ASYMMETRY PARAMETERS AND DIFFERENTIAL CROSS SECTIONS FOR PHOTOIONIZATION-EXCITATION OF HELIUM

Asymmetry parameter β₂ for photoionization of helium leaving the He⁺ ion in the n=2 level and differential cross sections(DCSs) in the n=2,3,4 levels with photon emission angle θ=90⁰ are provided at photon energies 69-76.8 eV employing the R-matrix method with a 20-term target representation, in which five polarization orbitals \bar{6}l are included. The asymmetry parameter β₂ and the DCS, in the n=2 level in the region of 69-73 eV, are in good agreement with available experimental and theoretical results. Above the n=3 threshold, we present new theoretical results along with measurements. No theoretical and experimental results can be used to compare with the present calculations of the DCSs for photoionization into the He⁺ ion in the n=3,4 levels.     

R-matrix calculation for photoionization

We have employed the R matrix method to calculate differential cross sections for photoionization of helium leaving helium ion in an excited state for incident photon energy between the N=2 and N=3 thresholds (69～73 eV) of He + ion. Differential cross sections for photoionization in the N=2 level at emission angle 0° are provide. Our results are in good agreement with available experimental data and theoretical calculations.     

电子碰撞激发氢原子和氦离子散射微分截面的计算

本文详细介绍计算电子碰撞激发散射截面的扭曲波玻恩近似（DWBA）理论模型,并对低能DWBA模型进行修正。利用修正的DWBA模型计算了电子碰撞激发氢原子和氦离子1s-2s 和1s-2p的散射微分截面。将关于氢原子由基态到n=2态的电子碰撞激发散射微分截面与绝对实验测量数据比较,发现二者符合得很好,这验证了我们对DWBA修正的正确性。本文工作为拟合强场诱导的氦原子非序列双电离关联电子动量谱提供了有效的理论方法。     

1,2-环氧辛烷的同步辐射光电离解离研究

利用同步辐射产生的真空紫外光和反射式飞行时间质谱仪,在超声冷却条件下测量了1,2-环氧辛烷在光子能量9.8~16.6 eV能区的光电离解离过程,获得了不同能量光子作用下的电离解离产物。通过测量各离子的光电离效率曲线,得到了主要碎片离子的出现势。结合G3理论计算得到了母体离子、中性碎片及离子碎片的结构与能量,通过对比实验测量值与理论值给出了1,2-环氧辛烷的光电离解离通道.     

A direct comparison between gas-phase and solid-state Hg 5d photoionization processes

We present an experimental study of the Hg 5d spin-orbit intensity branching ratio in the photon energy range 18–100 eV. The results obtained on cleaved α-HgS exhibit strong differences with respect to the recent gas-phase data by Shannon and Codling. This is the first time that $\text{experimental}$ results on gas-phase photoionization and $\text{experimental}$ results on solid-state photoionization of the same core levels are directly compared to each other. The evidence we provide for solid-state effects in the photoionization process is, therefore, independent of any particular theoretical interpretation of the data.     

Dirac R-matrix calculations of photoionization cross sections of Ni XII and atomic structure data of Ni XIII

We performed R-matrix calculations for photoionization cross sections of the two ground state configuration 3s²3p⁵ (²P^o_3/2,1/2) levels and 12 excited states of Ni XII using relativistic Dirac Atomic R-matrix Codes (DARC) across the photon energy range between the ionizations thresholds of the corresponding states and well above the thresholds of the last level of the Ni XIII target ion. Generally, a good agreement is obtained between our results and the earlier theoretical photoionization cross sections. Moreover, we have used two independent fully relativistic GRASP and FAC codes to calculate fine-structure energy levels, wavelengths, oscillator strengths, transitions rates among the lowest 48 levels belonging to the configuration (3s²3p⁴, 3s3p⁵, 3p⁶, 3s²3p³3d) in Ni XIII. Additionally, radiative lifetimes of all the excited states of Ni XIII are presented. Our results of the atomic structure of Ni XIII show good agreement with other theoretical and experimental results available in the literature. A good agreement is found between our calculated lifetimes and the experimental ones. Our present results are useful for plasma diagnostic of fusion and astrophysical plasmas.     

