Electron kinetic energies from vibrationally promoted surface exoemission: evidence for a vibrational autodetachment mechanism

We report kinetic energy distributions of exoelectrons produced by collisions of highly vibrationally excited NO molecules with a low work function Cs dosed Au(111) surface. These measurements show that energy dissipation pathways involving nonadiabatic conversion of vibrational energy to electronic energy can result in electronic excitation of more than 3 eV, consistent with the available vibrational energy. We measured the dependence of the electron energy distributions on the translational and vibrational energy of the incident NO and find a clear positive correlation between final electron kinetic energy and initial vibrational excitation and a weak but observable inverse dependence of electron kinetic energy on initial translational energy. These observations are consistent with a vibrational autodetachment mechanism, where an electron is transferred to NO near its outer vibrational turning point and ejected near its inner vibrational turning point. Within the context of this model, we estimate the NO-to-surface distance for electron transfer.     

Many-electron problems. I. Energy relations in the theory of neutral atoms

Many-electron atom calculations are analyzed. It is proved that for neutral atoms to a good approximation the electron–electron interaction energy is a constant multiple of the electron–nucleus interaction energy. A consequence of the above theorem is that the total energy of these atoms shows a very simple dependence on the atomic number.     

推/拉电子取代基对PCBM结构及光谱性质影响的理论研究

应用密度泛函理论(DFT)方法计算[6,6]-苯基-C₆₁-丁酸甲酯(PCBM)及其苯环对位取代得到的4种衍生物的几何和电子结构. 采用第一激发能校正了分子的最低未占据分子轨道(LUMO)能级, 探讨了推/拉电子基团对分子前线轨道的影响. 在全优化几何构型的基础上, 采用含时密度泛函理论(TD-DFT)方法研究了电子吸收光谱特征和电荷转移态性质, 并讨论了推/拉电子基团对体系电子吸收光谱性质的影响. 通过对重组能和电子亲和势的计算, 预测了PCBM与4种衍生物的电子能力及电子迁移率大小的关系. 结果表明, 在PCBM中, 在苯环的对位引入推电子基团可以提高分子的前线轨道能级, 改变前线轨道电子云分布, 明显增强可见光范围内的吸收强度, 增加可见光范围内的电荷转移吸收, 且激发态的电荷转移随着引入基团推电子能力的增加而增强. 化合物5的激发态分子内电荷转移性质最强, 且具有较独特的光伏性质. 而在同样位置引入拉电子基团, 则降低了分子前线轨道能级对电子吸收光谱的影响.     

Fundamental properties characterizing low-pressure microwave-induced plasmas as excitation sources for spectroanalytical chemistry

Experimental measurements of the spectroscopic temperature and the electron temperature in low-pressure rare gas plasmas sustained by a microwave generator operating at 2450 MHz have revealed divergent values. These measurements have been interpreted on the basis of a radiative recombination model originally proposed by Schlüter. The importance of Penning ionization by metastable rare gas atoms in the excitation of foreign atoms has been discussed in terms of this model.On the basis of the radiative recombination model for these plasmas, the parameters of analytical importance are the concentration and energy of electrons in a high energy electron group, the concentration and energy of electrons in a low energy electron group, and the concentration of metastable rare gas atoms. Measurements of the spectroscopic temperature of an argon plasma have revealed that the energy of electrons in the low energy electron group is not greatly affected by applied microwave power and pressure over the range from 1–25 torr. The energy of electrons in the high energy electron group is not greatly affected by pressure and applied microwave power over the range studied, but has been shown to depend on the ionization potential of the plasma gas. The total electron concentration is not greatly affected by gas pressure for low applied powers, but varies with applied power, particularly at low pressures. The concentration of metastable argon atoms has been shown to depend on both the applied power and pressure. Studies of the excitation of mercury by these plasmas have led to results which are consistent with the radiative recombination model.     

