The positive limit of photoemission of electrons into electrolytic solutions

The photoemission of electrons into electrolytic solutions was studied from metals bearing positive charge, when the metal is polarized in the potential range positive of the p.z.c. The dependence on potential of the energy of the red limit of photoemission is given by a curve which increases due to increasing positive charge of the electrode up to a limit at about +0.35 V vs. SCE, where the photoemission ceases.     

Influence of core electrons on plasmon oscillations

Core electrons constitute a polarizable background which tends to screen the plasma oscillations. The influence of core electrons on plasmon dispersion is studied with sum rule techniques. Analytical expressions are derived for the surface plasmon of flat surfaces and of small metal particles. Core polarization explains semiquantitatively the blue-shift of the surface plasmon recently observed in silver systems.     

The influence of the nuclear shape and of the muonic vacuum polarization on strongly bound electrons

We calculate the energy levels ofK- andL-shell electrons in lead, uranium, fermium and in the super-heavy systemZ=169 taking into account various nuclear charge distributions and the muonic vacuum polarization.     

Spin polarization of electrons elastically scattered from argon atoms in the Ramsauer-Townsend region

Spin polarization of elastic electron scattering from argon atoms in the Ramsauer-Townsend region (0.1–0.6 eV) is calculated using a relativistic method. Via the energy-angle dependent surfaces, the differential cross section and the spin polarization parametersS, T andU are presented as functions of both the electron energy and the scattering angle. It is shown that the spin polarization effect is significant along a projected curve in the energy-angle plane.     

Raman spectrum modification in the presence of solvated electrons. Observation by polarization CARS

The multiple-scattering series for depolarized light scattering from simple liquids is investigated. It is found that the leading term in the series, due to double—double scattering, is probably not large compared to double—triple (DT) scattering contributions. Existing theories, which neglect DT scattering, have concluded that the experimental data cannot be explained using the dipole—induced dipole (DID) model for the pair polarizability. It is suggested that a theory which includes DT scattering will not lead to rejection of the DID model.     

Influence of polarization functions on molecular electrostatic potentials

We study the effects of d-polarization functions, centered on the heavy atoms, on the SCF molecular electrostatic potentials calculated for some molecules. The positions and energies of the minima found are very sensitive to the inclusion of polarization functions in the basis set used. The variations depend on the heavy atom involved and on the possible anisotropy of its charge distribution. These variations are particularly important for second-row atoms.     

Influence of polarization fluctuations on the electronic structure of molecular solids

Fluctuations in the intermolecular polarization energies of charge states in molecular solids can lead to broad (ΔE ≈ 0.5 eV) distributions of localized states, especially in polymers. Such fluctuations are caused by defects (e.g. surfaces) and thermal vibrations in molecular crystals and also by variations in the local structure in polymers. The resulting energy distributions yield natural interpretations of such diverse observations on the broadening of the photoemission spectra of molecular solids and the contact charge exchange spectra of polymers.     

Influence of the polarization effect on the donor properties of 1-phenylsilatrane

A new mechanism of the dipole—induced dipole polarization interaction was considered to explain an anomalous increase in the +I effect in a 1-phenylsilatrane molecule with the pentacoordinated Si atom. The interaction through the space between the silatrane cage having a high dipole moment and easily polarizable aromatic phenyl substituent induces dipole that adds the influence of the inductive effect along the bonds, which is characteristic of usual silatranes. The high inductive constant σ_I calculated for 1-phenylsilatrane on the basis of chemical shifts δ_C of aromatic carbon atoms was interpreted as a superelectronodonor effect in silatranes. However, the polarization effect through the space leads to a substantial change in the chemical shifts δ_C for the phenyl group in the direction that formally corresponds to the enhancement of the +I effect transmitted via the bonds. It is shown for the first time that the inductive effect in silatranes is not constant and varies according to the polarizability of the substituent at the Si atom.     