Elastic scattering of electrons from tetrahydrofurfuryl alcohol

Differential cross-sections (DCSs) for elastic scattering of electrons from tetrahydrofurfuryl alcohol (THFA), which can be considered as an analogue molecule to DNA sugar deoxyribose, were determined using crossed beam measurements for incident energies from 40 eV to 300 eV and scattering angles from 30° to 110°. The relative DCSs were measured both as a function of incident electron energy and scattering angle, allowing absolute calibration of the whole data set via normalization to a single point. The absolute calibration has been performed according to calculated absolute DCSs obtained by the corrected independent atom method using an improved quasifree absorption model. The calculated data-set includes DCSs and integral elastic and inelastic cross-sections in the incident energy range from 5 eV to 5000 eV. The theoretical results agree very well with the experimental ones, regarding the shape of DCSs. Moreover, the same theoretical procedure has been used to obtain DCSs for elastic electron scattering from a simpler deoxyribose analogue tetrahydrofuran (THF), which agree very well, both in shape and on the absolute scale, with the recent experimentally obtained absolute DCSs [A.R. Milosavljević et al., Eur. Phys. J. D 35, 411 (2005)]. The present results are also compared with the recent theoretical data for THF and THFA. Finally, according to both experimental and theoretical data, the DCSs for elastic electron scattering from THFA and THF molecules appear to be very similar both in shape and absolute scale.     

Partial photoionization cross sections and angular distributions for double excitation of helium up to the N = 13 threshold

Partial photoionization cross sections sigmaN(Egamma) and photoelectron angular distributions betaN(Egamma) were measured for the final ionic states He+ (N > 4) in the region between the N = 8 and N = 13 thresholds (Egamma > 78.155 eV) using the cold target recoil ion momentum spectroscopy technique (COLTRIMS). Comparison of the experimental data with two independent sets of theoretical predictions reveals disagreement for the branching ratios to the various HeN(+) states. The angular distributions just below the double ionization threshold suggest an excitation process for highly excited N states similar to the Wannier mechanism for double ionization.     

Enhanced nonlinear double excitation of He in intense extreme ultraviolet laser fields

Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields (～24.1 eV, ～5 TW/cm2) is presented. Resonances to the doubly excited states converging to the He+ N=3 level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schr?dinger equation for the two-electron atom under a laser field. It is shown that the three-photon double excitation is enhanced by intermediate Rydberg states below the first ionization threshold, giving a greater contribution to the photoionization yields than the two-photon process by more than 1 order of magnitude.     

Double photoionization of C2+

The time-dependent close-coupling method is used to study the single photon double ionization of C²⁺ in support of possible experiments at FLASH/DESY using an EBIT. Energy and angle differential cross sections are calculated to fully investigate the correlated motion of the two photoelectrons. Single energy differential as well as total cross sections are calculated for different incident photon energies in the range of 125–225 eV. Good agreement is found between our results and available R-matrix results for the double ionization of C²⁺. The study is also extended to the double photoionization along the Be-like isoelectronic sequence (Be-F⁵⁺), where good agreement is found when compared with available theoretical calculations and experimental measurements.     

Photoelectron imaging of helium droplets

The photoionization and photoelectron spectroscopy of He nanodroplets (10(4) atoms) has been studied by photoelectron imaging with photon energies from 22.5-24.5 eV. Total electron yield measurements reveal broad features, whose onset is approximately 1.5 eV below the ionization potential of atomic He. The photoelectron spectra are dominated by very low energy electrons, with less than 0.6 meV. These results are attributed to the formation and autoionization of highly vibrationally excited He(*)(n) Rydberg states within the cluster, followed by strong final state interactions between the photoelectron and the droplet.     

Photoionization cross section of Xe+ ion in the pure 5p5 2P3/2 ground level

Coupling an ion trap with synchrotron radiation is shown here to be a powerful approach to measure photoionization cross sections on ionic species relaxed in their ground state. The photoionization efficiency curve of Xe+ ions stored in a Fourier transform ion cyclotron resonance ion trap was recorded at ELETTRA in the 20-23 eV photon energy range. Absolute cross sections were derived by comparison of the photoionization yield of Xe+ with measurements from the ASTRID merged-beam experiment. Multiconfiguration Dirac-Fock calculations were performed for the interpretation of these new data.     

Triple coincidence (e,gamma2e) experiment for simultaneous electron impact ionization excitation of helium

Simultaneous ionization and excitation of helium atoms by 500 eV electron impact is observed by a triple coincidence of an ionized slow electron, the recoiling He+ ion, and the radiated vacuum ultraviolet photon (lambda< or =30.4 nm). Kinematically complete differential cross sections are presented for the He+(2p)2P final ionic state, demonstrating the feasibility of a quantum mechanically complete experiment. The experimental data are compared to predictions from state-of-the-art numerical calculations. For large momentum transfers, a first-order treatment of the projectile-target interaction can reproduce the experimental angular dependence, but a second-order treatment is required to obtain consistent magnitudes.     

Experimental study of single- and double-electron-capture by Ne ions from He gas

In the present work, the total cross-sections for single- and double-electron-capture by low-energy highly-charged Ne^q+ (q =2-6) recoil ions from He atomic gas have been measured in the energy range 600–1000 eV/q. These measurements are compared with other available experimental results. The charge state dependence of the single-electron-capture cross-sections for Ne^q+ (q = 2-6) recoil ions incident on He at energy of 1000 eV/q is compared with the theoretical predictions of the absorbing sphere model and the classical model.     

