Source Of Low-energy Electrons Research Articles

Development of a low energy e− gun for studies of e−–molecule interactions using a recoil-ion and electron momentum spectrometer

To study the interaction between low-energy electrons and molecules, a recoil-ion and electron momentum spectrometer (RIMS) has been developed at Shanghai EBIT laboratory. As the low-energy electron source, the ELG-2 electron gun (Kimball Physics) coupled with an ultrafast pulse generator has been tested. The energy of the electron beam ranges from a few eV to 2 keV, and the pulse width of the beam is about 1 ns.

Intermolecular Coulombic Decay in Small Biochemically Relevant Hydrogen-Bonded Systems

Intermolecular Coulombic decay (ICD) is a very fast and efficient relaxation pathway of ionized and excited molecules in environment. The ICD and related phenomena initiated by inner-valence ionization are explored for H(2)O···HCHO, H(2)O···H(2)CNH, H(2)O···NH(3), NH(3)···H(2)O, H(2)O···H(2)S, H(2)S···H(2)O, and H(2)O···H(2)O (p-donor···p-acceptor). This set of small hydrogen-bonded systems contains seven types of hydrogen bonding, which are typical for biochemistry, and thus its investigation provides insight into the processes that can take place in living tissues. In particular, an estimate of the ICD in biosystems interacting with water (their usual medium) is made. This decay mode is expected to be a source of low-energy electrons proven to be of extreme genotoxic nature. For the purpose of our study, we have used high-precision ab initio methods in optimizing the geometries and computing the single- and double-ionization spectra of formaldehyde-, formaldimine-, ammonia-, hydrogen sulfide-, and water-water complexes. The energy range of the emitted ICD electrons, as well as the kinetic energy of the dissociating ions produced by ICD, is also reported.

On the intermolecular Coulombic decay of singly and doubly ionized states of water dimer

A semiquantitative study of the intermolecular Coulombic decay (ICD) of singly and doubly ionized water dimer has been carried out with the help of ab initio computed ionization spectra and potential energy curves (PECs). These PECs are particular cuts through the (H(2)O)(2), (H(2)O)(2) (+), and (H(2)O)(2) (++) hypersurfaces along the distance between the two oxygen atoms. A comparison with the recently published experimental data for the ICD in singly ionized water dimers [T. Jahnke, H. Sann, T. Havermeier et al., Nat. Phys. 6, 139 (2010)] and in large water clusters [M. Mucke, M. Braune, S. Barth et al., Nat. Phys. 6, 143 (2010)] shows that such a simplified description in which the internal degrees of freedom of the water molecules are frozen gives surprisingly useful results. Other possible decay channels of the singly ionized water dimer are also investigated and the influence of the H-atom participating in the hydrogen bond on the spectra of the proton-donor and proton-acceptor molecules in the dimer is discussed. Importantly, the decay processes of one-site dicationic states of water dimer are discussed and an estimate of the ICD-electron spectra is made. More than 33% of the dications produced by Auger decay are found to undergo ICD. The qualitative results show that the ICD following Auger decay in water is also expected to be an additional source of low-energy electrons proven to be extremely important for causing damages to living tissues.

A hitherto unrecognized source of low-energy electrons in water

Most of the low-energy electrons emitted from a material when it is subjected to ionization radiation are believed to be directly ionized secondary electrons. Coincidence measurements of the electrons ejected from water clusters suggests many are produced by a quantitatively new mechanism, known as intermolecular Coulombic decay. Low-energy electrons are the most abundant product of ionizing radiation in condensed matter. The origin of these electrons is most commonly understood to be secondary electrons1 ionized from core or valence levels by incident radiation and slowed by multiple inelastic scattering events. Here, we investigate the production of low-energy electrons in amorphous medium-sized water clusters, which simulate water molecules in an aqueous environment. We identify a hitherto unrecognized extra source of low-energy electrons produced by a non-local autoionization process called intermolecular coulombic decay2 (ICD). The unequivocal signature of this process is observed in coincidence measurements of low-energy electrons and photoelectrons generated from inner-valence states with vacuum-ultraviolet light. As ICD is expected to take place universally in weakly bound aggregates containing light atoms between carbon and neon in the periodic table2,3, these results could have implications for our understanding of ionization damage in living tissues.

