Ejection Of Secondary Electrons Research Articles

Surface charge dynamics on air-exposed ferroelectric Pb(Zr,Ti)O3(001) thin films.

Probing of the free surface ferroelectric properties of thin polar films can be achieved either by estimating the band bending variance under the top-most layer or by studying the extent of the extrinsic charge accumulated outside the surface. Photoemitted or incoming low-energy electrons can be used to characterize locally both properties in a spectromicroscopic approach. Thin ferroelectric lead zirco-titanate (PZT) is investigated by combining low energy/mirror electron microscopy (LEEM/MEM) with photoemission electron microscopy (PEEM) and high-resolution photoelectron spectroscopy (XPS). Significant extrinsic negative compensation charge is proven to accumulate on the surface of the outward polarized thin film, indicated by high MEM-LEEM transition values, up to 15.3 eV, and is correlated with the surface electrostatic potential, which can be partially screened either by electrons interacting with the sample or by soft X-rays through the ejection of secondary electrons and generation of positive charge under the surface. A radiation-induced surface charge compensation effect is observed. The study indicates that air-exposed high quality ferroelectric thin films show large negative surface potentials, determined locally on the surface, which are nevertheless sensitive to beam damage and molecular desorption. These values represent a confirmation of previously estimated surface potential energy values determined from the LEED data on clean surfaces.

Secondary electron-induced biomolecular fragmentation in fast heavy-ion irradiation of microdroplets of glycine solution.

The influence of secondary electrons on radiation damage of biomolecules in water was studied by fast heavy-ion irradiation of biomolecular solutions. Water microdroplets containing the amino acid glycine under vacuum were irradiated by fast carbon projectiles with energies of 0.8-8.0 MeV. A variety of fragments from the droplets were observed by time-of-flight secondary-ion mass spectrometry: methylene amine cation and formate anion originating from the cleavage of C-Cα bonds, cyanide anion generated by cleavage of multiple bonds, and protonated and deprotonated glycine. The dependence of the yield of each fragment on projectile energy was examined; different behavior was observed for positive and negative fragments. Considering that biomolecular fragmentation may be induced by secondary electrons ejected from the water molecules surrounding biomolecules, we calculated the cross section for ejection of secondary electrons from liquid water. We found that the formation of both positive and negative glycine fragment ions correlated with the predicted emission of secondary electrons at different projectile energies. The formation of [Gly-H]- fragments, typical for gas phase dissociative electron attachment to amino acids, is shown to be caused by electrons from the low-energy part of the secondary electron distribution.

Defect-induced discriminative modulation of the highest occupied molecular orbital energies of graphene

Defects are capable of modulating various properties of graphene, and thus controlling defects is useful in the development of graphene-based devices. Here we present first-principles calculations, which reveal a new avenue for defect engineering of graphene: the modulation by defects on the highest occupied molecular orbital (HOMO) energy of a charged monolayer graphene quantum dot (GQD) is discriminative. When the charge of a GQD increases its HOMO energy also increases. Importantly, when the GQD contains one particular class of defects its HOMO energy is sometimes higher and sometimes lower than that of the corresponding GQD without any defects, but when the GQD contains another class of defects its HOMO energy is always higher or lower than that of the corresponding intact GQD as its excess charge reaches a critical value. This discriminative modulation could allow defect engineering to control secondary electron ejection in graphene, leading to a new way to develop graphene-based devices.

Effects of dielectric substrate thickness on plasma immersion ion implantation

A one-dimensional plasma fluid model is developed for investigating the effects of dielectric substrate thickness on plasma immersion ion implantation. By considering the effects of secondary electron emission from the dielectric substrate and using finite difference schemes, evolution of plasma sheath, ion fluence and dielectric surface potential versus time and substrate thickness are evaluated. It was demonstrated that with the increasing dielectric thickness, sheath width and ion fluence over the dielectric surface decrease and surface potential reduces. These effects can be attributed to the accumulation of positive ions and ejection of secondary electrons from the dielectric surface and thereby lessening the strength of the electric field over the dielectric substrate. It is also shown that the secondary electrons have a profound effect on implantation results and must be considered in plasma immersion ion implantation of dielectric materials.