卟啉-酞菁分子内能量传递和电子转移的溶剂效应

用稳态荧光光谱研究了以氧原子和哌嗪作为连接基的卟啉酞菁二元分子在不同溶剂中的分子内能量传递和电子转移过程结果表明;分子内的能量传递和电子转移是两个相互竞争的过程，在非极性溶剂中，激发单重态的能量传递是主要过程，而在极性溶剂中则以电子转移为主运用Rehm-Weller公式计算了两种二元化合物在不同溶剂中的电子转移反应的自由能变化△G0ET，表明溶剂的极性对电子转移反应的自由能变化△G0ET影响很大极性越大;体系中的电子转移反应的△G0ET、越负，电子转移反应越易进行由于电子转移过程较能量传递过程进行得快，所以表现为体系中能量传递效率降低而电子转移效率增大。两种二元化合物的能量传递效率（φEnT）利和电子转移效率（φET）随溶剂的极性的变化具有相同的变化趋势     

Electron Transfer in Frozen Media

Classical theories of electron transfer are modified to take into account the differences between electron transfer in a rigid medium and in a fluid. Intramolecular vibrations and part of the dielectric polarization are assumed to remain dynamic in rigid media while the remaining part of the polarization, arising from dipole reorientations, is frozen. In rigid media, electron transfer occurs with the solvent locked into the dipole orientations of the initial state. This causes an increase in the free energy change and a decrease in the solvent reorganizational energy. It also increases the activation free energy for electron transfer. For photoinduced electron transfer, the analysis is more complex because multiple states are involved. The activation free energy can either be greater or less than in a fluid depending on charge distributions before and after electron transfer. The same analysis can be applied to interconversion between excited states in rigid media.     

Materials and structures for the electron transport layer of efficient and stable perovskite solar cells

The electron transport layer plays a vital function in extracting and transporting photogenerated electrons, modifying the interface, aligning the interfacial energy level and minimizing the charge recombination in perovskite solar cells. This review summarizes the recent research progress on electron transport materials of metal oxides, organic molecules and multilayers. The doped metal oxides as electron transport materials in regular perovskite solar cells show improved device performance relative to their non-doped counterpart due to enhanced electron mobility and energy level alignment. The non-fullerene organic electron transport materials with better electron mobility and tunable energy level alignment need to be further designed and developed despite their advantages of mechanical flexibility and wide range tunability. The multilayer electron transport materials are suggested to be an important direction of research for efficient and stable perovskite solar cells because of their favorable synergistic interaction.     

A parallel acquisition charged particle energy analyser using a magnetic field

A new form of charged particle energy analyser is proposed. It is broadly based on the 180° magnetic spectrograph, but is intended to detect charged particles moving out of the dispersion plane with a helical motion. The analyser has the capability to acquire charged particle energy spectra over a large energy range, similar to those acquired in Auger electron spectroscopy, ca. 2500 eV and large angular range, up to 90°, in parallel. These conditions are more favourable for surface analysis by electron spectroscopy at high vacuum, where for example an electron energy resolution of 0.2% to 0.5% is typical. Expressions showing how the landing positions of the charged particles on the detector vary as a function of energy and polar take off angle are determined as well as the conditions for optimum energy resolution at a range of polar take off angles. The equations reveal that in general, the device obtains the highest resolution at angles of revolution greater than 180°. The design is simple and could be easily put into practice using available material and technologies and be used to analyse the energies of electrons emitted from a sample placed in a scanning electron microscope. It can be made to function with a primary electron beam of any desired energy and could fit in to the small space between the sample and the end of an electron column. However, the device is difficult to retrofit into existing SEMs and ideally an SEM column needs to be designed to work in association with the analyser. The direction of the magnetic field of the analyser is coincident with the axis of the electron gun so that the primary beam is little influenced by the magnetic field and symmetry can be maintained in the primary beam electron column. Because the device is intended to acquire electron spectra in parallel, any movement of the primary beam on the sample because of a ramping field in the analyser is avoided. The field of view and the effect of the analyser upon the operation of the SEM are discussed. Spectra including elastic and Auger peaks reveal an energy resolution of ~4 eV at 900‐eV electron energy. Copyright © 2016 John Wiley & Sons, Ltd.     

Investigation of dopant effect on some TL dosimeters containing boron

Effective atomic numbers, effective electron densities for photon mass energy absorption and kerma values of soft tissue and some thermoluminescence dosimeter containing boron are calculated in the energy range from 1 keV to 20 MeV. It is investigated that the variation of effective atomic numbers, effective electron densities and kerma with energy. The TL dosimeters studied are compared with the soft tissue for calculated values. In addition, dopant effect on the effective atomic numbers and electron densities for photon mass energy absorption and kerma values of the TL dosimeter containing boron is presented. Effect of the concentrations of the element H on effective electron density of the soft tissue is discussed.     

