用软X射线飞秒脉冲研究气相光电子发射：空间电荷效应的研究
Photoemission from the gas phase using soft x-ray fs pulses: An investigation of the space-charge effects
摘要：
对气相超快光电子能谱学中的空间电荷效应进行了实验和计算研究。目标样品CF $_3$ I由自由电子激光产生的超短（100 fs）远紫外辐射脉冲激发。光电子的能量分布的修改，即光谱结构的移位和加宽，被监测为脉冲强度的函数。实验结果进行了比较，采用Barnes-Hut算法计算的粒子之间的个别库仑力的影响与计算模拟。在所提出的模型中，在低辐射注量下获得的测量谱被用来确定光电发射事件后电子的初始能量分布。由空间电荷效应修改的光谱，然后再现由$N$体计算，模拟的动力学的光电子受到个人相互库仑排斥和吸引力的正离子。所采用的数值方法占的能量分布上的空间电荷的影响，并允许再现完整的光电子谱，而不仅仅是一个特定的光电发射结构。模拟还提供了皮秒尺度上的空间电荷效应的时间演化信息。与固体样品的光电子的情况下的差异突出和讨论。所提出的模拟过程，虽然它省略了角分布的分析，构成了一个有效的简化模型，允许预测和占空间电荷的影响，在时间分辨的光电发射实验与高强度脉冲源的光电子能谱。
英文摘要：
An experimental and computational investigation of the space-charge effects occurring in ultrafast photoelectron spectroscopy from the gas phase is presented. The target sample CF$_3$I is excited by ultrashort (100 fs) far-ultraviolet radiation pulses produced by a free-electron laser. The modification of the energy distribution of the photoelectrons, i.e. the shift and broadening of the spectral structures, is monitored as a function of the pulse intensity. The experimental results are compared with computational simulations which employ a Barnes-Hut algorithm to calculate the effect of individual Coulomb forces acting among the particles. In the presented model, a survey spectrum acquired at low radiation fluence is used to determine the initial energy distribution of the electrons after the photoemission event. The spectrum modified by the space-charge effects is then reproduced by $N$-body calculations that simulate the dynamics of the photoelectrons subject to the individual mutual Coulomb repulsion and to the attractive force of the positive ions. The employed numerical method accounts for the space-charge effects on the energy distribution and allows to reproduce the complete photoelectron spectrum and not just a specific photoemission structure. The simulations also provide information on the time evolution of the space-charge effects on the picosecond scale. Differences with the case of photoemission from solid samples are highlighted and discussed. The presented simulation procedure, although it omits the analysis of angular distribution, constitutes an effective simplified model that allows to predict and account for space-charge effects on the photoelectron energy spectrum in time-resolved photoemission experiments with high-intensity pulsed sources.
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