Surface Photoeffect Research Articles

Hot electron photoemission in metal–semiconductor structures aided by resonance tunneling

Enhancement of the surface photoemission from metal into semiconductor by resonance tunneling of photoexcited electrons through (quasi-) discrete level in quantum well, located within Schottky barrier of the metal–semiconductor interface, is studied theoretically taking into account the difference between the electron masses in metal and semiconductor. It is shown, in particular, that resonance tunneling through the discrete level can lead to the redshift of the threshold wavelength of surface photoeffect, higher slope linear growth in photocurrent near the threshold (in contrast to quadratic growth, i.e., Fowler's law), and the possibility to increase substantially the photoemission efficiency similarly to recent experimental results on hot carrier generation in plasmonic structures with a discrete energy level at metal interface. The difference in the effective masses is shown to significantly affect the results. Double-barrier tunneling structures with resonant tunneling may become attractive for applications in photochemistry and in plasmonic photodetectors in near IR and middle IR regions of the spectrum.

Character of the outgoing wave in soft x-ray photoemission

Bloch-waves scattering theory is applied to photoemission from Mg(0001) up to photon energies of 320 eV. The quality of various approximations to the photoemission final state is analyzed based on the Fourier decomposition of the ab initio time-reversed scattering states. The relative importance of bulk and surface photoeffect in a one-dimensional pseudopotential model and in a real crystal is studied. The crucial role of the lateral umklapp scattering at high energies is revealed, and its implications for the stationary as well as for the attosecond time-resolved photoemission are discussed. The fine structure of the photoemission intensity distribution at high energies observed in earlier experiments is reproduced and explained.

Internal photoemission from plasmonic nanoparticles: comparison between surface and volume photoelectric effects

We study the emission of photoelectrons from plasmonic nanoparticles into a surrounding matrix. We consider two mechanisms of electron emission from the nanoparticles--surface and volume ones--and use models for these two mechanisms which allow us to obtain analytical results for the photoelectron emission rate from a nanoparticle. Calculations have been carried out for a step potential at the surface of a spherical nanoparticle, and a simple model for the hot electron cooling has been used. We highlight the effect of the discontinuity of the dielectric permittivity at the nanoparticle boundary in the surface mechanism, which leads to a substantial (by ∼5 times) increase of the internal photoelectron emission rate from a nanoparticle compared to the case when such a discontinuity is absent. For a plasmonic nanoparticle, a comparison of the two photoeffect mechanisms was undertaken for the first time which showed that the surface photoeffect can in the general case be larger than the volume one, which agrees with the results obtained for a flat metal surface first formulated by Tamm and Schubin in their pioneering development of a quantum-mechanical theory of photoeffect in 1931. In accordance with our calculations, this possible predominance of the surface effect is based on two factors: (i) effective cooling of hot carriers during their propagation from the volume of the nanoparticle to its surface in the scenario of the volume mechanism and (ii) strengthening of the surface mechanism through the effect of the discontinuity of the dielectric permittivity at the nanoparticle boundary. The latter is stronger at relatively lower photon energies and correspondingly is more substantial for internal photoemission than for an external one. We show that in the general case, it is essential to take both mechanisms into account in the development of devices based on the photoelectric effect and when considering hot electron emission from a plasmonic nanoantenna.

Surface photoeffect and optical absorption: Maxwell equations in tensor and nonlocal isotropic approximation

The surface photoeffect is studied for a þ polarized continuous laser with an angular frequency ω in the long wave length (LWL) domain, i.e. λ≥ 100Å. In this LWL domain the laser fields are usually taken to be constant in space, an approximation called LWL approximation. However, perpendicular to a gas-solid interface, the potential, the electron density and the laser fields vary on a sub-nanometric scale. Here, these varying laser fields are solutions of the combined Ampère-Maxwell and material equations. The material functions are taken to be either scalar or tensor. They are a function of the unperturbed electron density of the material system calculated from the Schr\"odinger equation. For a planar structureless gas-solid interface and a þ polarized laser, the Ampère-Maxwell equation in real space reduces to a set of coupled first and second order ordinary differential equations in one dimension. For an Al(001) surface calculation using model jellium and DFT/LDA potentials, the total absorbed power density of the laser and the number of electrons escaping from the surface, display monopole and multipole surface plasmon resonances and a minimum near the bulk plasmon resonance found by other calculations and in experiments. [DOI: 10.1380/ejssnt.2009.249]

Surface photoeffect: photoabsorption of surface and image states

The surface photoeffect of Al(001) is studied for a p continuous laser in the long wavelength (LWL), i.e. λ ≥ 100 Å. Perpendicular to a gas–solid interface the potential, the electron density, and consequently the laser fields, vary at the atomic scale and the LWL approximation, corresponding to constant in space laser fields, does not apply. The vector potential associated with these laser fields, written in the temporal gauge U = 0, is obtained by solving the Maxwell and the material equations. In the present model the dielectric function, entering these material equations, is dependent on the electron density ρ(z) perpendicular to the surface calculated from the Schrödinger equation and on the bulk dielectric constant. Using these calculated spatially varying laser fields, the photoabsorption spectrum of the surface, the Fermi and the first image states of Al(001) are calculated using the Fermi golden rule. For the first time in the case of a realistic potential, the photoabsorption yield is analysed in terms of the standard velocity, the surface and the interference contributions. All these terms contribute significantly to the total yield and this yield, calculated up to 25 eV, compares fairly well with the experiment.

