Optical signatures of the Charge of a Dielectric Particle in a Plasma

With an eye on dust particles immersed into an ionized gas, we study the effect of a negative charge on the scattering of light by a dielectric particle with a strong transverse optical phonon resonance in the dielectric constant. Surplus electrons alter the scattering behavior of the particle by their phonon limited conductivity in the surface layer (negative electron affinity) or in the bulk of the particle (positive electron affinity). We identify a charge-dependent increase of the extinction efficiency for low frequencies, a shift of the extinction resonance above the transverse optical phonon frequency, and a rapid variation of the polarization angles over this resonance. These effects could be used for non-invasive optical measurements of the charge of the particle.Comment: 10 pages, 7 figure

Mixed-valence correlations in charge-transferring atom-surface collisions

Motivated by experimental evidence for a mixed-valence state to occur in the neutralization of strontium ions on gold surfaces we analyze this type of charge-transferring atom-surface collision from a many-body theoretical point of view using quantum-kinetic equations together with a pseudo-particle representation for the electronic configurations of the atomic projectile. Particular attention is paid to the temperature dependence of the neutralization probability which--experimentally--seems to signal mixed-valence-type correlations affecting the charge-transfer between the gold surface and the strontium projectile. We also investigate the neutralization of magnesium ions on a gold surface which shows no evidence for a mixed-valence state. Whereas for magnesium excellent agreement between theory and experiment could be obtained, for strontium we could not reproduce the experimental data. Our results indicate mixed-valence correlations to be in principle present, but for the model mimicking most closely the experimental situation they are not strong enough to affect the neutralization process quantitatively.Comment: 9 pages, 10 figures, corrected version to be published in Phys. Scr.

Ion-induced secondary electron emission from metal surfaces

Using a helium ion hitting various metal surfaces as a model system, we describe a general quantum-kinetic approach for calculating ion-induced secondary electron emission spectra at impact energies where the emission is driven by the internal potential energy of the ion. It is based on an effective model of the Anderson-Newns-type for the subset of electronic states of the ion-surface system most strongly affected by the collision. Central to our approach is a pseudo-particle representation for the electronic configurations of the projectile which enables us, by combining it with two additional auxiliary bosons, to describe in a single Hamiltonian emission channels involving electronic configurations with different internal potential energies. It is thus possible to treat Auger neutralization of the ion on an equal footing with Auger de-excitation of temporarily formed radicals and/or negative ions. From the Dyson equations for the projectile propagators and an approximate evaluation of the self-energies, rate equations are obtained for the probabilities with which the projectile configurations occur and an electron is emitted in the course of the collision. Encouraging numerical results, especially for the helium-tungsten system, indicate the potential of the approach.Comment: Substantially revised version including now additional charge-transfer channels which make the emission scenario proposed in the first version less likely. It is thus no longer emphasized. Comparision with experiments is now rather favorable. 23 pages, 11 figure

Phonon-mediated desorption of image-bound electrons from dielectric surfaces

A complete kinetic modeling of an ionized gas in contact with a surface requires the knowledge of the electron desorption time and the electron sticking coefficient. We calculate the desorption time for phonon-mediated desorption of an image-bound electron, as it occurs, for instance, on dielectric surfaces where desorption channels involving internal electronic degrees of freedom are closed. Because of the large depth of the polarization-induced surface potential with respect to the Debye energy multi-phonon processes are important. To obtain the desorption time, we use a quantum-kinetic rate equation for the occupancies of the bound surface states, taking two-phonon processes into account in cases where one-phonon processes yield a vanishing transition probability, as it is sufficient, for instance, for graphite. Besides producing an estimate for the desorption time of an electron image-bound to a graphite surface, we investigate the desorption scenario and show that desorption via cascades over bound states dominates unless direct one-phonon transitions from the lowest bound state to the continuum are possible.Comment: 20 pages, 8 figure

Mie scattering analog in graphene: lensing, particle confinement, and depletion of Klein tunneling