Vacuum problems of miniaturization of vacuum electronic components: a new generation of compact photomultipliers

Recently, a new generation of compact photomultipliers exhibiting high sensitivity, fast time response, extremely low background and extended dynamic range has been introduced. The electron multiplication is enabled by a high-gain and extremely low-background, closed-end, channel electron multiplier serving also as a body of the new mini electron tube. By preventing ultra-fine leaks by the introduction of ultra-sensitive leak detection, and incorporating a high performance getter material, the new channel photomultiplier becomes a very small volume static vacuum system with a relatively large inner surface, evolving gases with different outgassing rates. For an extended channel photomultiplier lifetime, it is of great importance to maintain extreme vacuum conditions and therefore the tube bodies have to be pre-conditioned by a thorough UHV baking followed by an effective electron and UV light scrubbing. Semitransparent photocathodes are remotely processed, onto many faceplates in one cycle, in order to maximize quantum efficiency at the chosen wavelength, minimize photocathode dark current and ensure excellent uniformity. The faceplates are hermetically sealed to the tube bodies during the course of a vacuum transfer technique. The tube shelf life is mainly dependent on the photocathode stability in an alkali-free environment inside the tube while the tube operational life is dependent on the intensity of ion bombardment of the photocathode reflecting both in the ejection of secondary electrons followed by the formation of spurious pulses and photocathode surface sputtering. Possible sources of chemically inert gases, which can affect the correct functioning of the tubes are discussed.

Asymmetric(e,2e)measurement of vibrational intensities in the 100-eV electron-impact ionization ofN2to theN2+X2Σg+andA2Πustates

Asymmetric $(e,2e)$ experiments have been carried out on ${\mathrm{N}}_{2}$ for incident energy 100 eV, incident electron scattering angle 4\ifmmode^\circ\else\textdegree\fi{}, secondary-electron energy 5 eV, and secondary-electron ejection angles from -32\ifmmode^\circ\else\textdegree\fi{} to 100\ifmmode^\circ\else\textdegree\fi{}. The experiments have sufficient energy resolution (\ensuremath{\sim}0.13 eV) to resolve the vibrational structure in the coincidence energy-loss spectrum. Ionization transitions to the $X{}^{2}{\ensuremath{\Sigma}}_{g}^{+}$ and $A{}^{2}{\ensuremath{\Pi}}_{u}^{+}$ states were observed. When the final state of the product ion was $X{}^{2}{\ensuremath{\Sigma}}_{g}^{+},$ no vibrational excitation was observed at secondary electron ejection angles of 100\ifmmode^\circ\else\textdegree\fi{} (recoil) or -80\ifmmode^\circ\else\textdegree\fi{} (binary). However, at -32\ifmmode^\circ\else\textdegree\fi{}, near the peak of the binary lobe of the cross section, a strong increase in the production of the ${v}^{\ensuremath{'}}=1$ state was observed. In contrast, production of the $A{}^{2}{\ensuremath{\Pi}}_{u}^{+}$ state was characterized by the expected Frank-Condon vibrational intensities at 100\ifmmode^\circ\else\textdegree\fi{} and 80\ifmmode^\circ\else\textdegree\fi{} on the recoil side but a decrease in the relative intensities of the ${v}^{\ensuremath{'}}=1,$ 2, and 3 vibrational states at -32\ifmmode^\circ\else\textdegree\fi{}. The A state deviations occur where the cross section is small, so we conclude that 100-eV electron impact does not produce significant vibrational excitation in the $A{}^{2}{\ensuremath{\Pi}}_{u}^{+}$ state. On the other hand, the $X{}^{2}{\ensuremath{\Sigma}}_{g}^{+}$ state is produced with significant vibrational excitation in the binary lobe where the cross section is large. This excitation is important for both a theoretical understanding of the ionization process as well as plasma and atmospheric phenomena.

Ejection of primary and secondary electrons (cascades) from the ion track centre

Ejection of primary and secondary electrons (cascades) from the ion track centre

Measurement of MeV Ion Track Structure in an Organic Solid.

Ion tracks, formed by the interaction of energetic charged particles with matter, have played an important role in the development of modern physics. More recently, MeV-GeV ion track production has been used to modify materials in a controlled manner [1,2], being a potential tool to engineer material properties in the nanometer to micrometer scale. MeV-GeV ions are finding recently wider use in radiation therapy for, e.g., treatment of different forms of cancer. In spite of this, the understanding of the evolution of energy deposited around the path of an ion has not progressed markedly since early works in the sixties and seventies [3‐5]. Ions with velocities higher than the Bohr velocity syB › 0.22 cmynsd deposit their energy in electronic excitation and ionizations in a cylindrically symmetric region around the path of the impacting ion, forming the track core. The incident ions cause ejection of energetic secondary electrons that transport part of this energy out of the core [3,4]. The mean deposited energy density, esrd, after the ion passage is approximated by a esrd ~ r 22 dependence, where r is the radial distance from the ion path. Such a dependence is obtained from

Vibrational distribution of N2+A ²Πu from 100‐eV electron impact on N2

Measurements of the electron excitation of the Meinel band system of N2+ produced by electron impact on N2 have been hampered by the long lifetime of the N2+ A 2Πu upper state, the spectroscopically inconvenient near‐infrared region where the bands are located, as well as overlap by the N2 first positive system. In the present work, an experiment in which the scattered projectile electron and the low‐energy secondary electron are detected in coincidence was used to allow the vibrational distribution of the A 2Πu state produced by 100‐eV electron impact on N2 to be determined directly in excitation without the need for optical measurements. The coincidence energy loss spectrum was measured for a typical secondary electron energy of 5 eV over a range of secondary electron ejection angles. The results of a band contour analysis show that the population of the ν′=0‐5 vibrational levels is consistent with that predicted by the Frank‐Condon factors for ionization of N2 to the N2+ A 2Πu state.

