Electron Emission Spectra Research Articles

Optical Properties of Blue Phosphor

(, 0.5, 1, 5, and 10 at.%) polycrystalline powders blue phosphors were prepared via the classical solid-state reaction method. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation, and emission spectra were used to characterize phosphors. By analyzing the excitation and emission spectra of samples, the result indicates that there exists the energy transfer only from the group to the energy level of ion. On the other hand, the influence of the thulium concentration on the blue emission transition and and the emission of group are investigated.

Structural and Bluish-White Luminescent Properties of Li+-Doped BPO4 as a Potential Environmentally Friendly Phosphor Material

Many efforts have been devoted to exploring novel luminescent materials that not contain expensive or toxic elements, or do not need a mercury vapor plasma source. In this paper, BPO4 and Li+-doped BPO4 powder samples were prepared by the Pechini-type sol−gel (PSG) process. The structure and optical properties of the resulting samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), photoluminescence (PL) excitation and emission spectra, kinetic decay, and X-ray photoelectron spectra (XPS), respectively. It was found that PSG -derived Li+-doped BPO4 annealed at 960 °C exhibited bright bluish-white emission centered at 416 nm. The luminescence decay curves analysis indicates that each sample has two kinds of lifetimes (5.9 ns and 0.529 ms) and two types of kinetic decay behaviors which can be fitted into a single-exponential function and a double-exponential function, respectively. This can be attributed to two kinds of emission centers in samples. The luminescent mechanisms of the samples are discussed in detail.

Young-type interference effect on angular distribution of secondary electrons emitted fromH2in collisions with fast electrons

The Young-type interference arising due to the spatial coherence has been investigated in the electron emission spectrum from fast electron impact ionization of the inversion symmetric homonuclear diatomic molecule ${\mathrm{H}}_{2}$. The evidence of the interference effect in the angular distribution of the double differential spectrum of the secondary electron is found. The signature of constructive interferences has been identified in the soft-collision regions as well as in binary encounters. The observed oscillation in the forward-backward asymmetry parameter is explained in terms of the Cohen-Fano-type interference coupled with the angular dependence of oscillation frequency. A comparative study indicates a marked difference between the angular asymmetry in the case of fast heavy ion $({\mathrm{F}}^{9+})$ and electron collisions with ${\mathrm{H}}_{2}$ at a similar velocity.

Low-Fluence Electron Yields of Highly Insulating Materials

Electron-induced electron yields of high-resistivity high-yield materials - ceramic polycrystalline aluminum oxide and polymer polyimide (Kapton HN) - were made by using a low-fluence pulsed incident electron beam and charge neutralization electron source to minimize charge accumulation. Large changes in the energy-dependent total yield curves and yield decay curves were observed, even for incident electron fluences of < 3 fC/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The evolution of the electron yield as charge accumulates in the material is modeled in terms of electron recapture based on an extended Chung-Everhart model of the electron emission spectrum. This model is used to explain the anomalies measured in highly insulating high-yield materials and to provide a method for determining the limiting yield spectra of uncharged dielectrics. The relevance of these results to spacecraft charging is also discussed.

Rare earth centers properties and electron trapping in SnO 2 thin films produced by sol–gel route

Some very relevant optical, electrical, and structural properties of SnO 2 doped with rare-earth ions Er 3+ and Eu 3+ are presented. Films are produced by the sol–gel-dip coating process, and may be described as a combination of nanoscopic dimension crystallites (about 3–10 nm) with their respective intergrain potential barriers. The Er 3+ and Eu 3+ ions are expected to act as acceptors in SnO 2, which is a natural n-type conductor, inducing a high degree of charge compensation. Electron trapping and emission spectra data are presented and are rather distinct, depending on the location of the rare-earth impurity. This behavior allows the identification of two distinct centers: located either in the SnO 2 lattice or segregated at the particles surface. Based on a model for thermally activated cross-section defects, the difference between the capture energy of the photo-excited electron and the intergrain potential barrier is evaluated, leading to distinct values for high and low symmetry sites. A higher distortion in the lattice of undoped SnO 2 and SnO 2:Eu (1 at.%) was evaluated from Rietveld refinements of X-ray diffraction data. This was confirmed by Raman spectra, which are associated with the particles size and disorder. By comparing the samples with the same doping concentration, it was found that this disorder is higher in Eu-doped SnO 2 than in Er-doped SnO 2, which is in agreement with a higher energy for the lattice relaxation in the trapping process by Eu 3+ centers.

