Ar原子序列双光双电离产生光电子角分布的理论计算

基于多组态Dirc-Fock方法和密度矩阵理论,给出了原子序列双光双电离光电子角分布的计算表达式,发展了相应的计算程序.利用该程序对Ar原子3p壳层序列双光双电离过程进行了理论研究,给出了光电离的总截面、磁截面、剩余离子取向以及光电子角分布的各向异性参数与入射光子能量的函数关系.结果显示在光电离截面的Cooper极小位置附近取向参数出现极大值,而光电子角分布的各向异性参数在该位置附近出现极小值.进一步给出了33.94和55.34 eV光子能量下序列双光双电离过程中第一步的Ar原子和第二步的Ar^+离子3p壳层光电子角分布,分析了序列双光双电离光电子角分布与单光电离光电子角分布的差异.将计算结果与文献已有的数据进行了比较,具有很好的一致性.本文的研究结果对揭示光与物质相互作用的非线性动力学机制具有重要的参考价值.     

多通道光离化的光电子干涉效应及空间角分布

利用光电子角分布成像光谱技术原理,以钠原子为例利用双光子共振三光子电离方法研究了从钠原子的4D态双能级电离后所产生的光电子角分布的状态.分析、讨论了多通道电离过程中所产生光电子的干涉效应.分析了光电子的空间角分布各部分的物理意义.结果表明该干涉图谱能够清楚地描绘出光电子能量和空间角分布状态.     

Angular distributions in the photoelectron spectroscopy of carbon monoxide

The angular distributions of the photoelectron intensity for CO have been investigated using a photoelectron spectrometer that has a rotatable detector and an HeI light source. The asymmetry parameter β was carefully measured for 24 vibrational transitions spanning three ionization bands. Its variation with final-state vibrational quantum number was found to have an unusual behavior. It is concluded that an autoionizing state of CO should exist at 584 Å in order to account for this behavior, and that the value of β is very sensitive to the presence of such a state.     

Angular distribution of the photoelectron spectrum of CO2, COS,CS2, N2O,H2O,and H2S

The photoelectron spectra of the triatomic molecules CO₂, COS, CS₂, N₂O, H₂O, and H₂S have been measured as a function of the angle θ between the direction of the incoming photon and outgoing photoelectron. The photoelectron spectra have been measured with a double-focusing electrostatic electron spectrometer to which has been attached a chamber containing a gas discharge lamp that can be freely rotated. (The photon source used was the 21.22 eV He I resonance line). From the dependence of intensity as a function of θ the angular parameter β was determined for each ionization band observed in the photoelectron spectra. A correlation was noted between the values of β and the molecular orbitals relative to the contributions of oxygen and sulfur atomic orbitals. Individual β values were also obtained for most of the vibrational bands seen in the photoelectron spectra. In most cases the vibrational structure showed little or no change in the angular parameter for a given electronic state. In certain cases, however, such as the fourth ionization band in CS₂, CO₂, and COS, rather sizeable changes in β were observed for the different vibrational bands.     

原子激发态在高频强激光作用下的光电离研究

本文通过数值求解动量空间的三维含时薛定谔方程, 研究了原子高激发态在高频激光脉冲作用下, 在电离阈值附近的光电子能谱和两维动量角分布. 研究结果表明: 在该能量范围内, 单光子电离过程的贡献是最主要的. 体系初态的主量子数可以由光电子能谱峰值的位置来确定; 体系初态的角量子数可以通过光电子的两维动量角度分布确定. 在比较宽泛的参数范围内, 这一规律不随入射激光的强度和脉冲时间宽度的改变而改变, 因此原则上可以利用它对原子的初态进行识别. 此外, 还研究了体系的初态为相干叠加态, 光电子动量谱随着叠加态相对相位的变化规律. 关键词： 阈上电离 激发态 高频激光脉冲 两维动量角度分布     

Nondipole parameters of TDCS for electron impact ionization and drag current

A comprehensive study is undertaken of angular distributions of electron knock-out from atomic targets by fast electrons with a small transfer of momentum. The general expressions for the parameters of the triple differential cross-section of impact ionization in the optical limit are derived. The calculated parameters are compared with those of the angular distribution of electrons ejected from an atom in the process of photoionization. In these processes, when the multipole transitions are involved, the one-to-one correspondence between the photoionization and impact ionization parameters disappears. The nondipole transitions lead to the backward/forward asymmetry of the angular distribution of ejected electrons that is absent in the dipole approximation for ionization by both fast electrons and photons. Using the He atom as an example, the character of the asymmetry for these two processes is qualitatively different and the backward/forward asymmetry results in macroscopic directed motion of secondary electrons accompanying the passing of a fast electron beam through gas or plasma. The general formulas for this drag current are derived and applied to gaseous He.     