Al原子K壳层光电离的理论研究

基于多组态Dirac-Fock理论方法及其相应的GRASP2K和修改后的RATIP2012程序包,计算了极化光子入射Al原子K壳层光电离截面,并利用最新发展的RPI-E计算程序研究了K壳层光电子角分布,重点讨论了辐射场的非偶极项对光电离截面和光电子角分布的影响.结果表明,在入射光子5000 e V能量范围内,辐射场的非偶极效应对光电离总截面的影响可以忽略不计.随着入射光子能量的增加,辐射场的非偶极效应对光电子角分布的影响越来越大,入射光子能量在5000 eV,一级非偶极参数γ的数值达到1.5.     

Dissociative multiple photoionization of Br2, IBr,and I2 in the VUV and X-ray regions: a comparative study of the inner-shell processes involving Br(3d,3p,3s) and I(4d,4p,4s,3d,3p)

Dissociative multiple photoionization of the bromine, the iodine monobromide, and the iodine molecules in the Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) inner-shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the ranges of 90∼978 eV for Br₂, 60∼133 eV for IBr, and 86∼998 eV for I₂. Total photoion and photoion–photoion coincidence (PIPICO) yields have been recorded as functions of the photon energy. Here, giant shape resonances have been observed beyond the thresholds of the inner-shells owing to the Br(3d¹⁰)→Br(3d⁹ϵf), I(4d¹⁰)→I(4d⁹ϵf), and I(3d¹⁰)→I(3d⁹ϵf) transitions. The dissociation processes of the multiply charged parent ions have also been evaluated from variations of photoelectron–photoion coincidence (PEPICO) and PIPICO spectra with the photon energy. From each Br(3p_3/2) (189.9 eV) and I(4p_3/2) threshold (129.9 eV), quintuple ionization of the molecules begins to play important roles in the photoionization, subsequently yielding ion pairs of X³⁺–X²⁺ (X=Br, I). From the I(3d_5/2) threshold (627.3 eV), loss of six electrons from iodine molecule additionally begins to play a minor role in the multiple photoionization, giving rise to the formation of ion pairs of either I³⁺–I³⁺ or I⁴⁺–I²⁺. A direct comparison of the strengths and the ranges of the I(4d) and Br(3d) giant resonances was successfully made from dissociative photoionization of IBr. Over the entire energy range examined, 60<E<133 eV, biased charge spread relevant to the specific core-hole states of IBr is observed, presumably reflecting the fact that charge localizes mostly in the excited atoms, which can be accounted for mainly by a two step decay via a fast dissociation followed by autoionization upon the VUV absorption.     

F原子光电离截面的理论计算

对F原子的光电离截面,人们已经做了大量实验研究,并观察到了大量的自电离共振结构.而与实验相比,理论计算仍存在一定的差异.本文通过对靶态的能量进行修正,利用R矩阵方法计算了F原子基态在3P-1D电离阈值能量范围内的分波光电离截面、总光电离截面以及自电离共振2P4(1D)ns2D,2P4(1D)md2P和2P4(1D)md2D系列的共振位置,并与最新的多组态Hartree-Fock理论计算及实验结果进行了比较,我们的结果与实验符合得更好.     

Dipole oscillator strengths for the photoabsorption,photoionization and fragmentation of molecular oxygen

The photoabsorption, photoionization and fragmentation of O₂ have been studied using electron impact coincidence methods to obtain branching ratios and dipole oscillator strengths (cross-sections). The photoabsorption measurements cover the energy range 5–300 eV while the formation of electronic states of O₂⁺ (photoelectron spectroscopy) and the resulting ionic fragmentation (photoionization mass spectrometry) are both measured from close to threshold up to photon energies of 75 eV. The binding energy spectra of O₂ show peaks at 33, 47 and 57 eV in addition to those reported elsewhere in the literature. These peaks are assigned to multiple final ion states arising from photoionization of the inner valence orbitals. Structure in the O₂⁺ electronic state partial oscillator strength curves is in good agreement with recent theoretical work which predicts the existence of several shape resonances. A quantitative picture of the dipole-induced breakdown of O₂ is obtained for the energy range 12–75 eV. The photoionization efficiency is found to be constant above 20 eV.     

Crossed beam photodissociation imaging of HeH+ with vacuum ultraviolet free-electron laser pulses

Molecular photofragmentation has been studied by event imaging on HeH+ ions at 32 nm (38.7 eV) in a fast ion beam crossed with the free-electron laser in Hamburg (FLASH), analyzing neutral He product directions and energies. Fragmentation into He(1snl,n > or = 2)+H+ was observed to yield significant photodissociation at 32 nm with an absolute cross section of (1.4+/-0.7) x 10(-18) cm2, releasing energies of 10-20 eV. A clear dominance of photodissociation perpendicular to the laser polarization was found in contrast to the excitation paths so far emphasized in theoretical studies.