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A novel optical effect can be observed in a thin gold foil due to the excitation of surface plasmons which permits a form of all-optical modulation at low pulse rates. Modulated excitation of surface plasmons by infrared photons is shown to couple to several beams at visible-photon energies. The coupling is manifested by the observation of the visible photons being pulsed by the action of the infrared pulses, and by the far field diffraction of the visible beams into concentric rings. When each visible beam also excites surface plasmons, then a quadratic dependence of the visible photon power upon the infrared incident power is measured. The decay of surface plasmons is implicated as the primary cause of thermally induced changes in the foil. The thermal effects dissipate in sufficiently small times so that operation up to the kilohertz range in pulse repetition frequency is obtained.

Effective energy-loss functions were derived for oxygen-adsorbed amorphous silicon surfaces from a reflection electron energy-loss spectroscopy analysis based on the extended Landau theory. This study has revealed that the intensity of the surface-plasmon-loss peak for a clean surface decreases and its peak position shifts towards the lower-energy losses as oxygen exposure proceeds (⩽1000 L). To understand the above behavior of the surface-plasmon-loss peak, the distribution of the energy losses was calculated using the hydrodynamic model. The decrease and shift of the surface-plasmon-loss peak has been described with considerable success by assuming that the quasifree static electron density in the vicinity of the silicon surface decreases as oxygen adsorption proceeds owing to oxygen’s high electron affinity.

It is well known that the hydrodynamical model of the electron gas is suitable when the electron density can be considered as the fundamental quantity, small wavelength effects being less important. This is the case when the electron gas is excited by the passage of swift charged particles. In this paper we make use of the quantum hydrodynamical model of the electron gas to derive, within this model, the nonlinear wake potential of fast charged particles passing through matter, the Barkas effect, and double-plasmon excitation probabilities. This is compared with results obtained by following many-body schemes within the random-phase-approximation.

We present a nonlinear modulation scheme based on coupling between optically excited surface plasmons in a thin gold film and the heating of the film by a pulsed direct current, and also for the reverse process. The reflected surface-plasmon excitation beam is shown to undergo a spatial deformation as a function of a pulsed current through the foil. Similarly, the current through the thin film is modulated by the action of periodic excitation of surface plasmons using infrared photons.

A survey is presented of the theoretical status in understanding some nonlinear aspects of charged particle interactions in matter. In particular, a diagrammatic analysis of the many-body interactions between a moving particle and the electron gas is made to investigate, within the random-phase-approximation, the Barkas correction to the electronic stopping power, the nonlinear wake potential generated by swift ions in matter, and processes involved in multiple excitations of plasmons.

A many-body perturbation-theoretic calculation of the double-plasmon excitation in an electron gas is presented, using Feynman diagrams. The low-q limit of the inverse mean free path for this second order process is evaluated in the random-phase-approximation, and it is compared to previous theoretical and experimental estimations.

The surface scattering of kilovolt-energy ions and atoms at glancing-incidence angles has contributed substantially to knowledge of elementary electronic excitations in the interface region. We consider the influence of inelastic exchange of mechanical energy, due to recoil of the target atoms, on the distribution of scattered projectile atoms. Although with a glancing-incident-angle geometry the relative energy loss by this mechanism is small, it can have a substantial effect on the angular distribution of the scattered particles. These conclusions are supported by numerical calculations of energy losses and scattered particle distributions.

Experimental work by Shea and Compton suggests that ${\mathrm{Ag}}^{+}$ ions emitted from a roughened Ag surface irradiated by a nanosecond or picosecond laser beam may absorb the full energy of the Ag surface plasmon (SP). We have modeled this process as an inverse bremsstrahlung-type absorption of the SP quantum by an ${\mathrm{Ag}}^{+}$ ion which undergoes a collision with the surface. We estimate the absorption probability and find it to be consistent with the Shea-Compton results.

Corrections to the Bethe-Bloch formula for the energy loss of charged particles in matter have been of much interest since the early work of Barkas and collaborators, who found differences between the ranges of positive and negative pions of the same energy. We here report the first rigorous many-body perturbation-theoretic calculation of the Z13 correction for a wide range of particle velocities in the electron gas, Z1 being the projectile charge. Our calculations are in good agreement with recent measurements of the energy loss of protons and antiprotons.

