External Photoelectric Effect Research Articles

External photoelectric effect and high-efficiency photocathodes in the soft-x-ray region of the spectrum

Two approaches to calculating the “true” x-ray photoemission from high-efficiency photocathodes are analyzed in the photon energy range 1–10 keV. Analytic expressions are derived for the quantum yield of true x-ray photoelectrons. The calculated values of quantum yield are compared with experimentally measured values.

Direct photodesorption of ions from the surface of a crystal

A model of the direct photodesorption of ions from the surface of a crystal, reminiscent of the ionization of an atom in the external photoelectric effect, is studied. The model is based on the concept of a quasiclassical ion, whose motion in a surface potential well is described by a wave packet. The choice of wave function in the form of a packet may be justified by a variational principle, by finding the parameters of the packet. These parameters may also be selected from known experimental data. These methods are standard in solid state physics and make it unnecessary to take into account the form and magnitude of the operator describing the interaction between the ion and the crystal surface.

Laser resonance external photoelectric effect in F 2 ? color centers of LiF crystals

The laser-resonance two-step external photoelectric effect is revealed in the F 2 − color centers of LiF crystals exposed to nanosecond laser pulses. The first photoionization step uses IR radiation tunable within the resonance absorption band of the color centers and the second step the second harmonic of a Nd:YAG laser. The photoionization cross sections of the F 2 − centers are estimated.

External photoelectric effect in CaF 2:Sm 2+ crystals under irradiation with visible light

External photoelectric effect is registered in CaF 2:Sm 2+ crystal under irradiation with visible light, the effect being explained by the photoionization of the impurity ions Sm 2+. The spectral dependence of the photoelectric effects is studied and its quantum yield estimated. The results obtained are compared with the previously known data of the photoconduction of fluoride crystals.

Role of the external photoelectric effect in surface microprocessing

An analysis is made of the photoemission of electrons in gases when the surface of a solid is subjected to high-intensity ultraviolet laser radiation which does not cause surface heating. Various situations are considered in which generation of high local electric fields and of a dense cloud of charged particles near the surface may alter and even determine the mechanism of laser-stimulated processes on surfaces of solids.

Détermination de l'épaisseur et de la concentration des couches minces épitaxiales Ga 1− xAl xAs/GaAs par la méthode des sauts de la photoémission

The analytical expression of the quantum efficiency of the external photoelectric effect is established for a photocathode formed by juxtaposition of a thin film and a substrate. The thickness and the atomic composition of the thin Ga 1− x Al x As films, deposited by liquid phase epitaxy onto a GaAs(100) substrate, are determined by the X-ray photoemission jumps method. This method is based on the measurement of photoemission jumps at the absorption K edges of the elements constituting the photocathode. The results of measurements of the thickness of thin Ga 1− x Al x As films and their concentration are compared with the technological conditions of fabrication on the one hand and the behaviour characteristics of photocurrent spectra on the other.

Double quantum external photoelectric effect in merocyanine

The double quantum external photoelectric effect. DQEPE. was found in a merocyanine dye. A modified Millikan apparatus was used to measure photoemission from a microcrystal of the dye suspended in a N2 atmosphere by an electric field. The crystal ionization energy was 5.25 eV and the band gap was 2.5 eV. The mechanism of the DQEPE in merocyanine is photoionization of a trapped electron. and is distinctly different from the DQEPE found previously in anthracene and tetracene.

ABSORPTION SPECTRA AND PHOTOEMISSION OF CHLOROPHYLL-B MULTILAYERS

Abstract The monolayer of chlorophyll (Chi) b at the air/water interface has been examined by pressure–area isotherm and film thickness. Furthermore, with the multilayer of Chi b on the solid surface the visible absorption spectra and the external photoelectric effect in the vacuum ultraviolet region have been studied. The energy diagram has been proposed.

The external photoelectric effect of semiconductors

The Fowler method is applied to semiconductors (or dielectrics) with some band structure. The dependence of both the quantum yield and the density of the photoelectric current upon the frequency of the light and the temperature are considered in detail in the case of a simple cubic structure. It is shown that the photoelectric emission characteristics depend substantially upon the direction in which the photoelectrons are emitted.