Photoelectron Emission from Granulated Gold Films Activated by Cesium and Oxygen

Granulated gold films activated by cesium and oxygen are found to produce an intense photoelectron emission band about 100 nm wide at a wavelength of 530 nm with a sensitivity of about 4 mA/W in the visible range. This band is likely to be associated with excitation of surface plasmons. The results obtained indicate that, owing to surface photoeffect, gold films activated by Cs and O can be used as fast photocathodes with a time constant of several femtoseconds.

Interplay between plasmons and the band structure for the Mo(112) surface

Recent photoemission and inverse photoemission results for the Mo(112) surface are discussed in the framework of the calculated band structure. For the Mo(112) surface, the main photoemission features combine contributions from both the surface and the bulk. Except for those photon energies near to excitations of the bulk and multipole surface plasmons, the comparison of the bulk band structure, along the k points normal to the surface, shows a good agreement with photoemission spectra in the position of the critical points. The dominant surface states at $\overline{\ensuremath{\Gamma}}$ are found to have the ${a}_{1}$ symmetry, while the band along $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{Y}$ at about 0.8 eV binding energy is found to be odd with respect to the $\overline{\ensuremath{\Gamma}}\ensuremath{-}\overline{X}$ mirror plane. The surface-induced enhancement of photoemission---the surface photoeffect---is indicated and is shown to be responsible for dramatic changes in the spectra when the photon energy falls into the region of the multipole surface plasmons.

Nonlinear surface photoeffect in multi-color laser field: effect of the finite width of the surface potential barrier

Nonlinear surface photoeffect in multi-color laser field: effect of the finite width of the surface potential barrier

High current density small divergence electron beam production by laser-induced various-order multiphoton photoeffect

The relative roles of (1) surface and volume type photoelectric effects, (2) slow and fast laser-induced thermionic electron emission, and (3) optical tunnel emission are discussed from the point of view of the production possibilities of efficient high current density, small divergency electron beams. Angular distribution of photoelectrons emitted from gold target surface by high-intensity pulsed laser radiation at wavelengths of 1060, 343, and 248 nm for the s and p polarization of the grazing incident radiation are investigated experimentally. For the longer wavelengths, where the order of the multiphoton photoelectric emission increases, the ratio of the yields of the surface photoeffect and of the volume photoeffect also increases. The comparative advantages of the surface multiphoton photoeffect of metals as a promising method for the realization of small emittance, high brightness, high current density electron sources are pointed out.

Influence of the Coulomb repulsion between electrons on their energy spectrum in the case of the nonlinear surface photoeffect

It is shown that the Coulomb interaction among electrons that is produced by the laser-induced surface multiphoton photoeffect may change the initial energy spectrum of the electrons dramatically. The expansion of a photoelectron bunch (during transit from the surface to a detector) results in an increase of the electron energy and a broadening of the energy spectrum. The theoretical estimates of this effect are in good agreement with experimental energy spectra [ Phys. Rev. A41, 4123 ( 1990)]. The unexpectedly high energy of the electrons can be explained by the Coulomb interaction between the electrons.

On the surface photoeffect in a heavily doped semiconductor

The surface photoeffect in a heavily doped semiconductor was investigated using the influences of band-gap narrowing and carrier degeneracy. Variation of the surface photovoltage with the normalized surface potential is presented graphically. A comparison with earlier work done under identical conditions is made. The temperature-dependent behaviour is also studied.

Experimental band structure of potassium as measured by angle-resolved photoemission.

The bulk band structure of potassium along the [110] direction was measured using angle-resolved photoemission from an epitaxial potassium film several thousand angstroms thick grown on a Ni(100) substrate. We find the occupied bandwidth to be 1.60\ifmmode\pm\else\textpm\fi{}0.05 eV, which is narrower than the free-electron bandwidth of 2.12 eV and agrees with recent calculations of the quasiparticle self-energy. A narrow peak near the Fermi level which did not disperse with photon energy was observed for photon energies which, according to the nearly-free-electron model, should yield no direct transitions. A comparison of the binding energy and intensity of the anomalous peak as functions of photon energy is made to the calculations of Shung and Mahan [Phys. Rev. B 38, 3856 (1988)]. The discrepancies found are discussed in terms of an enhanced surface photoeffect in the photon energy range 20\ensuremath{\le}\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\le}30 eV. For low photon energies, a bulk peak was also observed due to a surface umklapp process with an intensity comparable to the standard bulk (110) peak. The possible contributions to this strong surface umklapp process from a shear instability at the first few (110) atomic planes is discussed.