Angular distributions for double ionization of Li- by an ultrashort, intense laser pulse.

We predict photoelectron angular distributions for double ionization of Li- by both weak and intense ultrashort, linearly polarized laser pulses by direct numerical integration of the three-dimensional, time-dependent Schr?dinger equation. Li- is treated as a two-active electron system. Near threshold, for low intensity we recover general features of angular distributions for one-photon double ionization. For the intense field (multiphoton) case, the photoelectron angular distribution changes significantly, particularly in directions parallel and perpendicular to the laser polarization axis.     

Relative intensities in photoelectron spectroscopy of atoms and molecules

The relative intensities of photoelectron lines is discussed. The relationship of observed intensities to angle of observation is considered as are the errors introduced by ignoring the fact that different lines may have different angular distributions. Tables of theoretical results for the angular distribution asymmetry parameter, β, are presented for incident Al K_α, Mg K_α, and Zr M_ζ radiation for all atomic ground state subshells of non-zero angular momentum. The application of these results to molecules is discussed.     

Asymmetry in photoelectron emission from chiral molecules induced by circularly polarized light

In photoionization of free, unoriented chiral molecules with circularly polarized radiation, a significant circular dichroism, i.e., an asymmetry in the forward-backward electron emission, has been observed in the photoelectron angular distribution. This leads also to an asymmetry in the momentum transfer to the photoions. The spectra for the left- and right-handed enantiomers of bromocamphor exhibit asymmetries up to several percent which vary as a function of orbital binding energy. This enantioselective effect can similarly occur for biomolecules with handedness, like amino acids, and may thus be a contributing factor related to the origin of the terrestrial biomolecular homochirality.     

阈上电离过程中光电子角分布的精细结构

应用非微扰散射理论并考虑自发辐射 ,研究了阈上电离过程中光电子的角分布。由于自发辐射的影响 ,电子的角分布具有精细结构 ,成功地解释了Nandor等人的实验观测。     

共振条件下载波包络相位效应对阈上电离谱的影响

本文通过求解三维含时薛定谔方程, 从理论上研究了共振条件下氢原子的光电子能谱与角分布随着激光脉冲载波包络相位(CEP)的变化规律. 研究结果表明: 在共振位置附近, 光电子能谱的强度与CEP有强烈的依赖关系; 进一步对各个分波的分析, 可以确定共振电离初态的角量子数. 此外, 利用光电子的角分布信息, 可以对多周期激光脉冲的初始相位信息进行高精度探测. 关键词： 阈上电离 超短脉冲 载波包络相位     

利用强场多光子电离技术实现对多原子分子离子振动量子态的光学操控

利用飞秒光电子影像技术研究了碘甲烷分子在飞秒强激光场作用下的多光子电离动力学,在实验上实现了运用飞秒强场多光子电离技术对多原子分子离子的振动量子态进行光学操控.提高了飞秒激光的强度,从1.6×1013W/cm2提高到2.5×1013W/cm2.在增大的激光强度范围内,发现了新的能量组分,并对此进行归属.通过采集光电离的光电子影像可以得到强场电离后光电子的动能分布和角度分布两方面的信息,前期的研究主要侧重于讨论光电子动能随光强的变化,重点讨论了光电子角度分布随光强的变化,通过观察光电子角度分布的变化趋势对振动量子态调控机理进一步认识.     

Photoionization of CF4 in the VUV region

We present calculated results of photoionization cross sections and photoelectron angular distributions for ionization out of the five outermost valence orbitals of CF₄ for photon energies ranging from near threshold to 55 eV. The Schwinger variational iterative method, using an exact static-exchange plus a model correlation–polarization potential, is applied to obtain the continuum photoelectron orbitals. The quantitative agreement between our calculated results and the experimental data is fair. Moreover, our study is capable of identifying most structures seen in experimental results for both cross sections and asymmetry parameters.     