For many decades swift electrons have provided a powerful tool for the study of the dynamical properties of condensed matter. The development of this technique has involved much important physics. Here the author sketches the historical background of the field and some important developments in theory and experiment. Possible directions for future research are indicated.

A diagrammatic analysis of many-body interactions between a moving ion and an electron gas is made. The Z 3 1 correction to the stopping power of the medium for the penetrating ion is derived using Feynman diagrams, and the contributing terms leading to significant Z 1 3 corrections to the stopping power at low velocities of the ion are evaluated, as a function of the velocity of the ion, in the random-phase-approximation.

The electronic stopping power of aluminum metal for protons has been calculated with explicit account taken of the different charge states of the photon inside the medium. The fraction of negative ions (H-) neutral atoms (H0) and bare protons (H+) in the beam as a function of ion speed are derived from the capture and loss cross-sections. The total stopping power is then obtained by weighting the calculated partial stopping powers with the appropriate charge state fractions. The energy loss per unit path length due to electronic exchange processes is also evaluated. Our calculations show that the relative contribution to the stopping power from capture and loss processes is of the order of 15% for the case of protons moving in aluminum. Good agreement with experimental data is found.

Chemical sensing using optical diffraction from an array of microcantilevers is demonstrated. Properly fashioned arrays of micromachined silicon-nitride cantilevers containing embedded deformable diffraction gratings are functionalized with chemically selective coatings. Adsorption of specific molecules on the cantilever leads to bending, which changes the diffraction pattern of a laser beam reflecting off the array. Quantitative chemical information can be obtained by monitoring the displacement of diffraction peaks as a function of analyte exposure.

We study the acceleration of deuterons bouncing forth and back between Ti atoms of the solid target and the heavy-water cluster projectiles impinging on a deuterated TiD surface. The multiple scattering sequence restricted to near head-on collisions closely resembles the Fermi acceleration map. Summation over all iterates of the map provides an estimate (possibly an upper limit) for the collisionally generated preequilibrium high-energy tail of the D distribution. Implications for the recent experimental study on cluster-impact fusion by Beuhler et al. are discussed.

We sketch developments in the theory of the self-energy of charged particles moving near condensed matter surfaces. Some applications to experimental results from spectroscopy with electrons localized in microprobe beams and to electrons tunneling across a gap between two metals are considered.

Developments in the theory of the image potential of charged particles moving near condensed matter surfaces are sketched. Some applications to electrons, (i) localized in image potential states, (ii) tunneling across a gap between two metals, (iii) localized in fast microprobe beams, are considered.

The cluster-orientation dependence of the emitted knock-on collision electron distributions when fast diclusters interact with condensed matter is investigated. The scattering by each centre of the dicluster is described by means of the phase shifts of a velocity dependent Yukawa potential describing the dynamic screening, and the multiple scattering effects are approximated within the second Born approximation. The applicability of this approximation as the charges and velocities of the diclusters are changed is also discussed, and numerical comparisons of our results with experiment are made.

Much experimental data have been accumulated relative to the emission of photons accompanying electron capture by swift, highly stripped channeled ions. Recent data suggest that the photon energies may be less than that expected from simple considerations of transitions from the valence band of the solid to hydrogenic states on the moving ion. We have studied theoretically the impact-parameter dependence of the radiative-electron-capture process, including the effect of the ion's wake and capture from inner shells of the solid on the photon-emission probability, and using a statistical local-density approach. Numerical comparisons of our results with experiment are made.

For the first time, a complete computer simulation of physical and chemical reactions at the molecular level has been used to calculate the yield of a chemical species resulting from irradiation of a biological molecule in aqueous solution. Specifically, when a solution of glycylglycine is irradiated anaerobically, an ammonia molecule is released by the action of a hydrated electron, which is produced by irradiation of water. In the computations, Monte Carlo techniques are used to simulate the statistical progression of molecular events as they are assumed to occur. These include the initial physical ionization and excitation of water molecules along a particle track in the liquid; the subsequent formation of free radicals and other species: and the random diffusion and chemical reactions of the species with each other, the solvent, and solute molecules. We have calculated and measured the yield of ammonia from irradiation of glycylglycine with 250 kVp x-rays as a function of glycylglycine concentration between 0.01 and 1.2 M. Excellent agreement is obtained between predicted and measured results. The literal simulation of events, combined with specific experimental measurements, offers a powerful new tool for studying mechanisms of radiation action and damage at the molecular level.

