pH condition (under simulated sunlight)