Ultrafast energy transfer between water molecules

Analysis of the electrons ionized from water dimers suggests that the energy absorbed by one molecule is rapidly transmitted to the second molecule from which the electron is ejected. This process, referred to as intermolecular Coulombic decay, is a qualitatively different source of low-energy electrons to conventional direct ionization processes. At the transition from the gas to the liquid phase of water, a wealth of new phenomena emerge, which are absent for isolated H2O molecules. Many of those are important for the existence of life, for astrophysics and atmospheric science. In particular, the response to electronic excitation changes completely as more degrees of freedom become available. Here we report the direct observation of an ultrafast transfer of energy across the hydrogen bridge in (H2O)2 (a so-called water dimer). This intermolecular coulombic decay leads to an ejection of a low-energy electron from the molecular neighbour of the initially excited molecule. We observe that this decay is faster than the proton transfer that is usually a prominent pathway in the case of electronic excitation of small water clusters and leads to dissociation of the water dimer into two H2O+ ions. As electrons of low energy (∼0.7–20 eV) have recently been found to efficiently break-up DNA constituents1,2, the observed decay channel might contribute as a source of electrons that can cause radiation damage in biological matter.

Vacuum-UV negative photoion spectroscopy of CF3Cl, CF3Br, and CF3I

Using synchrotron radiation, negative ions are detected by mass spectrometry following vacuum-UV photoexcitation of trifluorochloromethane (CF(3)Cl), trifluorobromomethane (CF(3)Br), and trifluoroiodomethane (CF(3)I). The anions F(-), X(-), F(2)(-), FX(-), CF(-), CF(2)(-), and CF(3)(-) are observed from all three molecules, where X = Cl, Br, or I, and their ion yields recorded in the range of 8-35 eV. With the exception of Br(-) and I(-), the anions observed show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Dissociative electron attachment, following photoionization of CF(3)Br and CF(3)I as the source of low-energy electrons, is shown to dominate the observed Br(-) and I(-) signals, respectively. Cross sections for ion-pair formation are put onto an absolute scale by calibrating the signal strengths with those of F(-) from both SF(6) and CF(4). These anion cross sections are normalized to vacuum-UV absorption cross sections, where available, and the resulting quantum yields are reported. Anion appearance energies are used to calculate upper limits to 298 K bond dissociation energies for D(o)(CF(3)-X), which are consistent with literature values. We report new data for D(o)(CF(2)I(+)-F) < or = 2.7+/-0.2 eV and Delta(f)H(o)(298)(CF(2)I(+)) < or = (598+/-22) kJ mol(-1). No ion-pair formation is observed below the ionization energy of the parent molecule for CF(3)Cl and CF(3)Br, and only weak signals (in both I(-) and F(-)) are detected for CF(3)I. These observations suggest that neutral photodissociation is the dominant exit channel to Rydberg state photoexcitation at these lower energies.

Vacuum-UV negative photoion spectroscopy of SF5CF3

Ion pair formation, generically described as AB-->A(+)+B(-), from vacuum-UV photoexcitation of trifluoromethyl sulfur pentafluoride, SF(5)CF(3), has been studied by anion mass spectrometry using synchrotron radiation in the photon energy range of 10-35 eV. The anions F(-), F(2)(-), and SF(x)(-) (x=1-5) are observed. With the exception of SF(5)(-), the anions observed show a linear dependence of signal with pressure, showing that they arise from ion pair formation. SF(5)(-) arises from dissociative electron attachment, following photoionization of SF(5)CF(3) as the source of low-energy electrons. Cross sections for anion production are put on to an absolute scale by calibration of the signal strengths with those of F(-) from both SF(6) and CF(4). Quantum yields for anion production from SF(5)CF(3), spanning the range of 10(-7)-10(-4), are obtained using vacuum-UV absorption cross sections. Unlike SF(6) and CF(4), the quantum yield for F(-) production from SF(5)CF(3) increases above the onset of photoionization.