Statistics of the external x-ray photoelectric effect of bulk cathodes: E P Savinov and V N Schemelov,Zh Eksper Teor Fiz, 60 (4), April 1971, 1371–1378 ( in Russian)

Statistics of the external x-ray photoelectric effect of bulk cathodes: E P Savinov and V N Schemelov,Zh Eksper Teor Fiz, 60 (4), April 1971, 1371–1378 ( in Russian)

Photoemission from an Anthracene Single Crystal Doped with Alkali-Metals

External Photoelectric effect is investigated with an anthracene single crystal doped superficially with alkali-metals; cesium, potassium and sodium. The threshold energy is found to be dependent upon the amount of metal doped. When the doping amount is low, the photoelectrons are excited via the first singlet exciton state and are emitted with the aid of the surface double layer formed between alkali-metal and anthracene. The photoemission yield of a highly doped crystal shows a cubic law dependence upon the incident photon energy. The threshold energy, 2.57 eV, obtained in this case can be interpreted as the energy of surface state below the vacuum level.

The Ionization energy of cationic cyanine dyes

Typical photosensitizing organic dyes are studied by a comparison of their measured and calculated ionization potentials. The ionization potential of the dye, adsorbed on a substrate, is measured by determining the threshold of the external photoelectric effect from the dye. The measured values are corrected for the effect of the substrate and an ionization potential, IP exp, is obtained which is independent of the substrate, and which may be compared to the calculated values. The Hartree-Fock LCAO-MO theory, for open and closed shell systems, is used in the π electron approximation with the assumption of zero-differential overlap. A first approximation to the ionization potential, IP n , is calculated from the difference between the total π electronic energies of the ground and ionized states. A method is described to correct this value, for the assumption of a static σ electron substructure, by computing the work done in the redistribution of the σ electrons upon ionization. The ionization potential calculated for one dye cation was almost twice the measured value, IP exp. It is concluded that the dye anion cannot be neglected and the HF-SCF-LCAO-MO theory is modified to include the Coulomb interaction of the anion. The ionization potential of one neutral dye molecule was more than 2 eV higher than IP exp. Agreement with experiment is obtained by calculating the ionization potential of the dye including the effects of the nearest neighboring molecules.

Recombination radiation of a semiconductor p-n junction

The recombination radiation of a p-n junction is treated as the process inverse to the external photoelectric effect. The probability for radiative recombination is found in first-order pertubation theory on the basis of the interaction of the carriers with the electromagnetic radiation. The eigenstates of the electrons and holes are constructed with account of the barrier at the boundary of the n- and r-regions. It is taken into account in the calculations that mn ≪ mp for real semiconductors; this leads to a predominance of the radiation from the p-region. A comparison with experiment may yield a new explanation for the short-wavelength peak in the spontaneous radiation of a GaAs p-n junction.

1056. Investigation of the external photoelectric effect in aluminium single crystals

Interaction between carbon disulfide and sulfur hexafluoride is excited by cw CO2 laser radiation and has been investigated for different ratios over a pressure range from 3.1 to 34.6 kPa. The reaction yields sulfur tetrafluoride, sulfur, carbon, thiocarbonyl fluoride, tetrafluoromethane and hexafluoroethane, the ratio of these latter products is dependent on the partial pressure of sulfur hexafluoride in the initial CS2-SF6 mixture. Interaction is considered to include both the SF6-sensitized decomposition of carbon disulfide and reaction between sulfur hexafluoride and carbon disulfide.

Photoemission from Polycyclic Aromatic Crystals in the Vacuum Ultraviolet Region. II

Abstract The external photoelectric effect of the molecular crystals in the vacuum ultraviolet region was observed. The ionization potential of tetrathiotetracene, indanthrone and perylene, as estimated by the threshold values of their spectral responses, were 4.55, 5.10 and 5.40 eV respectively. The band-gap energies, calculated on a band model, were 0.62, 0.68 and 1.72 eV respectively. These values were in fairly good agreement with those obtained from the temperature dependency measurements of their electrical resistivities. By analyzing the photocurrent-voltage relation for tetracene and pentacene, the kinetic energy distribution curves were obtained. From an analysis of those curves, it turned out that the one-electron process plays a dominant role in the emission processes and that illumination with a short-wavelength light (1200 Å—1500 Å) can excite electrons from the deeper levels of the valence band to a vacuum. The observed positions of the filled π-electron levels were almost consistent with the results obtained using the LCAO method; for tetracene the depths of lower levels in order from the uppermost one, were 1.52, 2.48, 2.96, 3.46 and 3.92 eV from the observations and 1.60, 2.34, 2.98, 3.32 and 3.89 eV from the calculations.

Photoemission from Polycyclic Aromatic Crystals in the Vacuum Ultraviolet Region

Abstract The external photoelectric effect of thin films of tetracene, pentacene, violanthrene, quater-rylene and tetrathiotetracene was studied in the vacuum ultraviolet region. The contact potential differences between the emitters, organic crystals, and the collector (Aquadaq) were determined from the saturation points of the current-voltage characteristics. It was observed that the quantum yields of photoemission increase markedly with an increase in the photon energy, reaching the order of 10−2 electrons/quantum at hv=10 eV. The threshold energies were lower by 1–2 eV. than the molecular ionization potentials. An energy diagram for the external photoelectric process of organic molecular crystals was introduced on the basis of the intrinsic band model. With the aid of this diagram, the band gap energies, Δε, were obtained from the observed values of photoemission. The values of Δε thus obtained were in fairly good agreement with those based on the results of the temperature dependence of semiconduction.