Electron and positron pair emission by low energy positron impact on surfaces

The emission of electron pairs from surfaces has the power to reveal details about the electron–electron interaction in condensed matter. This process, stimulated by a primary electron or photon beam, has been studied both in experiment and theory over the last two decades. An additional pathway, namely positron–electron pair emission, holds the promise to provide additional information. It is based on the notion that the Pauli exclusion principle does not need to be considered for this process.We have commissioned a laboratory based positron source and performed a systematic study on a variety of solid surfaces. In a symmetric emission geometry we can explore the fact that positron and electron are distinguishable particles. Following fundamental symmetry arguments we have to expect that the available energy is shared unequally among positron and electron. Experimentally we observe such a behavior for all materials studied. We find an universal feature for all materials in the sense that on average the positron carries a larger fraction of the available energy. This is qualitatively accounted for by a simplified scattering model. Numerical results, which we obtained by a microscopic theory of positron–electron emission from surfaces, reveal however that there are also cases in which the electron carries more energy. Whether the positron or the electron is more energetic depends on details of the bound electron state and of the emission geometry. The coincidence intensity is strongly material dependent and there exists an almost monotonic relation between the singles and coincidence intensity. These results resemble the findings obtained in electron and photon stimulated electron pair emission. An additional reaction channel is the emission of an electron pair upon positron impact. We will discuss the energy distributions and the material dependence of the coincidence signal which shows similar features as those for positron–electron pairs.     

Density functional theory studies of electron interaction with DNA: can zero eV electrons induce strand breaks?

The discovery of DNA strand breaks induced by low energy secondary electrons sparks a necessity to elucidate the mechanism. Through theoretical studies based on a sugar-phosphate-sugar model that mimics a backbone section of the DNA strand, it is found that bond cleavages at 3' or 5'C-O sites after addition of an electron are possible with a ca. 10 kcal/mol activation barrier. Moreover, the potential energy surfaces show that dissociation at both sites is highly favorable thermodynamically. Although the phosphate group in DNA is not a favored site for electron attachment because of competitive electron transfer to the bases, any electrons which attach to phosphates on first encounter may induce strand breaks even when the electron energy is near zero eV. These findings have profound implication as low energy secondary electrons are abundantly generated in all types of ionization radiation.     

Optimization of electron beam crosslinking of wire and cable insulation

The computer simulations based on Monte Carlo (MC) method and the ModeCEB software were carried out in connection with electron beam (EB) radiation set-up for crosslinking of electric wire and cable insulation. The theoretical predictions for absorbed dose distribution in irradiated electric insulation induced by scanned EB were compared to the experimental results of irradiation that was carried out in the experimental set-up based on ILU 6 electron accelerator with electron energy 0.5–2.0 MeV.The computer simulation of the dose distributions in two-sided irradiation system by a scanned electron beam in multilayer circular objects was performed for various process parameters, namely electric wire and cable geometry (thickness of insulation layers and copper wire diameter), type of polymer insulation, electron energy, energy spread and geometry of electron beam, electric wire and cable layout in irradiation zone. The geometry of electron beam distribution in the irradiation zone was measured using CTA and PVC foil dosimeters for available electron energy range. The temperature rise of the irradiated electric wire and irradiation homogeneity were evaluated for different experimental conditions to optimize technological process parameters. The results of computer simulation are consistent with the experimental data of dose distribution evaluated by gel-fraction measurements. Such conformity indicates that ModeCEB computer simulation is reliable and sufficient for optimization absorbed dose distribution in the multi-layer circular objects irradiated with scanned electron beams.     

Femtosecond diffraction with chirped electron pulses

Here, we report on the possible achievement, in ultrafast electron diffraction and imaging, of temporal resolution of tens of femtoseconds through the use of chirped electron packets in combination with energy filtering. Space–charge forces in multi-electron packets accelerate leading electrons and retard trailing ones, thus inducing correlations of momentum and time. By resolving the diffraction images with an energy analyzer, well-defined temporal slices of the long electron packet can be selected. Numerical simulations show that conventional electron sources are sufficient to reach the 30-fs domain of resolution without electron packet compression. They also reveal the influence of packet shape, electron density and photoemission bandwidth on the achievable time resolution.     