Scanning Tunneling Microscope Excited Cathodoluminescence from ZnS Nanowires

The electronic excitation spectrum of ZnS nanowires (NWs) is characterized by cathodoluminescence (CL) excited using the tip of a scanning tunneling microscope as a highly localized and bright source of low-energy (150-350 eV) electrons with impinging power densities in the range of up to 60 kW/cm(2) or more. The CL spectra reveal significant differences when compared with the photoluminescence spectra of ZnS NWs. The differences can be associated with the properties of the surface region of the NWs, which are preferentially emphasized in the CL spectra owing to the small probing depth of low-energy electrons.

Motion of an Electron from a Point Source in Parallel Electric and Magnetic Fields

Negative ions undergoing near-threshold photodetachment in a weak laser field provide an almost pointlike, isotropic source of low-energy electrons. External fields exert forces on the emitted coherent electron wave and direct its motion. Here, we examine the spatial distribution of photodetached electrons in uniform, parallel electric and magnetic fields. The interplay of the electric and magnetic forces leads to a surprising intricate shape of the refracted electron wave, and multiple interfering trajectories generate complex fringe patterns in the matter wave. The exact quantum solution is best understood in terms of the classical electron motion.

Proteomics-Grade de Novo Sequencing Approach

The conventional approach in modern proteomics to identify proteins from limited information provided by molecular and fragment masses of their enzymatic degradation products carries an inherent risk of both false positive and false negative identifications. For reliable identification of even known proteins, complete de novo sequencing of their peptides is desired. The main problems of conventional sequencing based on tandem mass spectrometry are incomplete backbone fragmentation and the frequent overlap of fragment masses. In this work, the first proteomics-grade de novo approach is presented, where the above problems are alleviated by the use of complementary fragmentation techniques CAD and ECD. Implementation of a high-current, large-area dispenser cathode as a source of low-energy electrons provided efficient ECD of doubly charged peptides, the most abundant species (65-80%), in a typical trypsin-based proteomics experiment. A new linear de novo algorithm is developed combining efficiency and speed, processing on a conventional 3 GHz PC, 1000 MS/MS data sets in 60 s. More than 6% of all MS/MS data for doubly charged peptides yielded complete sequences, and another 13% gave nearly complete sequences with a maximum gap of two amino acid residues. These figures are comparable with the typical success rates (5-15%) of database identification. For peptides reliably found in the database (Mowse score > or = 34), the agreement with de novo-derived full sequences was >95%. Full sequences were derived in 67% of the cases when full sequence information was present in MS/MS spectra. Thus the new de novo sequencing approach reached the same level of efficiency and reliability as conventional database-identification strategies.

A porous silicon diode as a source of low-energy free electrons at milli-Kelvin temperatures

We have developed a porous silicon (PS) diode that yields free-electron currents with energies <0.1eV below 77 K. The power dissipated during emission is low so that pulses of electrons can be produced below 100 mK without raising the temperature of the system. Free electrons were generated in liquid He4 and He3 as well. The device was developed as a source of electrons for a quantum computing system using electrons on the surface of a dielectric film. The results suggest that a Poole-Frenkel type of mechanism accounts for the observed electric-field-enhanced conduction but the electron emission mechanism is not well understood in the present models of PS.

A maximum likelihood method for linking particle-in-cell and Monte-Carlo transport simulations

The expectation-maximization (E-M) algorithm [Dempster et al., J. R. Stat. Soc. B 39 (1977) 1–38] is a maximum likelihood technique to estimate the probability density function (PDF) of a set of measurements. A high performance implementation of the E-M algorithm to characterize multidimensional data sets using a PDF parameterized as a Gaussian mixture was developed. The resulting PDFs compare favorably to histogram based techniques—no binning artifacts and less noisy (especially in the tails). The motivation, the mathematical properties and the implementation details will be discussed. The PDF estimator is used extensively in the radiographic chain model [Kwan et al., Comput. Phys. Comm. 142 (2001) 263–269] in simulations which quantify bremsstrahlung X-ray emission from rod-pinch diodes and other devices. In these devices, electrons hit an anode and produce X-ray photons. The PIC code MERLIN [Kwan and Snell, in: Lecture Notes in Physics, Springer, 1985] is used to model the dynamics of a low-energy (up to ∼2.25 MeV) radiographic electron source. The photon production is modeled with the Monte-Carlo transport code MCNP [Briesmeister, ed., MCNP—A General Monte Carlo N-Particle Transport Code, 2000]. The estimator is used to upsample and uniformly weight the PIC electrons to provide a suitable population for the Monte-Carlo calculation that would be computationally prohibitive to generate directly.

