Determination of electron affinity of electron accepting molecules

The unoccupied π ^* states of the solid film of electron accepting organic molecules, 7,7,8,8-tetracyanoquinodimethane (TCNQ), fluorinated TCNQ derivatives, 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TNAP), C₆₀, and 6,6-phenyl-C₆₁-butyric acid methyl ester (PCBM) have been studied by inverse photoemission spectroscopy. The assignment of the π ^* affinity levels of these typical electron accepting molecules provides the basic information for the organic electronics and the new electronic functional molecular design. The comparison with density functional theory calculations enables understanding how the electron affinity evolves in terms of molecular orbitals. The correlation between the film morphology and the irradiation damage on the TCNQ derivative samples by electron impact during the inverse photoemission measurements is also discussed.     

Measurement of the electron affinity of cerium

The electron affinity of cerium has been measured using laser photodetachment electron spectroscopy. The electron affinity of Ce(1G4) was determined to be 0.955 +/- 0.026 eV. The data also show that Ce- has at least two bound excited states with binding energies of 0.921 +/- 0.025 eV and 0.819 +/- 0.027 eV relative to the (1G4) ground state of the cerium atom. The present experimental measurements are compared to recent calculations of the energy levels of Ce-. Strong disagreement with the most recent theoretical atomic structure calculations highlights the complicated nature of this particular lanthanide.     

负电子亲和势GaN光电阴极的研究进展

GaN材料由于其优良的性能,成为紫外探测和真空电子源领域极具发展潜力的材料之一;目前制备的反射式GaN光电阴极的量子效率已达到70%以上,透射式也达到了30%.本文对GaN光电阴极的结构设计、表面清洗和Cs/O激活三大方面进行了综述,分析了影响量子效率的关键因素,并对今后可能的发展方向进行了展望. 关键词： GaN光电阴极 负电子亲和势 量子效率 进展     

反射式负电子亲和势GaN光电阴极的光电发射及稳定性研究

利用在线多信息紫外光电阴极激活评估系统,测试了真空室内两个GaN 光电阴极Cs,O激活后及衰减6 h和18 h后补Cs的光谱响应特性曲线和量子效率曲线;并绘制了光纤光源波长为300 nm的光电阴极响应电流衰减变化曲线.实验结果证明,GaN 光电阴极较GaAs阴极具有更好的稳定性,量子效率可保持相对稳定达10 h,然后缓慢衰减,衰减速率较窄禁带半导体材料低得多.补Cs后光电流最大值较刚激活完有16.8%的增长,这充分证明阴极表面量子效率衰减的原因是Cs的脱附,而不是O的吸附.这些现象可由双偶极层模型来解释, 关键词： 光学 光电阴极 量子效率 稳定性     

Accurate calculation of electron affinity for S_3

The accurate equilibrium structures of S_3 and S_3~- are determined by the coupled-cluster method with single, double excitation and perturbative triple excitation(CCSD(T)) with basis sets of aug-cc-pV(n+d)Z(n = T, Q, 5, or 6), complete basis set extrapolation functions with two-parameters and three-parameters, together with considering the contributions due to the core-valence electron correlation, scalar relativistic effects, spin–orbit coupling, and zero-point vibrational corrections. Our calculations show that both the neutral S_3 and anion S_3~- have open forms with C_(2r) vsymmetry. On the basis of the stable geometries, the adiabatic electron affinity of S_3 is determined to be 19041(11) cm~(-1), which is in excellent agreement with the experimental data(19059(7) cm~(-1)). The dependence of geometries and electron affinity on the computation level and physical corrections is discussed. The present computational results are helpful to the experimental molecular spectroscopy and bonding of S_3.     

负电子亲和势GaN光电阴极激活机理研究

利用自行研制的光电阴极激活评估实验系统,给出了GaN光电阴极Cs激活及Cs/O激活的光电流曲线.针对GaN光电阴极的负电子亲和势（NEA）特性的成因,结合激活过程中光电流变化规律和成功激活后的阴极表面模型,研究了NEA GaN光电阴极激活机理.实验表明：GaN光电阴极在单独导入Cs激活时就可获得明显的NEA特性,Cs/O激活时引入O后光电流的增长幅度不大.用双偶极层模型［GaN（Mg）：Cs］：O—Cs较好地解释了激活成功后GaN光电阴极NEA特性的成因. 关键词： 负电子亲和势 GaN 激活 光电流     

Evidence for electron emission stimulated desorption from negative electron affinity GaAs surfaces

While desorption from surfaces caused by impinging electrons (electron stimulated desorption) is a well-established effect, electrons to be emitted also may give rise to desorption from an emitting surface (electron-emission stimulated desorption). Evidence for this effect is derived from data on the degradation of electron emission from negative electron affinity GaAs surfaces. The time dependence of the degradation is calculated, and agreement with the observed linear time dependence is found. Using the experimental degradation ata, the desorption cross section for the electron-emission stimulated desorption is obtained as 2 × 10^?25 cm².     

Surface states and negative electron affinity in polyethylene

First-principles calculations are used to investigate the electronic properties of the surfaces of polyethylene. The calculations support the experimental evidence of a negative electron affinity, with calculated values of -0.17 eV and -0.10 eV for surfaces with chains perpendicular and parallel to the surface normal, respectively. Both surfaces exhibit a surface state with binding energy -1.2 +/- 0.5 eV with respect to the bulk polyethylene conduction band minimum. Implications of these findings on spectroscopy, as well as on the transport and aging properties of polyethylene for high-voltage applications, are discussed.     