Field emission spectra of single-atom tips with thermodynamically stable structures

Energy spectra of field emitted electrons from well-characterized tungsten nanoemitters covered with different metals have been measured in detail while changing the electric field and the topmost atomic structure. At very high electric fields, additional humps appear in the spectra of a single-atom tip. Their energy positions depend on both the coated material and structure termination but not on the electric field. On the other hand, their intensities increase with increasing field. The current spectra did not include either peculiar features attributable to resonant tunneling or electric-field penetration, or significantly narrow FWHM, but are rather analogous to those of the conventional metallic field emitters. The spectral features along with a recent ab-initio theory indicate a large reduction in the tunneling barrier height in front of the single-atom electron source.

Exciton Autoionization in Ion-Induced Electron Emission

We report on measurements of electron emission spectra from surfaces of highly oriented pyrolytic graphite (HOPG) excited by 1-5 keV He+ and Li+ which, for He+, exhibit a previously unreported high-energy structure. Through a full quantum dynamic description that allows for the calculation of neutralization and electron-hole pair excitation, we show that these high-energy electrons can arise from autoionization of excitons formed by electron promotion to conduction band states close to the vacuum level. The same calculation explains the observed absence of high-energy excitons for Li+ on HOPG.

Electron emission from foils and biological materials after proton impact

Electron emission spectra from thin metal foils with thin layers of water frozen on them (amorphous solid water) after fast proton impact have been measured and have been simulated in liquid water using the event-by-event track structure code PARTRAC. The electron transport model of PARTRAC has been extended to simulate electron transport down to 1 eV by including low-energy phonon, vibrational and electronic excitations as measured by Michaud et al. [Michaud, M., Wen, A., Sanche, L., 2003. Cross sections for low-energy (1–100 eV) electron elastic and inelastic scattering in amorphous ice. Radiat. Res. 159, 3–22] for amorphous ice. Simulated liquid water yields follow in general the amorphous solid water measurements at higher energies, but overestimate them significantly at energies below 50 eV.

Preparation and luminescence properties of nano-polycrystalline Cr 3+:Lu 3Ga 5O 12

Nano-polycrystalline Cr 3+:Lu 3Ga 5O 12 (Cr:LGG) has been obtained by sintering its precursor, which was synthesized via coprecipitation method. The obtained nanopowders were charactered by X-ray diffraction (XRD), scanning electron microscopy (SEM), excitation and emission spectra and fluorescence decay curve measurements. XRD result shows that Cr:LGG precursor has translated into polycrystalline Cr:LGG after being sintered at 1000 °C for 3 h. Three broad excitation bands locate around 462, 608 and below 300 nm, respectively. The emission spectrum shows a broadband from 600 to 840 nm with maximum at 708 nm. The fluorescence decay time is determined as 890 μs.

Comparison study of two aldehyde group finishing agents in surface modification of up-conversion luminescence materials

Surface modification of up-conversion luminescence materials (Na[Y 0.57Yb 0.39Er 0.04]F 4 modified by amino groups) by grafting and modifying with aldehyde groups was studied by means of Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and emission spectrum (EM). The surface modification effect was compared using two different finishing agents, p-phthalaldehyde and glutaraldehyde. It was found that the surface of up-conversion luminescence materials could be modified by aldehyde groups of the two finishing agents, the systematic dispersibility and the thermostability of the up-conversion luminescence material modified by p-phthalaldehyde were better than those of the material modified by glutaraldehyde, and the luminous intensity of the material modified by p-phthalaldehyde was increased. The AI (the ratio of the suspended segmental quality in the specimen to the total mass of the specimen) of the material modified by p-phthalaldehyde was higher than that of the material modified by glutaraldehyde. It is obviously seen that the embellishment effect of p-phthalaldehyde as a finishing agent was better than that of glutaraldehyde. In addition, the reasons why p-phthalaldehyde is a good finishing agent are also explained.