Charge-Transfer Exciton State and Energy Levels in Tetracene Crystal

An experimental study of the double-quantum external photoelectric effect in tetracene reveals evidence of an exciton state of energy 2.9 eV different from the normal exciton. This mobile, bulk-generated state is considered to be a charge-transfer (CT) exciton and is most efficiently produced at an energy ≥Ec=3.7 eV. We associate Ec with the lower edge of a relatively broad upper conductivity band. Presumably, there are other conducting states Ec′ such that ECT<Ec′<Ec. The calculated value for the polarization energy of the free electron in the crystal, Pe=Ic—Ec=1.6 eV, where Ic is the threshold energy for external electron emission. This is the same as the polarization energy of the positive hole Ph=Ig—Ic, where Ig is the ionization energy of the tetracene molecule. The electron affinity of the isolated molecule must be less than 0.8 eV. The diffusion length of the charge-transfer exciton is less than 5000 Å.

Investigation of the External Photoelectric Effect in HgTe

AbstractResults are presented of investigations on the external photoelectric effect in HgTe samples containing a few monocrystalline grains. The investigations use the retarding field method in a spherical vacuum condenser. Monochromatic light of wavelengths 2537 and 2480 Å is used for excitation and the current‐voltage characteristics are measured. Both newly split surfaces and surfaces which have been subjected to mechanical treatment are examined. Mean values of the thermoelectric and photoelectric work function are measured and found to be respectively: (4.120 ± 0.005) eV and (4.130 ± 0.005) eV. Values of the absolute quantum yield for samples at λ = 2537 Å are within the range 1.58 × 10−7 to 6.9 × 10−6 electrons/quantum. Changes are observed in the photoelectric characteristics of surfaces exposed to air for a period of time. The concentration of electrons and holes in the samples and the effective mass ratio mh/me are computed for energy gaps of +0.01, 0, and −0.01 eV.

Photoconductivity and the external photoelectric effect in PbS

We have measured the yields for photoconductivity and external photoernission in the vacuum ultraviolet spectral region, 584–3000 Å, using polycrystalline layers and natural crystals. The photoconductive yield in PbS layers rises approximately linearly with incident photon energy hv up to 7.5 eV; further extension of the measurement was thwarted by the magnitude of the external photocurrent. The rise in yield, as previously reported by us, is attributed to collision multiplication beginning at hvt = 2 eV. The observed photoelectric threshold for galena crystals was ∽ 5 eV, and for layers ∽ 6 eV. Both kinds of specimen exhibit very large external photoyields at high hv. 10–20 per cent at hv = 14.5 eV. The energy distribution of emitted electrons was examined using retarding-potential methods. The high photoelectric thresholds imply that any electron with sufficient energy to escape would be subject to scattering due to collision multiplication, an effect expected normally to limit the attainable photoelectric yield. From the high ultimate yields that were observed, and from the changes in the energy distribution of the emerging electrons with increasing hv, we infer that scattering by collision multiplication, considered as a function of hv, is not a monotonically rising function, but has a maximum. The data suggest strongly that this occurs for incident photon energies of slightly more than 9 eV.

Theory of the Electronic Polaron and Ionization of a Trapped Electron by an Exciton

Introducing the concept of the electronic polaron which is an analogue of the usual lattice polaron, we have considered its motion under the influence of a trapping potential, by using a simplified model for the electronically polarizable crystal such that the excited states of crystal electrons consist of a single exciton band. By a suitable transformation we obtain an expression for the Hamiltonian which consists of the following three parts: the first part describes the motion of an electron under the potential in an dielectric medium in the classical way, the second part represents the energy of the electronic polarization field the quanta of which are nothing but free excitons, and the last part is the interaction energy between these two parts which causes the production or annihilation of excitons in the neighbourhood of the potential by exchanging energy with the electron. Treating the last part as a perturbation, we have calculated the probability for the ionization of a traped electron by an exciton. After correcting the oversimplification of the model, we have applied the obtained result to actual crystals, leading to a conclusion that in alkali-halides the excitons annihilate rather by ionizing F-electrons than by spontaneous emission if the density of the F-centers is larger than 1016 per c.c., in consistence with the experimental results of Apker and Taft on the external photoelectric effect.