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Temporal- and angular-resolved photoionization experiments are essential for probing the geometric configuration and electronic state evolution of atoms and molecules, which requires measuring the full spatial angular distributions of electrons and ions in free electron laser (FEL) experiments. Here, we present the first experimental results from the composite velocity imaging spectrometer (CpVMI) on the Shanghai soft X-ray free electron laser facility (SXFEL). The study demonstrates its ability to capture energy and angular information of electrons and ions with high resolution and full solid-angle collection. Krypton (Kr) atoms and carbon tetrachloride (CCl4) molecules are ionized using FEL pulses at 263.8 eV. Electron momentum images are recorded with an Andor Zyla 4.2 PLUS camera, and ion time-of-flight mass spectra and momentum distributions are acquired using a TPX3CAM. For Kr, the electron spectrum contains peaks from 3p, 3d, and 4p photoionization, as well as the Auger electrons from 3d and 3p levels. The measured anisotropy parameters (β) of these electrons show good agreement with previous theoretical Hartree-Fock calculations. The ion abundance in the time-of-flight mass spectra of Kr is consistent with the ratio derived from the intensities of the corresponding photoelectron peaks. For CCl4, the electron spectrum contains Cl 2p photoelectrons, 2p Auger electrons, and valence-shell photoelectrons, with their angular distribution parameters also aligning with theoretical predictions. The TPX3CAM can directly measure the momenta of fragment ions without the need of inverse Abel transformation. By integrating the high-resolution flight time mass spectrometry and momentum imaging data obtained from TPX3CAM, we successfully visualize and analyze the key photodissociation pathways of CCl4 molecules under the action of soft X-ray FEL. In particular, it can distinguish between direct two-body dissociation and multi-step dissociation processes, and observe the unique angular distributions and kinetic energy release characteristics of different dissociation channels. In conclusion, the experimental results clearly show that the CpVMI fully meets the technical requirements for FEL user experiments in terms of energy, angular distribution, and momentum measurement, providing a platform for FEL light-induced dynamics research. Future enhancements, including improved light focusing and the use of supersonic molecular beams, are expected to further improve the performance of the instrument. -
Keywords:
- X-ray free electron laser /
- velocity imaging spectrometer /
- angular distribution of charged particles
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图 1 复合速度成像谱仪示意图, 其中气体样品通过超声分子束或者空心针注入主腔, 气体样品与光相互作用后, 产生的电子和离子在电场的引导下飞行到两端的荧光屏探测器, 利用相机记录实验图像
Figure 1. Schematic diagram of the velocity imaging spectrometer. The gas sample is injected into the main chamber through an ultrasonic molecular beam or a hollow needle. After interacting with the light, the electrons and ions produced by the gas sample fly to the fluorescent screen detectors at both ends under the guidance of an electric field. The experimental images are recorded by two cameras.
图 2 Kr的光电子图像 (a) 图像左半部分是原始图像, 右半部分是反阿贝尔变换后的动量谱, 中间红色的双箭头表示自由电子激光的极化方向; (b) 将动量谱全角度积分后得到的电子能谱
Figure 2. Photoelectron images of Kr: (a) The left half of the image is the raw image, and the right half is the momentum spectrum obtained after the inverse Abel transformation. The red double-headed arrow in the middle indicates the polarization direction of the free electron laser. (b) The electron energy spectrum obtained by integrating the momentum spectrum over all angles.
图 3 Kr的飞行时间质谱
Figure 3. ToF mass spectrum of Kr.
图 4 CCl4的光电子图像 (a) 图像左半部分是原始图像, 右半部分是经反阿贝尔变换后得到的动量谱, 中间红色的双箭头表示自由电子激光的极化方向; (b) 将动量谱全角度积分后得到的电子能谱
Figure 4. Photoelectron images of CCl4: (a) The left half of the image is the raw image, and the right half is the momentum spectrum obtained after the inverse Abel transformation. The red double-headed arrow in the middle indicates the polarization direction of the free electron laser. (b) The electron energy spectrum obtained by integrating the momentum spectrum over all angles.
图 5 (a) CCl4的飞行时间质谱; (b)—(g) 灰色区间内的离子二维动量切片
Figure 5. (a) The ToF mass spectrum of CCl4; (b)–(g) the ion momentum images within the gray area of the mass spectrum.
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