Monte Carlo computer models that simulate the detailed, event-by-event transport of electrons in liquid water are valuable for the interpretation and understanding of findings in radiation chemistry and radiation biology. Because of the paucity of experimental data, such efforts must rely on theoretical principles and considerable judgment in their development. Experimental verification of numerical input is possible to only a limited extent. Indirect support for model validity can be gained from a comparison of details between two independently developed computer codes as well as the observable results calculated with them. In this study, we compare the transport properties of electrons in liquid water using two such models, PARTRAC and NOREC. Both use interaction cross sections based on plane-wave Born approximations and a numerical parameterization of the complex dielectric response function for the liquid. The models are described and compared, and their similarities and differences are highlighted. Recent developments in the field are discussed and taken into account. The calculated stopping powers, W values, and slab penetration characteristics are in good agreement with one another and with other independent sources.

We investigate the wake binding energies of electrons trapped in the first potential well caused by a swift proton and an antiproton in two different solids. Binding is possible in aluminium for both protons and antiprotons in a large range of velocities. In carbon the binding is drastically reduced when one takes into account realistic values of the damping.

Three different thicknesses of aluminum targets, 0.0025, 0.025, and 0.32 cm were irradiated with 50-MeV neutral hydrogen particles and the resultant bremsstrahlung radiation measured. The experimental results are compared with calculations that include bremsstrahlung by the projectile's electron and its cascade, bremsstrahlung by the cascade electrons that are generated by the projectile nucleus, and quasi-free-electron bremsstrahlung. The theoretical and experimental values of the total yield agree within a factor of 3 for all target thicknesses and all bremsstrahlung energies.

Calcul des corrections d'ordre superieur a l'energie de liaison d'un electron dans un etat de surface induit par potentiel d'image par l'approche generale de Zheng

We have studied the resonant coupling of surface plasmons in curved thin-film tunneling geometries by obtaining the dispersion relations for the system. The surface plasmon dispersion relations are calculated for a metal-coated dielectric probe above a dielectric half space with and without metal coating. The system is modeled in the prolate spheroidal system, and the dispersion relations are studied as functions of the parameter that defines the boundaries of the tip and the corresponding coating, and as functions of the involved coating thicknesses. Using this type of probe-substrate configuration, the nonradiative surface plasmon coupling mechanism is investigated in the visible spectrum at frequencies relevant to scanning probe microscopy. The simulations of the results predict optical access to the resonant surface modes of the system.

We have reviewed the theoretical and experimental results on linear and non-linear screening and stopping power for 3D and 2D systems. In the linear case (Z1/v≪1), dielectric formalism has been used to characterize such systems, while the non-linear aspects are described within the density functional theory (DFT) framework. Differences between proton and antiproton probe particles are discussed, both in the quadratic response function theory and in the DFT. For heavy ions, the main results on Z1 oscillations are revised.

Recent experiments (R. A. Baragiola and C. A. Dukes, Phys. Rev. Lett. {\bf 76}, 2547 (1996)) with slow ions incident at grazing angle on metal surfaces have shown that bulk plasmons are excited under conditions where the ions do not penetrate the surface, contrary to the usual statement that probes exterior to an electron gas do not couple to the bulk plasmon. We here use the quantized hydrodynamic model of the bounded electron gas to derive an explicit expression for the probability of bulk plasmon excitation by external charged particles moving parallel to the surface. Our results indicate that for each ${\bf q}$ (the surface plasmon wave vector) there exists a continuum of bulk plasmon excitations, which we also observe within the semi-classical infinite-barrier (SCIB) model of the surface., 4 pages, 3 figures, o appear in Phys. Lett. A

We present semiclassical and quantized hydrodynamic models to obtain the quadratic electronic response of a plane-bounded electron gas. Explicit expressions for the dynamic image potential experienced by charged particles moving near a jellium surface are derived, up to third order in the projectile charge. These expressions are employed to compute the image potential at all distances outside the surface. Though nonlinear corrections are found to be more important far inside the solid than outside, our results indicate that the nonlinear image potential is enhanced with respect to the linear image potential by a factor that is for Al as large as ∼1.15 near the surface in the case of a stationary particle (→v0) with positive unit charge e., A.B. and J.M.P. wish to acknowledge partial support by the Basque Unibertsitate eta Ikerketa Saila and the Spanish Ministerio the Educacion y Cultura.