High current, small divergence electron beams produced by laser-induced surface photoelectric effect

The potential application of the laser induced linear surface photoelectric effect for high current and small divergence electron beam formation is examined. The temporal evolution of the current pulses and the angular distribution of the photoelectrons produced following KrF laser irradiation of a gold surface is studied as a function of laser intensity. A threshold laser intensity of (140±20) MW/cm2 was determined, below which the single photon photoeffect dominates any laser induced thermionic emission process. Electron current measurements for vertical and horizontal polarization of the incident laser radiation showed that the current contribution from the surface photoeffect is about three times larger than that from the volume photoeffect. Under these conditions, an angular distribution of the produced electrons of 17°±2° was determined. The estimated normalized transverse beam emittance is 20π mm mrad.

Photofield emission spectroscopy of surface electronic states of tungsten

A novel photofield emission spectrometer has been used to observe departures from free electron behaviour in the total energy distributions of photofield emission from the (100), (110), (111), (211), and (510) facets of a tungsten field emitter. Measurements with p-polarized light at grazing incidence at seven photon energies in the visible and near ultraviolet show departures from free electron behaviour having the same form as those observed in field emission distributions, but shifted to higher energy by one photon energy. The shapes and polarization dependence of the distributions are precisely those expected for surface photoexcitation from electron states near the surface to vacuum tunnelling final states. These observations provide strong evidence that the surface photoeffect is the dominant mechanism of excitation by p-polarized light in photofield emission from tungsten.

Observation of surface photoeffect for an adsorbate: Oxygen on W(1 0 0) and W(1 1 1)

Two strong peaks are observed, one above and one below the tungsten plasmon energy, hv p′ in the spectral dependence of the photoemission from one 2 p-like orbitals of oxygen chemisorbed on W(1 1 1) and W(1 0 0) surfaces. The peaks are interpreted in terms of a surface photoeffect arising from the dielectric response of the surface region. The occurrence of the resonance above hv p correlates with oxygen being located below the W surface plane. Local dielectric model calculations qualitatively reproduce the observations.

Energy-distribution studies of photoexcitation in photofield emission

Total energy distributions of the photocurrent in photofield emission from tungsten have been measured for $s$- and $p$-polarized light. The results show that for photon energies below 3.5 eV the dominant mechanisms of photoexcitation are the surface photoeffect and the indirect bulk photoeffect. In the photofield-emission configuration the surface and indirect bulk distributions can be separated by suitably choosing the polarization and angle of incidence of the illumination.

Surface photoeffect in small spheres

Surface photoeffect in small spheres

Surface photoeffect with non specular surface scattering of electrons

Surface photoeffect with non specular surface scattering of electrons

The surface photoeffect

Angle-resolved photoelectron spectroscopy coupled with the polarized continuum of synchrotron radiation has been used to identify unambiguously a large enhancement in the photoexcitation from A(100) due to the surface photoeffect. This large enhancement in the cross section occurs only when the excitation field has a component perpendicular to the surface and is due to the dielectric response in the surface region, i.e., to the spatially varying electromagnetic field at the metal surface. An absolute determination of the differential photoionization cross section for excitation of Fermi-energy electrons shows quantitative agreement with Feibelman's self-consistent calculation. A simple picture of the surface photoeffect will be presented.

Surface photoeffect for metals: Energy and angular electron distributions

The nonlocal theory of the author for the surface photoeffect produced in metals by $p$-polarized light is used to obtain energy and angular electron distributions for a range of low to moderate excitation energies. Considering only the effects from single-particle excitations, the energy distribution curves have, in general, a roughly triangular shape with the peak occurring for initial states near the Fermi energy. There is also a high-energy tail due to damping. Features in the angular distribution curves include confinement to decreasing angles with increasing light frequency and low-angle structure resulting from damping. Very striking structure appears in the energy-resolved angular distributions when the light frequency $\ensuremath{\omega}$ is such that $\ensuremath{\hbar}\ensuremath{\omega}g{E}_{F}+\ensuremath{\Phi}$ where ${E}_{F}$ is the Fermi energy and $\ensuremath{\Phi}$ the work function. A number of significant changes, including a sharp increase in the total yield, occur in the various distributions when Landau damping sets in, that is, when the plasmon enters the single-particle-excitation region. Arguments are presented that there is no reason to expect that the nonlocal surface contributions to the yield should vanish above the plasma frequency.