Luminescent properties of Eu 3+-doped La 2Mo 2O 9 red phosphor by the flux method

A new red phosphor tervalent europium ion (Eu 3+)-doped lanthanum molybdate (La 2Mo 2O 9) was prepared by a flux method at 850 °C. The samples were characterized by application of X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) patterns, excitation and emission spectra. XRD analysis showed that single cubic phase La 2Mo 2O 9 crystal was synthesized. The spectra analysis indicated that the phosphors could be excited by UV light of 395 nm and visible light of 470 nm, and also emit intense red light with a maximum at 620 nm. The effects of different flux on the luminescence properties were studied in this experiment. The results showed that NH 4Cl is the appropriate flux and the optimal weight percent is 5.

Electron Energy Loss Spectroscopy study of the near surface region of the ternary Co–Cr–Mo alloy

Nature of the characteristic electron energy losses in the second electron emission spectra from the ternary Co–Cr–Mo alloy surface are studied in the low energy range of the primary electron energy E 0. The main types of losses were found: surface and bulk plasmons and their hybrid modes, interband transitions and ionization losses. For Co, Cr, Mo and Co–Cr–Mo alloy the experimental values of the plasmon energy were established to be less than it was predicted by free-electron gas model. Excess of conductive electrons in the surface layers for Co, Cr and Mo was observed by dependence of the surface plasmon dispersion from E 0, while for Co–Cr–Mo alloy the situation is quit opposite. Such behavior is explained by the complex phase structure of the ternary alloy. The analysis of intensity lines of plasmons from E 0 showed deeply changed alloy profile. Ionization Spectroscopy was used for studying the alloy elements distributing on the depth. Mo atoms preferred segregation in the outermost layers of Co–Cr–Mo alloy and enrichment with Cr competitive atoms in underlayers is displayed.

Bluish-White Emission from Radical Carbonyl Impurities in Amorphous Al2O3 Prepared via the Pechini-Type Sol−Gel Process

Many efforts have been devoted to exploring novel luminescent materials that do not contain expensive or toxic elements, or do not need mercury vapor plasma as the excitation source. In this paper, amorphous Al2O3 powder samples were prepared via the Pechini-type sol-gel process. The resulting samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), photoluminescence (PL) excitation and emission spectra, kinetic decay, and electron paramagnetic resonance (EPR). The obtained amorphous Al2O3 powder samples annealed at 500 and 600 degrees C exhibit bright bluish-white emission centered at 430 and 407 nm, respectively. The luminescent mechanisms of the amorphous Al2O3 powder samples can be ascribed to the carbon-related impurities such as radical carbonyl species. The calculated band structure of the defective amorphous Al2O3 agrees well with the results of spectral analysis and the proposed luminescent mechanism.

Mesoscopic pattern formation during initial oxidation of Ni(1 1 1) observed by electron emission microscopy

An apparatus available for metastable-atom electron emission microscopy (MEEM) and photoelectron emission microscopy (PEEM) was constructed for the present study, based on our electron trajectory simulations. In MEEM, information on the local electronic states at the topmost layer is selectively obtained. The mesoscopic (μm-scale) images during initial oxidation of Ni(1 1 1) at temperature of 300–700 K were monitored and compared with the spectroscopic data. The MEEM images obtained upon thermal collision of He ∗(2 3S) are shaded in ascending order of brightness: the chemisorbed, clean, and oxidized surfaces. Their brightness is in good agreement with the emission yield estimated from the corresponding electron emission spectra. During nucleation and lateral growth of oxide at 300 K, the PEEM and MEEM images reveal dappled and sometimes periodic patterns, probably due to surface defects that lingered even after the surfaces were cleaned. The evolution of the images suggests that the initial oxidation proceeds via successive multinucleation. It is also found that the μm-scale patterns of oxide strongly depend on the substrate temperature and dose pressure of gaseous oxygen. The oxide formation in the mesoscopic scale is discussed in terms of a diffusion-reaction field.