Most of the energy absorbed in the cell nucleus from a radiation field goes into the aqueous medium that surrounds macromolecules, like DNA, which are critical to the normal function of cells. This part of the energy deposition produces numerous reactive species that can diffuse to DNA sequences and induce chemical changes. The average diffusion distance of the free radicals that mediate this indirect mode of DNA damage is only a few nanometers because the cellular medium contains a high concentration of molecules that rapidly scavenge the radiation-induced species. Under these conditions, direct interaction of the radiation field with DNA can not be neglected as a potential mode of damage induction. Two aspects of the direct effect will be discussed in this paper: (1) screening of the interaction between DNA and charged particles by the dielectric response of the aqueous medium and (2) the impact-parameter dependence of these interactions.

Here we review the state of knowledge about the transport of subexcitation electrons in water. Longitudinal optical phonon and acoustical phonon interactions with an electron added to the system can give rise to an increase in the effective mass of the electron and to its damping. We generalize the pioneering treatment of thermalization given by Frolich and Platzman to apply to both long-range, polarization-type interactions, and as well to short-range interactions of low-energy electrons in water. When an electron is ejected from a molecule in condensed matter quantum interference may take place between the various excitation processes occuring in the medium. We have estimated the magnitude of this interference effect, together with the effect of Coulombic interactions on the thermalization of subexcitation electrons generated in the vicinity of a track of positive ions in water.

Detailed Monte Carlo calculations provide a powerful tool for understanding mechanisms of radiation damage to biological molecules irradiated in aqueous solution. This paper describes the computer codes, OREC and RADLYS, which have been developed for this purpose over a number of years. Some results are given for calculations of the irradiation of pure water. Comparisons are presented between computations for liquid water and water vapor. Detailed calculations of the chemical yields of several products from X-irradiated, oxygen-free glycylglycine solutions have been performed as a function of solute concentration. Excellent agreement is obtained between calculated and measured yields. The Monte Carlo analysis provides a complete mechanistic picture of pathways to observed radiolytic products. This approach, successful with glycylglycine, will be extended to study the irradiation of oligonucleotides in aqueous solution.

Detailed Monte Carlo calculations provide a powerful tool for understanding mechanisms of radiation damage to biological molecules irradiated in aqueous solution. This paper describes the computer codes, OREC and RADLYS, which have been developed for this purpose over a number of years. Some results are given for calculations of the irradiation of pure water. Comparisons are presented between computations for liquid water and water vapor. Detailed calculations of the chemical yields of several products from X-irradiated, oxygen-free glycylglycine solutions have been performed as a function of solute concentration. Excellent agreement is obtained between calculated and measured yields. The Monte Carlo analysis provides a complete mechanistic picture of pathways to observed radiolytic products. This approach, successful with glycylglycine, will be extended to study the irradiation of oligonucleotides in aqueous solution.

We report the observation of long-lived ({tau}{gt}10{sup {minus}3} sec) doubly charged negative ions of C{sub 60} in the gas phase. C{sub 60}{sup {minus}} and C{sub 60}{sup 2{minus}} ions are produced by laser desorption from a surface covered with C{sub 60} molecules and are believed to be produced by negative surface ionization from the laser-heated surface. Pseudopotential calculations indicate that C{sub 60}{sup 2{minus}} is slightly bound (electron affinity of C{sub 60}{sup {minus}}{congruent}0.1--0.4 eV) relative to C{sub 60}{sup {minus}}.

The spatial distributions of ionizations and other inelastic events in charged-particle tracks are important quantities that influence the final outcome of radiation interaction. Calculations of such distributions are presented for the tracks of electrons in the energy range 100 eV to 10 keV in liquid water and water vapor, and the results are compared. The distributions include the frequency of nearest-neighbor distances for all inelastic events, the mean nearest-neighbor distances for ionizations and for all inelastic events as a function of electron energy, the frequency of distances between all ionizations and all inelastic events, and the farthest distances between all inelastic events in electron tracks. The physical differences between liquid water and water vapor are discussed in terms of the respective inverse mean free paths, the collision spectra, and the nonlocalization of energy losses that are likely to occur in the liquid.