Spin polarization and cross sections of electrons elastically scattered from heavy alkaline-earth atoms

Semi-relativistic approach is employed to compute the differential and integrated cross sections, spin polarizationP and the spin polarization parametersT andU for the scattering of electrons from barium and strontium atoms in the energy range from 2.0–300 eV. The projectile-target interaction is represented both by real and complex optical potential in the solution of Dirac equation for the scattered electrons. The real optical potential includes the static, a parameter free correlation polarization and a modified semi classical exchange potentials. The complex optical potential is constructed by adding a model absorption potential as its imaginary part to the real optical potential.     

Influence of submonolayer sodium adsorption on the photoemission of the Cu(111)/water ice surface

Photoemission from an ice film deposited on Cu(111) as a function of thickness has been observed in the presence and absence of sodium atoms at the surface-vacuum interface. For either adsorbate alone and photon energies below 4 eV, two-photon photoemission from the Cu(111) substrate dominates. The Cu(111) photoelectron spectrum is perturbed by low coverages of Na, and its intensity is strongly attenuated by a few monolayers of ice. For a low density amorphous ice film, strong charging effects are observed. For ice films annealed to yield either the dense amorphous or crystalline phase, this effect is absent. Deposition of only 0.02 monolayer of Na leads to a dramatic decrease in the threshold for photoemission to 2.3+/-0.2 eV. Thus, photoelectrons are generated by visible radiation in a one-photon process with a cross section that exceeds 10(-18) cm(2). The initial state for the photoemission is identified as a metastable surface trapped electron, which decays thermally with an activation energy of 10+/-2 kJ mol(-1). Quantum calculations are described which support this model and show that the Na atom is accommodated in the first layer of the ice surface.     

Simulations of photoemission and equilibrium redox processes of ionic liquids: the role of ion pairing and long-range polarization

We have studied oxidation and reduction of ionic liquids using methods of theoretical chemistry. In particular, we have modeled photoelectron spectra of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf(2)N]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf(2)N]) ionic liquids and their components. We have considered ion pairs in the gas phase and solvated in the water represented by a dielectric continuum. We have also characterized the isolated cation and anion of the ionic liquids. The calculated quantities have been compared with available experiments. The photoelectron spectra were modeled within linearized reflection approximation, using a composite ab initio approach based on combination of the MP2 method for the ground state, the PMP2 method for the first ionized electronic state and the TDDFT method to estimate the higher ionization energies. We have also briefly explored energetics of processes following the vertical electron attachment/detachment. Our calculations were able to reproduce the measured photoelectron spectra. We have shown that the valence photoelectron spectrum results from simultaneous ionization from many molecular orbitals. In the threshold region, the electron is ejected from the anion in the gas phase ion pair while the cationic moiety is ionized when the ion pair is solvated. We have investigated the effect of both short-range and long-range interactions on the ionization characteristics, showing that the long-range polarization of the solvent effectively screens the specific short-range interactions. The large difference between the HOMO-LUMO gap and the electrochemical window has been rationalized in terms of relaxation following the redox processes, i.e. solute and solvent geometric relaxation and reactions, e.g. dimerization of the imidazolium radical formed during the reduction.     

Chemically induced electron spin polarization from photolysis with plane polarized light

Experiments confirm the prediction that the magnitude of electron polarization in radicals produced by photolysis of a carbonyl compound depends upon the polarization of the incident light and thereby provide conclusive evidence for the triplet electron polarization mechanism.     

Influence of the cathodic polarization on the stability of Ti surface in concentrated KOH solutions

The stability of spontaneous thin layers and thin layers formed upon cathodical polarization of Ti in KOH solutions have been studied by potentiostatic and ellipsometric methods. At open circuit potential (OCP) the strongly adherent films, whose thickness depends on the concentration of the KOH solution, were formed. During the cathodic polarization the transformation of these films to weakly adsorbed precipitated layers on the electrode surface was observed. Comparing the theoretically computed curves with the experimental Ψ vs Δ loci measured ellipsometrically, the complex indices of refraction and the thickness of the generated films, from 3.6 to 60 nm in 1 M KOH and from 36 to 105 nm in 5 M KOH (adherent to the electrode surface), were determined. At OCP the rate of film growth increases with increasing the concentration of KOH solution. Cathodic polarizations change the chemical composition and retard the rate of film growth. Based on the ellipsometric and electrochemical data the chemical compositions of the formed films consisted of TiO₂, Ti₂O₃, TiO₂·H₂O, Ti(OH)₃ and TiOOH·nH₂O.     

