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. A novel computational approach is presented in which 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 mutual Coulomb repulsion and to the attractive force of the positive ions. The employed numerical method 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 constitutes an effective tool to predict and account for space-charge effect in time-resolved photoemission experiments with high-intensity pulsed sources.

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使用软 X 射线 fs 脉冲的气相光发射：空间电荷效应的研究

介绍了在超快光电子能谱中发生的气相空间电荷效应的实验和计算研究。目标样品 CF$_3$I 由自由电子激光器产生的超短 (100 fs) 远紫外辐射脉冲激发。光电子能量分布的改变，即光谱结构的位移和加宽，作为脉冲强度的函数进行监测。提出了一种新的计算方法，其中使用在低辐射通量下获得的勘测光谱来确定光电发射事件后电子的初始能量分布。然后通过 $N$-body 计算再现由空间电荷效应修改的光谱，该计算模拟受相互库仑排斥和正离子吸引力影响的光电子的动力学。所采用的数值方法允许重现完整的光电子光谱，而不仅仅是特定的光发射结构。模拟还提供了关于空间电荷效应在皮秒尺度上的时间演变的信息。强调并讨论了与固体样品光电发射情况的差异。所提出的模拟程序构成了一种有效的工具，可以在高强度脉冲源的时间分辨光电发射实验中预测和解释空间电荷效应。所采用的数值方法允许重现完整的光电子光谱，而不仅仅是特定的光发射结构。模拟还提供了关于空间电荷效应在皮秒尺度上的时间演变的信息。强调并讨论了与固体样品光电发射情况的差异。所提出的模拟程序构成了一种有效的工具，可以在高强度脉冲源的时间分辨光电发射实验中预测和解释空间电荷效应。所采用的数值方法允许重现完整的光电子光谱，而不仅仅是特定的光电发射结构。模拟还提供了关于空间电荷效应在皮秒尺度上的时间演变的信息。强调并讨论了与固体样品光电发射情况的差异。所提出的模拟程序构成了一种有效的工具，可以在高强度脉冲源的时间分辨光电发射实验中预测和解释空间电荷效应。