Velocity map imaging (VMI) spectrometry is widely used to measure the momentum distribution of charged particles with the kinetic energy of a few tens of electronVolts. With the progress of femtosecond laser and X-ray free-electron laser, it becomes increasingly important to extend the electron kinetic energy to 1 keV. Here, we report on a recently built composite VMI spectrometer at the Shanghai soft X-ray free-electron laser, which can measure ion images and high-energy electron images simultaneously. In the SIMION simulation, we extended the electron kinetic energy to 1 keV with a resolution <2% while measuring the ions with the kinetic energy of 20 eV. The experimental performance is tested by measuring Ar 2p photoelectron spectra at Shanghai Synchrotron Radiation Facility, and O+ kinetic energy spectrum from dissociative ionization of O2 by 800 nm femtosecond laser. We reached a resolution of 1.5% at the electron kinetic energy of 500 eV. When the electron arm is set for 100 eV, a resolution of 4% is reached at the ion kinetic energy of 5.6 eV. This composite VMI spectrometer will support the experiment, such as X-ray multi-photon excitation/ionization, Auger electrons emission, attosecond streaking.
Velocity Map Imaging (VMI) spectrometer is a powerful tool to measure the two-dimensional momenta of the charged particles in the dissociative photoionization processes. By combining a time-position sensitive detector with a three-dimensional (3D) source focusing mode, 3D momentum can be measured with a much higher resolution. However, due to the side effects of the non-uniform electric field in this mode, it becomes complicated to retrieve the 3D momentum. Here, we describe a method to retrieve the 3D momentum from the time of flight and the position with a numerical accuracy better than 0.1%, much below the best achievable relative energy resolution of 1% in the reported experiments. The method is consistently tested in simulated data, including ions with different masses and charges. A scaling relationship is established among them.
Molecular cationic states with two valence holes and one n-Rydberg electron can be created after spectator Auger decay. Unraveling these states' dissociation is often very challenging due to the frequent occurrence of conical intersections between cationic potential energy curves. Here, based on an advanced analysis of the experimental multicoincidence data obtained after O1s core excitation in O 2 , we achieved an energy-resolution better than we recently exploited in Phys. Rev. A 99, 022511 (2019). We therefore revealed a group of weak channels in the two-dimensional energy-correlation map between the coincident resonant Auger electron and ion in addition to the previously reported strong ones. The fragments in the identified weak channels contain only outmost electrons in valence orbitals; in contrast, the fragments in the strong channels contain an outmost electron in a n -Rydberg orbital. Compared with the strong channels, the weak channels preferentially occur at smaller principal quantum number n. It indicates that the electron orbital size tends to be conserved during the dissociation process. These weak features are suggested to be created by the Rydberg-valence mixing between the molecular spectator Auger final state and the very dissociative molecular cationic states without the Rydberg electron. A tendency to orbital selection is also suggested in the Rydberg-valence mixing.
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