On the history of photoemission

Historical aspects of science are usually confined to special conferences, and receive only a brief mention in the introduction to talks at meetings addressing current topics. However, at the previous Seminar, several theoretical and experimental contributions were made, which discussed the historical development of surface science. Because of the considerable interest aroused by these presentations, we decided to keep the historical spirit alive, by telling the story of photoemission.

While field emission is the oldest area of surface science, photoemission is certainly one of the most important branches. Here, emphasis is placed on the early investigations, which are often scarcely mentioned, and on more recent work that has had significant impact on the progress of surface science.     

Lone pair versus bonding pair electrons: the mechanism of electronic polarization of water in the presence of positive ions

It is commonly accepted that the water molecules in the first solvation shell of a positive ion are strongly polarized because of an elongation of the oxygen lone pair orbitals along the ion-oxygen direction and this is commonly considered the dominant effect. Recent experimental and theoretical works have instead suggested that this is not the dominant aspect and that the problem is by far more complicated. Consistent with the picture given above, here we show that, in particular, an equally important role into the polarization process is played by the bonding pair electrons located along the internal oxygen-hydrogen bond. We also provide some arguments which suggest that the main reason of such a behavior is due to the distortion of the molecular orbitals caused by the interaction between non-hydrogen-bonded water molecules in the first solvation shell of the ion.     

Influence of boundary on the effect of double-layer polarization and the electrophoretic behavior of soft biocolloids

The electrophoresis of a soft particle comprising a rigid core and a charged porous membrane layer in a narrow space is modeled. This simulates, for example, the capillary electrophoresis of biocolloids such as cells and microorganisms, and biosensor types of device. We show that, in addition to the boundary effect, the effects of double-layer polarization (DLP) and the electroosmotic retardation flow can be significant, yielding interesting electrophoretic behaviors. For example, if the friction coefficient of the membrane layer and/or the boundary is large, then the DLP effect can be offset by the electroosmotic retardation flow, making the particle mobility to decrease with increasing double layer thickness, which is qualitatively consistent with many experimental observations in the literature, but has not been explained clearly in previous analyses. In addition, depending upon the thickness of double layer, the friction of the membrane layer of a particle can either retard or accelerate its movement, an interesting result which has not been reported previously. This work is the first attempt to show solid evidence for the influence of a boundary on the effect of DLP and the electrophoretic behavior of soft particles. The model proposed is verified by the experimental data in the literature. The results of numerical simulation provide valuable information for the design of bio-analytical apparatus such as nanopore-based sensing applications and for the interpretation of relevant experimental data.     

Electron photoemission from charged films: absolute cross section for trapping 0-5 eV electrons in condensed CO2

The electron trapping or attachment cross section of carbon dioxide (CO2) condensed as thin films on a spacer of Ar is obtained using a simple model for electron trapping in a molecular film and then charge releasing from the same film by photon absorption. The measurements are presented for different electron exposures and impact energies, film thicknesses, and probing photon energies. The cross section for trapping an electron of incident energy between 0 and 5 eV reveals three different attachment processes characterized by a maximum at about 0.75 eV, a structured feature around 2.25 eV, and a shoulder around 3.75 eV. From the measurement of their dependence with the probing photon energy, the two lowest processes produce traps having a vertical electron binding energy of approximately 3.5 eV, whereas the highest one yields a slightly higher value of approximately 3.7 eV. The 0.75 eV maximum corresponds to the formation of vibrational Feshbach resonances in (CO2)n- anion clusters. The 2.25 eV feature is attributed to the formation of a vibrationally excited 2Piu anion in (CO2)n- clusters, followed by fast decay into its vibrational ground state without undergoing autodetachment. Finally, 3.75 eV shoulder is assigned to the well-known dissociative electron attachment process from 2Piu anion state producing the O- anion in the gas phase and the (CO2)nO- anions in clusters.