Role of rediffused primary electrons in secondary-electron ejection from a Cu(100) surface.

The energy and angle dependence of the spin polarization of secondary electrons ejected from a Cu(100) surface by a polarized incident electron beam of energy 14, 50, and 100 eV has been investigated. At energies approaching the incident beam energy the secondary-electron polarization is comparable to that of the incident beam. The polarization decreases with decreasing secondary-electron energy, however, and at the lowest energies approaches zero. The data are interpreted in terms of the fractional contributions of rediffused primary and true secondary electrons to the total secondary-electron current.

Near-threshold structure in the atomic K-shell spectra for ionisation by photons or fast charged particles

The differential optical oscillator strength df/de for ionization, which results in an ejected electron of kinetic energy e, is proportional to the photoionization cross section.1 It has been known for some time2–4 that df/de does not necessarily decrease monotonically with e from the threshold for outer atomic subshells. This is also true for the generalized oscillator strength df(K,e)/de for ionization at momentum transfer ℏK, when K is small. The quantity df(K,e)/de is the essential factor in the Born cross section for secondary-electron ejection by charged particles,5 and it tends to df/de in the optical limit K → 0. The non-monotonic e-dependence, or structure in the spectral strength, is related primarily to angular momentum barriers to the escape of electrons near the threshold.1,6 These barriers are known to be especially important for continuum waves with angular momentum l ≥ 2 near the outer edge of the atom. Thus, it has been thought that the K-shell ionization should be unaffected by the barrier effect because the K-electron wave functions are quite compact and because the final state is limited to p waves only, by the dipole selection rule.

The interaction of helium metastable atoms with a clean and co covered polycrystalline tungsten surface

An investigation has been made of secondary electron ejection resulting from the impact of helium metastable atoms on a clean and CO covered polycrystalline tungsten surface respectively. The ejected election energy distribution from the clean tungsten surface is found to differ from that resulting from the neutralization of slow incident ions. In the present case the distribution is narrow and peaked at approximately 2 eV. When the tungsten surface is exposed to CO it is found that the total yield of secondary electrons increases, but that the energy distribution remains essentially the same. The increase in the total yield following CO exposure is shown to be associated with bonding of the CO in the α-state. A tentative explanation for the de-excitation mechanism is proposed, based on Auger de-excitation of the helium metastable atom.

Absolute doubly differential cross sections for ejection of secondary electrons from gases by electron impact. II. 100-500-eV electrons on neon, argon, molecular hydrogen, and molecular nitrogen

Absolute doubly differential cross sections for secondary electron production by electron impact have been measured for static gas targets of neon, argon, hydrogen, and nitrogen. Electron impact energies were from 100 to 500 eV. An electrostatic analyzer was used to analyze secondary electrons with energies between 4 eV and the primary electron energy minus the first ionization potential of the target. Angles of emission were 10\ifmmode^\circ\else\textdegree\fi{} to 150\ifmmode^\circ\else\textdegree\fi{}. The present data agree well with the data of Opal, Beaty, and Peterson at 90\ifmmode^\circ\else\textdegree\fi{} but, as was observed previously for helium, the agreement becomes increasingly poorer for larger and smaller angles. This angular disagreement, which is independent of target gas and impact energy, is approximately given by $a+(1\ensuremath{-}a)sin\ensuremath{\theta}$, where $a=0.10\ifmmode\pm\else\textpm\fi{}0.12$. Previously we compared the experimental data of Opal, Beaty, and Peterson with calculations by Manson for helium and obtained a similar correction but with $a=0.53$. Recent Born-approximation calculations of Manson are compared with our 500-eV argon data. The calculations reproduce the angular distributions of the measured cross sections quite well for small secondary-electron energies. For intermediate energies the agreement is still quite good near the momentum-conservation peak but poorer for large scattering angles.