An inverse photoemission system with large solid angle of detection and adjustable optical bandpass

A high-brightness, low energy electron source and dual Geiger–Müller-type isochromat photon detectors are combined to create a versatile new inverse photoemission system. The bandpass of the photon detector can be set to one of the following discrete values: 0.37±0.02, 0.43±0.02, 0.56±0.02, or 0.73±0.04 eV by using ethanol, 1-propanol, 1-butanol, or a dimethyl ether/ethanol mixture, respectively, as the detection gas(es). All of the alcohols are self-quenching and do not require the addition of a quench gas. The design of the photon detectors, the electron gun, and the circuits that perform the dead time gating are described in detail. The capabilities of the new system are illustrated using spectra from both metal (Cu) and semiconductor (Si) surfaces.

Excitation of two interacting electrons as a plasmon-decay mechanism in proton-aluminum collisions

Projectile-induced plasmon excitation in an electron gas has been studied by several authors who proposed two possible mechanisms for these plasmons to decay. In a previous work we considered one of these mechanisms in which the plasmon transfers its energy to a nearly free electron that makes an interband transition. In this paper, the other mechanism is analyzed. A simple model is developed to describe plasmon decay in aluminum via the excitation of two interacting electrons. Results for the transition probability and the excitation power are presented. When contributions from both mechanisms are considered, they account for more than 60% of the excited plasmons. Also, the slope of the plasmon excitation curve is correctly reproduced. The study of first and second differential spectra in angle and energy show that plasmon decay into two interacting electrons is the main source of low-energy electrons moving in the forward direction (with respect to the projectile initial velocity).

Surface area overestimation within three-dimensional digital images and its consequence for skeletal dosimetry.

The most recent methods for trabecular bone dosimetry are based on Monte Carlo transport simulations within three-dimensional (3D) images of real human bone samples. Nuclear magnetic resonance and micro-computed tomography have been commonly used as imaging tools for studying trabecular microstructure. In order to evaluate the accuracy of these techniques for radiation dosimetry, a previous study was conducted that showed an overestimate in the absorbed fraction of energy for low-energy electrons emitted within the marrow space and irradiating the bone trabeculae. This problem was found to be related to an overestimate of the surface area of the true bone-marrow interface within the 3D digital images, and was identified as the surface-area effect. The goal of the present study is to better understand how this surface-area effect occurs in the case of single spheres representing individual marrow cavities within trabecular bone. First, a theoretical study was conducted which showed that voxelization of the spherical marrow cavity results in a 50% overestimation of the spherical surface area. Moreover, this overestimation cannot be reduced through a reduction in the voxel size (e.g., improved image resolution). Second, a series of single-sphere marrow cavity models was created with electron sources simulated within the sphere (marrow source) and outside the sphere (bone trabeculae source). The series of single-sphere models was then voxelized to represent 3D digital images of varying resolution. Transport calculations were made for both marrow and bone electron sources within these simulated images. The study showed that for low-energy electrons (<100 keV), the 50% overestimate of the bone-marrow interface surface area can lead to a 50% overestimate of the cross-absorbed fraction. It is concluded that while improved resolution will not reduce the surface area effects found within 3D image-based transport models, a tenfold improvement in current image resolution would compensate the associated errors in cross-region absorbed fractions for low-energy electron sources. Alternatively, other methods of defining the bone-marrow interface, such as with a polygonal isosurface, would provide improvements in dosimetry without the need for drastic reductions in image voxel size.

Local photon emission of self-assembled metal nanoparticles

The tip of a scanning tunneling microscope (STM) may be used as an extremely localized source of low-energy electrons to locally excite photon emission from a variety of metal films. The detection of locally excited luminescence at the junction of a STM tip provides access to electron dynamic properties at the surface, which makes it possible to study luminescence phenomena of nanometer-sized structures. We analyzed maps of the photon intensity emitted from electrically isolated silver nanoparticles self-organized as a 2D network on a gold (111) substrate. We observed unexpectedly strong variations of photon-emission efficiency from isolated nanoparticles depending on how tightly they are embedded within the network site. The quenching site observed in the STM photon emission map is interpreted as an enhanced interaction of electrons with surface phonon modes.