A self-energy formalism is used to determine the quantal image potential for an electron near an impenetrable solid surface. The electron is considered to be in a Coulombic potential outside a repulsive wall. The complete, orthonormal basis set of wave functions for an electron in such a model potential is employed in the calculation of the image potential.

Experimental work by Shea and Compton1 suggests that Ag+ ions emitted from a roughened Ag surface irradiated by a nanosecond or picosecond laser beam may absorb the full energy of the Ag surface plasmon (SP). We have modeled this process under the assumption that it proceeds through an inverse bremsstrahlung-type absorption of the SP quantum by an Ag+ ion which also undergoes a small-impact parameter collision with another ion or atom in the vicinity of the surface. We give a quantitative estimate of the absorption probability and find reasonable agreement with the Shea-Compton results.

These lectures cover special topics about the interaction of swift electrons with condensed matter. The focus is on inelastic processes via the linear response function of the medium and sum rules that it satisfies. The spin dependence of low-energy electron interactions and exchange scattering in an electron gas, and the low-energy end of electron-hole cascades in metals are discussed. Aspects of the localization of initially unlocalized, coherent excitations are entered into briefly.

We review the state of knowledge about inelastic interactions of swift charged particles with condensed matter. Emphasis is placed on the properties of the dielectric response function and its representation for biologically interesting materials. Progress toward the goal of modeling electron transport and track structure in these materials is described.

The spatial distributions of ionizations and other inelastic events in charged-particle tracks are important quantities that influence the final outcome of radiation interaction. Calculations of such distributions are presented for the tracks of electrons in the energy range 100 eV to 10 keV in liquid water and water vapor, and the results are compared. The distributions include the frequency of nearest-neighbor distances for all inelastic events, the mean nearest-neighbor distances for ionizations and for all inelastic events as a function of electron energy, the frequency of distances between all ionizations and all inelastic events, and the farthest distances between all inelastic events in electron tracks. The physical differences between liquid water and water vapor are discussed in terms of the respective inverse mean free paths, the collision spectra, and the nonlocalization of energy losses that are likely to occur in the liquid.

It is shown that a charged particle moving with velocity $\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}$ in a medium of resonance frequency ${\ensuremath{\Omega}}_{0}$ may set up two types of electron-density fluctuations. Collective fluctuations trail the particle, composing a conical pattern in a relatively extended periodic wake of wavelength $\ensuremath{\sim}\frac{2\ensuremath{\pi}v}{{\ensuremath{\Omega}}_{0}}$. They constitute a mode of energy transport from the particle track leading eventually to particle-hole excitations. Single-particle interactions give rise to bow waves ahead of the particles of wavelength $\frac{2\ensuremath{\pi}\ensuremath{\hbar}}{\mathrm{mv}}$. The gradient along $\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}$, at the site of the ion, of the wake potential set up by density fluctuations, multiplied by the ionic charge, yields an expression for the retarding force of the medium on the projectile in exact agreement with the Bethe stopping-power formula appropriate to the medium. The wake of a dicluster causes forces between the constituent ions which account quantitatively for measurements of the breakup behavior of swift molecular ions in thin foils. The increase in the energy straggling of a test charge, when moving in the wake of a leading ion, is shown to be small compared with the straggling induced by its own wake under normal conditions.

Relativistic corrections to the nonrelativistic Bethe-Bloch formula for the stopping power of matter for charged particles are traditionally computed by considering close collisions separately from distant collisions. The close-collision contribution is further divided into the Mott correction, appropriate for very small impact parameters, and the Bloch correction, computed for larger values. This division of the region of close collisions leads to a very cumbersome result if one generalizes the original Bloch procedure to relativistic energies. We avoid the resulting poorly specified scattering angle ${\ensuremath{\theta}}_{0}$ that divides the Mott and Bloch correction regimes by using the procedure suggested by Lindhard and applied by Golovchenko, Cox, and Goland to determine the Bloch correction for relativistic velocities.

Quantal aspects of spatial resolution in combined energy-loss—electron microscopy are considered. Working in the first Born approximation, it is shown that for coherent broad-beam irradiation the probability of stimulating a localized excitation in matter may be calculated as if the electron executed classical rectilinear motion at constant velocity, v, as v becomes very large. Corrections to this expression may be expressed as a power series in inverse powers of v and may be considered to originate in the quanta1 properties of the electron. The first correction term is displayed explicitly. The impact-parameter dependence of excitation of some localized systems is studied numerically.