Absolute doubly differential cross sections for ejection of secondary electrons from gases by electron impact. I. 100- and 200-eV electrons on helium

We have measured absolute values of the cross sections for ejection of electrons from helium gas by 100- and 200-eV electrons. These cross sections were measured for emission angles from 10\ifmmode^\circ\else\textdegree\fi{} to 150\ifmmode^\circ\else\textdegree\fi{} and for electron energies from 3 eV to a value equal to the primary energy minus the ionization potential. The measurements were made using a static-gas target and an electrostatic analyzer. The resulting cross sections agree reasonably well with those of Opal, Peterson, and Beaty near 90\ifmmode^\circ\else\textdegree\fi{}, but show the need for an adjustment of their angular distributions. Comparison with Born-approximation calculations of Manson et al. at 2 keV indicate that the cross sections of Opal et al. should be divided by $0.53+0.47sin\ensuremath{\theta}$. A somewhat larger correction is needed to bring their data into agreement with the results of this experiment. The adjustment in the angular distributions also affects the cross sections integrated over all angles.

Some aspects of channel electron multiplier operation (UV detection)

The use of wide aperture channel electron multipliers for the detection of ultraviolet radiation is described and the performance of similar Mullard and Bendix tubes compared. In particular, it is shown how the efficiency of these detectors can be improved by biasing a grid placed close to the entrance cone, but this improvement is achieved only at the expense of timing resolution. These characteristics are interpreted in terms of ejection of secondary electrons near the cone periphery.

Absolute fast neutral beam flux measurements using a pyroelectric detector

An inexpensive, sensitive pyroelectric bolometer for use as a fast neutral beam detector is described. Experimentally determined responsivity and noise equivalent power are presented, and are shown to agree with theoretical predictions. The detector has been used to measure the power absorption coefficient and the secondary electron ejection coefficients for 150 eV to 10 keV hydrogen and helium atoms impinging on a silver surface.

Determination of secondary electron ejection coefficients for rare gas metastable atoms

Secondary electron ejection coefficients for rare gas metastable atoms striking a stainless steel surface have been determined using an improved gas cell technique. Data are presented for He(2 1S), He(2 3S), and Ne(3P0,2) atoms, and limitations of this technique are discussed. An alternative method for γ determination is described which utilizes laser induced photoionization rather than chemiionization, and results are presented for He(2 1S) and Xe(3P0).

Angular distributions of electrons ejected from atoms and molecules by fast charged ions

Doubly differential cross sections for ejection of secondary electrons by fast ions have been calculated using linear combinations of products of atomic orbitals with 'inter nuclear' wavefunctions which asymptotically represent incoming and outgoing states. Numerical results for incident protons on He and H2 targets are compared with experiments. The behaviour of the cross sections in the forward peak of the experimental angular distribution, previously unaccounted for in the Born and binary-encounter calculations, is reproduced quantitatively.

Secondary Electron Ejection from Contaminated Metal Surface by He and Ar Atoms

Secondary electron yields γ0 on a contaminated Cu–Be surface for neutral He and Ar atoms of 19eV to 500eV have been measured. The neutral beams are produced from the ground state ion beams by charge transfer with the same kind of atoms. The yields γ0 obtained are confirmed to be of the ground state atoms. The values of γ0 are 0.58 for He and 0.24 for Ar at 500eV, and rapidly decrease to 6×10-6 for He and 2×10-6 for Ar at 19eV. The ejection mechanism is discussed. In the case of Ar, the secondary electron yield increases notably when the electron energy producing the primary ion beam is increased above 32eV. This indicates that metastable atoms Arm are produced from metastable ions Ar+m by the charge transfer process Ar+m+Ar→Arm+Ar+. This fact can be used as a new technique of detecting metastable ions. An example of its application is presented.

284. Radiation properties of secondary electron multipliers: A A Soloveva and L F Gavrilov, Rep Ural Polytech Inst, 215, 1973, 111–114 ( in Russian)

Recently, a new generation of compact photomultipliers exhibiting high sensitivity, fast time response, extremely low background and extended dynamic range has been introduced. The electron multiplication is enabled by a high-gain and extremely low-background, closed-end, channel electron multiplier serving also as a body of the new mini electron tube. By preventing ultra-fine leaks by the introduction of ultra-sensitive leak detection, and incorporating a high performance getter material, the new channel photomultiplier becomes a very small volume static vacuum system with a relatively large inner surface, evolving gases with different outgassing rates. For an extended channel photomultiplier lifetime, it is of great importance to maintain extreme vacuum conditions and therefore the tube bodies have to be pre-conditioned by a thorough UHV baking followed by an effective electron and UV light scrubbing. Semitransparent photocathodes are remotely processed, onto many faceplates in one cycle, in order to maximize quantum efficiency at the chosen wavelength, minimize photocathode dark current and ensure excellent uniformity. The faceplates are hermetically sealed to the tube bodies during the course of a vacuum transfer technique. The tube shelf life is mainly dependent on the photocathode stability in an alkali-free environment inside the tube while the tube operational life is dependent on the intensity of ion bombardment of the photocathode reflecting both in the ejection of secondary electrons followed by the formation of spurious pulses and photocathode surface sputtering. Possible sources of chemically inert gases, which can affect the correct functioning of the tubes are discussed.