Influence of Ga and Hg on microstructure and electrochemical corrosion behavior of Mg alloy anode materials

The effects of Hg and Ga on the electrochemical corrosion behavior of Mg-5%Hg (molar fraction) alloys were investigated by the measurement of polarization curves and galvanostatic test. The microstructure of the alloys and the corroded surface of the specimens were investigated by scanning electron microscopy, X-ray diffractometry and emission spectrum analysis. It can be concluded that the addition of 1%Ga (molar fraction) reduces corrosion current density from 26.98 mA/cm2 to 2.34 mA/cm2; while the addition of 1%Hg (molar fraction) increases corrosion current density. The addition of Ga and Hg both promotes the electrochemical activity of the alloys and the influence of Ga is more effective than Hg. Mg-5%Hg-1%Ga alloy has the best electrochemical activity, showing mean potential of -1.992 V. The activation mechanism of the magnesium alloy produced by Hg and Ga was put forward. Magnesium atoms are dissolved in liquid Hg and Ga to form amalgam and undergo severe oxidation at the amalgam/electrolyte interface.

Preparation of Monodispersed Eu3+:CaMoO4 Nanocrystals with Single Quasihexagon

This paper reports the preparation of Eu3+:CaMoO4 nanocrystals by the microemulsion-mediated hydrothermal method. The synthesized Eu3+:CaMoO4 nanocrystals were characterized by XRD, transmission electron microscopy, high-resolution transmission electron microscopy, excitation, and emission spectra. The result shown that Eu3+:CaMoO4 nanocrystals with sizes of 40–90 nm exhibited a monodispersed and single quasihexagon. Eu3+:CaMoO4 nanocrystals exhibit a strong red fluorescence at 614 nm. This result may bring opportunity for the development of the other nanocrystals.

Interference of electrons ionized by short laser pulses

We analyze the two-dimensional momentum distribution of electrons ionized by short laser pulses by solving the time-dependent Schrödinger equation. Lindner et al., [Phys. Rev. Lett. 95, 040401 (2005)] identified oscillations in the electron emission spectrum for ionization by a few-cycle pulse as a time-double slit interference. We extend this analysis to interference fringes in the momentum distributions. For longer pulses we find a complex two-dimensional interference pattern that resembles ATI rings at higher energies and displays Ramsauer - Townsend - type diffraction oscillations in the angular distribution near threshold.

Local Electronic Properties Induced at the Molecule−Metal Interface

Electron emission spectra resulting from thermal collisions of He*(23S) metastable atoms with alkanethiols (CnH2n+1SH, n = 1−3) on a Pt(111) surface were measured to clarify the electronic properties induced at the molecule−metal interface. Upon chemisorption, the S 3p-derived states are broadened significantly by admixture with the Pt bands, yielding a sharp Fermi edge, whereas such a metallic state is missing in weakly bound species. The spectral feature is in agreement with first-principles calculations using density functional theory. Furthermore, our data show that the local density of states at the Fermi level decreases rapidly on going from the S atom to the outer C atoms, indicating that the chemisorbed species are metallic in character at the S end and insulating at the alkyl end. A new classification of adsorbates into insulating, metallic, and amphibious molecules would be useful for understanding ordinary charge transport at the molecule−metal interface.

Neutralization mechanisms in He +–Al surface collisions

From a quantum mechanical calculation where the populations of He ground and first excited states are properly taken into account, we can identify for the first time the neutralization to the He first excited state as an operative mechanism in He +–Al surface collisions. This identification allows us to understand the presence of high energy electrons in the ion induced electron emission spectra, through the inclusion of Auger deexcitation as an electron emission source, as well as to suggest a possible cause for the disagreement still found between theory and experiments in low energy ion scattering (LEIS) for this system.

Modelling of a Potential Relief on a Surface of Charged Polar Dielectrics

Expressions for calculating the potential distribution on the surface of a polarized electret depending on the polarization direction, the shape of nonuniformity and sign of the charge on this surface were obtained. These expressions allow one to calculate the potential distribution above the inhomogeneities that are of various sizes and forms, the latter can be represented by an “infinite” strip, a circle, a square or a ring. Within the limits of simple approximations, spectra of electron emission from a surface of poled dielectrics were calculated for the minimum and the maximum in the potential distribution and a comparison was drawn with the experimental spectra. A qualitative conformity was shown to exist between the theoretical and the experimental spectra.