Guided by the analogy to Mie scattering of light on small particles we show that the propagation of a Dirac-electron wave in graphene can be manipulated by a circular gated region acting as a quatum dot. Large dots enable electron lensing, while for smaller dots resonant scattering entails electron confinement in quasibound states. Forward scattering and Klein tunneling can be almost switched off for small dots by a Fano resonance arising from the interference between resonant scattering and the background partition.Comment: 6 pages, 4 figures, references correcte

Electron surface scattering kernel for a plasma facing a semiconductor

Employing the invariant embedding principle for the electron backscattering function, we present a strategy for constructing an electron surface scattering kernel to be used in the boundary condition for the electron Boltzmann equation of a plasma facing a semiconducting solid. It takes the microphysics responsible for electron emission and backscattering from the interface into account. To illustrate the approach, we consider silicon and germanium, describing the interface potential by an image-step and impact ionization across the energy gap as well as scattering on phonons and ion cores by a randium-jellium model. The emission yields deduced from the kernel agree sufficiently well with measured data, despite the simplicity of the model, to support its use in the boundary condition of the plasma's electron Boltzmann equation.Comment: 14 pages, 10 figure

Phonon-mediated sticking of electrons at dielectric surfaces

We study phonon-mediated temporary trapping of an electron in polarization-induced external surface states (image states) of a dielectric surface. Our approach is based on a quantum-kinetic equation for the occupancy of the image states. It allows us to distinguish between prompt and kinetic sticking. Because the depth of the image potential is much larger than the Debye energy multi-phonon processes are important. Taking two-phonon processes into account in cases where one-phonon processes yield a vanishing transition probability, as it is applicable, for instance, to graphite, we analyze the adsorption scenario as a function of potential depth and surface temperature and calculate prompt and kinetic sticking coefficients. We find rather small sticking coefficients, at most of the order of $10^{-3}$, and a significant suppression of the kinetic sticking coefficient due to a relaxation bottleneck inhibiting thermalization of the electron with the surface at short timescales.Comment: 10 pages, 7 figure

Scattering of infrared light by dielectric core-shell particles

We study the scattering of infrared light by small dielectric core-shell particles taking a sapphire sphere with a CaO core as an example. The extinction efficiency of such a particle shows two intense series of resonances attached, respectively, to in-phase and out-of-phase multipolar polarization-induced surface charges build-up, respectively, at the core-shell and the shell-vacuum interface. Both series, the character of the former may be labelled bonding and the character of the latter antibonding, give rise to anomalous scattering. For a given particle radius and filling factor the Poynting vector field shows therefore around two wave numbers the complex topology of this type of light scattering. Inside the particle the topology depends on the character of the resonance. The dissipation of energy inside the particle also reflects the core-shell structure. It depends on the resonance and shows strong spatial variations.Comment: 18 pages, 12 figures, final versio

Surface mode hybridization in the optical response of core-shell particles

We present an exact rewriting of the Mie coefficients describing the scattering of light by a spherical core-shell particle which enables their interpretation in terms of an hybridization of the two surface modes arising, respectively, at the core-shell and the shell-medium interface. For this particular case we thus obtain from the Mie theory--analytically for all multipole orders and hence for arbitrarily sized particles--the hybridization scenario, which so far has been employed primarily for small particles in the electrostatic approximation. To demonstrate the strength of the rewriting approach we also extract the hybridization scenario for a stratified sphere directly from the expansion coefficients for the electromagnetic fields.Comment: 12 pages, 7 figures, published versio

Particle-based modeling of oxygen discharges

We present an one-dimensional particle-in-cell Monte-Carlo model for capacitively coupled radio-frequency discharges in oxygen. The model quantitatively describes the central part of the discharge. For a given voltage and pressure, it self-consistently determines the electric potential and the distribution functions for electrons, negatively charged atomic oxygen, and positively charged molecular oxygen. Previously used collision cross sections are critically assessed and in some cases modified. Provided associative detachment due to metastable oxygen molecules is included in the model, the electro-negativities in the center of the discharge are in excellent agreement with experiments. Due to lack of empirical data for the cross section of this process, we propose a simple model and discuss its limitations.Comment: 4 pages, 5 figures, accepted contribution to 28th ICPIG, Prag (2007