Photoexcitation of a dipole-forbidden resonance in helium

We have observed photoexcitation of the dipole-forbidden 1s(2) 1S0-->2p(2) 1D2 resonance in helium by measuring the nondipolar forward-backward asymmetry of photoelectron angular distributions in the 2l2l(') autoionizing region. By exploiting the electric dipole-quadrupole interference in the excitation of both the 2s2p 1P1 and 2p(2) 1D2 levels, we have observed the quadrupole resonance in photoabsorption and extracted its Fano line shape parameters and the relative phase of the 1sEp and 1sEd continua. We find the quadrupole line profile index q(2) to be markedly different from theoretical expectations.     

Generation of attosecond pulses in molecular nitrogen

We investigate the carrier-envelope phase dependence of the total ionization yield for single and double ionization of xenon. We compare our results to theoretical calculations and to the phase dependent asymmetry in photoelectron emission. We observe that the phase dependence of the photoion yields, regardless if single or double ionization, is at least 2-3 orders of magnitude below the photoelectron emission signal. We conclude that total photoionization yields are only very weakly dependent on the carrier envelope phase, and that they are not a useful means for measurement of the phase. It seems possible that the broad bandwidth of few-cycle pulses facilitates multiphoton ionization, which leads to a randomization of strong field ionization phase dependencies. Besides, we observe that the spatial asymmetry in photoelectron emission appears to be useful as an indicator for the laser pulse duration in the few cycle regime.     

Nondipole Effects in Atomic Ionization Induced by an Intense Counter-Rotating Bicircular Laser Field

Nondipole effects occurring in the process of atomic ionization by an intense, mid-infrared, counter-rotating bicircular laser field are investigated using the strong-field approximation with leading-order nondipole corrections. The time integrals appearing in the expression for the differential ionization rate are computed in two ways: numerically, and by applying the saddle-point approximation. The nondipole corrections introduce an asymmetry in the photoelectron momentum distribution along the field propagation direction. The asymmetry is quantified by the partial average value of the propagation-direction momentum component of the photoelectrons and by the normalized difference of the differential ionization rates computed including and excluding the nondipole corrections. Using the saddle-point approximation, it is investigated how the nondipole corrections change the solutions for direct photoelectrons and how this affects the momentum spectra. The impact of nondipole corrections increases with increasing photoelectron energy. Analysis of the complete photoelectron spectra including both direct and rescattered photoelectrons shows that, in the low-energy region, a shift against the propagation direction occurs. The partial average of the propagation–direction momentum component in the rescattering region exhibits a plateau structure and also a local minimum structure that was recently observed in an experiment with a linearly polarized laser field (Lin et al., Phys Rev. Lett. 128, 023201 (2022)).     

Proton excitation of the isovector giant quadrupole resonance in 208Pb

The angular distribution of γ-rays from fast proton capture in ²⁰⁸Pb has been examined in detail by the direct-semidirect model. The interference of the dipole, quadrupole and octupole amplitudes was considered. The excitation of the isovector giant quadrupole resonance was assumed to occur at the excitation energy of 22.5 MeV, having the width of 5 MeV. The excitation of this resonance strongly influences the asymmetry of the angular distribution but the major part of the asymmetry is a consequence of the direct capture of the incident proton. Several groups of optical-model parameters were used in the calculation in order to see how they affect the results. The complex interaction coupling between the incident proton and the quadrupole giant resonance is essential to obtain good fits to the experimental asymmetry of the angular distribution of γ-rays.     

Molecular frame photoelectron emission in the presence of autoionizing resonances

We have measured the angular distribution of valence-shell photoelectrons excited by circularly polarized light from fixed-in-space N2O molecules, near to and on top of resonances due to Rydberg states embedded in the ionization continuum. The sign of the circular dichroism for ionization into the N2O+ (B2Pi, (1pi)-1) state is reversed on top of the lowest dominant resonances. Measured angular distributions are well predicted by state-of-the-art multichannel configuration interaction calculations. The change in sign of the circular dichroism at the peak of the resonance is the result of a rapid change in the phases of resonant dipole matrix elements by a factor of 2pi as the energy is scanned across the resonance.     

The effects of self-induced resonance in multiphoton ionization

The ionization of atoms in the presence of an intermediate two-photon resonance is considered. The ionization probability and spectral distribution of the ionized electrons are obtained for the case when the resonant is turned on adiabatically. The limiting cases of small and large ionization widths are investigated. At sufficiently large intensities of the resonant field an essential asymmetry in the ionization probability depending on the sign of the two-photon detuning is obtained.