Photoemission of spin-polarized electrons from negative electron affinity GaAsP

GaAs_0.62P_0.38 activated to negative electron affinity and irradiated with circularly polarized light of a wavelength shorter than approximately 680 nm emits electrons whose spins are mainly oriented antiparallel to the photon spins. At 650 nm a degree of electron spin polarization around 40% is observed. The spectrum of spin polarization of the photoelectrons is similar to that known from GaAs cathodes but is shifted to shorter wavelengths. 25 μA current of spin-polarized electrons were achieved with a 5 mW He-Ne laser.     

Experiment and theory of the elastic electron reflection coefficient from tungsten

The elastic reflection coefficient of 0–20 eV electrons incident normally on clean (110) and (001) tungsten surfaces has been measured accurately with a novel type of spectrometer. Corresponding calculations have been performed using various potential approximations. Agreement between theory and experiment is very good and clearly identifies surface-sensitive features.     

The electron affinity of Al13H cluster: high level ab initio study

Al₁₃H clusters have been considered candidates for cluster assembled materials. Here we have carried out benchmark calculations for the Al₁₃H cluster, both neutral and anionic, with the aim of verifying the nature of stationary points on the potential energy surface, studying dynamics of H atom and determining an adiabatic electron affinity. A range of correlated methods applied include second-order perturbation theory (MP2), spin-component-scaled MP2, coupled electron pair (CEPA) and coupled cluster singles and doubles with perturbative triple corrections (CCSD(T)). These methods are used in combination with the correlation consistent basis sets through aug-cc-pVTZ including extrapolation to the complete basis set (CBS) limit. Performance of several different flavours of density functional theory (DFT) such as generalised gradient approximation (GGA), hybrid GGA, meta-GGA and hybrid-meta-GGA is assessed with respect to the ab initio correlated reference data. The harmonic force constant analysis is systematically performed with the MP2 and DFT methods. The MP2 results show that for neutral Al₁₃H only the hollow structure is a potential energy minimum, with the bridged structure being a transition state for the H shift from the hollow site to the adjacent hollow site. The CCSD(T)/aug-cc-pVTZ (CCSD(T)/CBS) estimate of the energy barrier to this H shift is 2.6 (2.9) kcal/mol, implying that the H atom movement over the Al₁₃H cluster surface is facile. By contrast, the DFT force constant analysis results suggest additional terminal and bridged minima structures. For the anion Al₁₃H^?, exhibiting ‘stiffer’ potential energy surface compared to the neutral, the existence of the hollow and terminal isomers is consistent with the earlier photoelectron spectroscopy assignment. The adiabatic electron affinity of Al₁₃H is determined to be 2.00 and 1.95 eV (the latter including the ΔZPE correction) based on the CCSD(T) energies extrapolated to the CBS limit, whereas the respective CCSD(T)/CBS thermodynamic EA values are 2.79 and 2.80 eV.     

The effect of Be-doping structure in negative electron affinity GaAs photocathodes on integrated photosensitivity

A new structure of GaAs photocathode was introduced. The Be-doping concentration is variable in the new structure compared with the constant concentration of Be in the normal photocathode. Negative electron affinity GaAs photocathodes were fabricated by alternate input of Cs and O. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathodes with the new structure is enhanced by at least 50% as compared to those with the monolayer structure. Accordingly, two main factors leading to the enhanced photosensitivity of the photocathodes were discussed.     

Field electron emission study of nitrogen adsorbed on individual tungsten planes

The adsorption of nitrogen on 211, 111, 100 and 110 tungsten planes has been studied by means of the probe-hole emission technique over a wide range of temperatures. The field emission tube was attached to a molecular beam system. This technique enabled deposition of strictly controlled doses of nitrogen. It has been found that on the 211 plane three states of nitrogen γ, α and β exist. In the γ state molecules of opposite polarity are present. These correspond to the γ⁺ and γ^?. The α state undergoes transformation at about 300 K to a more stable β state. β nitrogen leaves the 211 plane through surface diffusion in the temperature region 600–700K. Results obtained on the 111 plane in the low temperature region confirm previous findings on the existence of γ and α states. At higher temperatures the concentration of nitrogen in the β state increases as a result of migration from the 211 plane. There is some evidence as to the existence of two high energetic states of nitrogen on the 111 and 100 planes. On the 110 plane only partial results were obtained due to field desorption.     

Fast electron energy loss spectrum of an amorphous tungsten trioxide film

The forward scattering energy loss spectrum of 300 KeV electrons transmitted through thin films of WO₃ has been measured in the energy range of 0.8–225 eV with an energy loss resolution of 0.1 eV. Energy losses due to excitation of color centers, inter band transitions and core elestrons in WO₃ have been identified. Measurement of the elastic electron difraction pattern revealed the amorphous nature of the film with structural differences depending upon the preparative methods.     

Study of photofield emission in tungsten by using free electron model

We present here the results of the calculations of photofield emission current by using the free electron model in which the appropriate wavefunctions are used. The transmission probability had been calculated by solving the Airy’s equation. The model developed is used to calculate photofield emission current from tungsten.     

Nonlocal density functional calculation of the electron affinity of atoms

The weighted-density approximation (WDA) to the exchange-correlation potential V_xc (r) is used to compute electron affinities of atoms within the context of density functional theory. Good results are obtained for the affinities of H, O and F, but it is critical that the WDA must be applied in such a way as to preserve the correct asymptotic behaviour of V_xc (r).