Fabrication and physical properties of self-organized silver nanocrystals

Abstract A simple method is used to prepare highly monodispersed silver nanoparticles in the liquid phase, which starts from an initial synthesis in functionalized AOT reverse micelles. To narrow the particle size distribution from 43 to 12.5% in dispersion, the particles are extracted from the micellar solution. The size-selected precipitation method is used. The nanocrystallites dispersed in hexane are deposited on a support. A monolayer made of nanoparticles with spontaneous compact hexagonal organization is observed. The immersion of the support on the solution yields to the formation of organized multilayers arranged as microcrystal in a face-centered-cubic structure. We compare the optical properties of spherical particles organized in a two- and three-dimensional structure with isolated and disordered particles. When particles, deposited on cleaved graphite, are arranged in a hexagonal array, the optical measurements under p-polarization show a new high-energy resonance, which is interpreted as a collective effect, resulting from optical anisotropy due to the mutual interactions between particles. We support this interpretation by numerical calculations performed for finite-size clusters of silver spheres. For disordered particles, a low-energy resonance appears instead of the high-energy resonance observed for spherical and organized particles. This is interpreted as optical shape anisotropy due to the asymmetrical arrangement of particles. The tip of a scanning tunneling microscope (STM) may be used as an extremely localized source of low-energy electrons to locally excite photon emission from a variety of metal films. The detection of locally excited luminescence at the junction of an STM tip provides access to electron dynamic properties at the surface, which makes it possible to study luminescence phenomena of nanometer-sized structures. The photon intensity emitted from electrically isolated silver nanoparticles self-organized as a 2D network on a gold (111) substrate is analyzed. We observed unexpectedly strong variations of photon-emission efficiency from isolated nanoparticles, depending on how tightly they are embedded within the network site. The quenching site observed in the STM photon emission map is interpreted as an enhanced interaction of electrons with surface photon modes.

A low stray light, high current, low energy electron source

A design of an electron gun system is presented whose stray light emission is reduced by about three orders of magnitude compared to a regular low-energy electron diffraction gun. This is achieved by a combination of a BaO cathode run at rather low temperature and a 30° tandem parallel-plate analyzer used as an optical baffle. The system provides a high beam current of several microampers at 50 eV beam energy. The system can be used down to ∼10 eV.

Electron Attachment Mass Spectrometry for the Detection of Electronegative Species in a Plasma

Electron attachment mass spectrometry (EAMS) has been implemented to detect electronegative species in a low pressure 13.56 MHz discharge. For this purpose a source of low energy electrons has been used in combination with a quadrupole mass spectrometer (QMS) and the signal of the negative ions, resulting from electron attachment to neutrals has been recorded as a function of the electron energy. Chemically active fluorocarbon gases like CF4 and CHF3 have been studied. EAMS provides much insight into chemistry in the plasma, especially into mechanisms of negative ion formation. It enables the detection of electronegative species, formed from the parent gas under plasma conditions, based on their different attachment cross sections. Moreover, the effective negative ion formation cross section in a plasma, taking into account the chemical conversion of the feed gas, can be determined. In fluorocarbon plasmas various species are formed, like C2F6 and C3F8 in a CF4 plasma and CF4 and C2F6 in a CHF3 plasma, which significantly influence the negative ion production mechanisms under discharge conditions. Because the active neutrals produced in the plasma have typically both a larger attachment cross section and a lower attachment energy threshold, negative ion formation is dominated by the plasma species and not by the parent gas.

Low-energy electron attachment to excited nitric oxide

Dissociative attachment of low energy electrons to the A2Σ+ state of nitric oxide has been measured quantitatively, and the cross section for this process has been determined. A magnetically collimated electron gun has been used as a controlled source of low energy electrons, and the mass analyzed detection system has been calibrated using the known dissociative attachment cross section of CCl4. The cross section for the reaction NO*(A2Σ+)+e−(0.5 eV)→N(4S)+O−(2P) is measured to be (2.0±0.5)×10−15 cm2 and is found to be independent of rotational quantum number within the statistical uncertainty.