New developments in theory of fast electron scattering in solids: Applications to microbeam analysis

The study of fast electron interaction with solids in the energy range from 100 eV to several tens of keV is prompted by quickly developing microbeam analysis techniques such as electron probe microanalysis, scanning electron microscopy, electron energy loss spectroscopy and so on. It turned out that for random solids the electron transport problem might be solved on the basis of the generalized radiative field similarity principle. The latter states that the exact differential elastic cross section in the kinetic equation may be replaced by an approximate one provided the conditions of radiative field similarity are fulfilled. Application of the generalized similarity principle to electron scattering in solids has revealed many interesting features of electron transport. Easy to use and effective formulae have been obtained for the angular and energy distribution of electrons leaving a target, total yields of characteristic photons and slow electrons escaping from a sample bombarded by fast primaries, escape probability of Auger electrons as a function of depth etc. The analytical results have been compared with Monte Carlo calculations and experiments in a broad range of electron energies and scattering properties of solids and good agreement has been observed.     

Mass spectrometric measurement of dissociation energies of the PbS,PbSe and PbTe diatomic molecules and determination of the lead electron affinity

From the appearance potential measurements by mass spectrometry and the kinetic energy measurements of the ions produced by electron dissociative attachment and ion-pair formation, during electron impact on PbS, PbSe and PbTe, we determine the dissociation energy of these molecules and the Pb electron affinity. Our values agree with those measured by other techniques.     

Electron kinetics of weakly ionized collision-dominated RF plasmas in CO

Based on the nonstationary electron Boltzmann equation this paper deals with the time-resolved electron kinetics in the rf plasma in CO, i.e., with the calculation of the temporal evolution of the energy distribution and of the resultant macroscopic quantities for the established steady state. A particular aspect of this plasma is the distinctly resonance-like behavior of the vibrational excitation of the CO molecules by electron collisions. This causes the lumped frequencies for energy and impulse dissipation in collisions, recently introduced in the study of the rf kinetics in Ne and H₂, to become extremely dependent on the electron energy. Despite this fact, it could be verified that the field frequency dependence of the temporal evolution of the electron kinetics can be interpreted by means of these two dissipation frequencies even under such complicated conditions as given by the atomic data in CO.     

Absolute partial cross sections and kinetic energy analysis for the electron impact ionization of ethylene

We present absolute partial electron impact ionization cross sections for ethylene in the electron energy range between threshold and 1000 eV measured with a two sector field double focusing mass spectrometer. Ion kinetic energy distribution functions have been measured at all electron energies by applying a deflection field method. Multiplication of the measured relative cross sections by the appropriately determined discrimination factors lead to accurate relative partial cross sections. Normalization of the sum of the relative partial cross sections to an absolute total cross section gives absolute partial cross section values. The initial kinetic energy distributions of several fragment ions show the presence of two or more contributions that exhibit different electron energy dependencies. Differential cross sections with respect to the initial kinetic energy of the ions are provided and are related to specific ion production channels. The electron threshold energies for the direct and numerous other dissociative ionization channels are determined by quantum chemical calculation and these allow the determination of the total kinetic energy release and the electron energy loss for the most prominent dissociative ionization channels.     

Identification of conjugate electron transitions in X-ray photoelectron spectra

The regularities of the electron energy dissipation found in the subsurface atomic layer are valid in the bulk of a solid, too. On the example of model graphite-based materials it is shown that energy losses in X-ray photoelectron spectra agree with the calculated valence electron excitation spectra in analogous unit cells. The control of conjugate electron transitions opens the way to gain new data on the geometry, character, and order of bonding between atoms in the sample by the conventional electron spectroscopy and quantum chemistry methods.     

Vibrational effects on the electron momentum distributions of valence orbitals of formamide

The ionization energy spectra and electron momentum distributions of formamide were investigated using the high-resolution electron momentum spectrometer in combination with high level calculations. The observed ionization energy spectra and electron momentum distributions were interpreted using symmetry adapted cluster-configuration interaction theory, outer valence Green function, and DFT-B3LYP methods. The ordering of 10a(') and 2a(") orbitals of formamide was assigned unambiguously by comparing the experimental electron momentum distributions with the corresponding theoretical results, i.e., 10a(') has a lower binding energy. In addition, it was found that the low-frequency wagging vibration of the amino group at room temperature has noticeable effects on the electron momentum distributions. The equilibrium-nuclear-positions-approximation, which was widely used in electron momentum spectroscopy, is not accurate for formamide molecule. The calculations based on the thermal average can evidently improve the agreement with the experimental momentum distributions.