Building on the well-established capabilities of photoelectron spectroscopy (PES) and photoionization mass spectrometry (PIMS), the method of photoelectron photoion coincidence （PEPICO） simultaneously collects both electrons and ions in photoionization and can provide more additional information. It has been shown that PEPICO is powerful to prepare and analyze state-selected ions and has been widely utilized in the field of chemical dynamics.

Through cooperation work with the French National Synchrotron Radiation Laboratory (Synchrotron SOLEIL), the research group of Prof. ZHANG Weijun at Anhui Institute of Optics and Fine Mechanics (AIOFM), Hefei Institutes of Physical Science, has made new progresses in the technical development of synchrotron-based PEPICO and revealed the detailed dissociation mechanisms of the CH₃Cl⁺ ions.

Dr. TANG Xiaofeng, one staff of the research group, has been focusing on the development of PEPICO and its applications in recent years. Previously, he and his co-workers successfully upgraded double imaging electron/ion coincidence end-station and have greatly enhanced the performances of the setup, especially on the vacuum, the mass resolution, and the energy resolution etc.

Just very recently, the group investigated the dissociation dynamics of energy-selected CH₃Cl⁺ ions utilizing the PEPICO setup and synchrotron radiation, which shows that the dissociation of CH₃Cl⁺ ions undergoes specific reactions into different products depending on the ions’ internal state. The appearance energies of the energetically accessible fragment ions, CH₂Cl⁺, CHCl⁺, CH₃⁺ and CH₂⁺, have been measured precisely from their respective mass-selected threshold photoelectron spectra (TPES) or photoionization efficiency (PIE) curves.

In particular, the internal energy distributions of the CH₃⁺ fragment ions have been assigned successfully via the high-resolution electron and ion kinetic energy correlation diagram. Then combined with accurate quantum chemical calculations, the detailed dissociation mechanisms of the CH3Cl+ ions have been clearly revealed. Their recent work, entitled Double imaging photoelectron photoion coincidence sheds new light on the dissociation of energy-selected CH₃Cl⁺ ions was published in Physical Chemistry Chemical Physics (18, 23923, 2016).

The work was supported by the National Natural Science Foundation of China and the Natural Science Foundation of Anhui Province.

Fig. 1 The vacuum ultraviolet (VUV) photoionization time of flight mass spectrum of CH₃Cl and (a) mass-selected electron image and (b) ion image corresponding to the CH₃⁺ fragment ions; (c) Electron and ion kinetic energy correlation diagram and the assignment along the CH₃^{+ 1}A₁,v2) + Cl(²P_1/2) dissociation channel. (Imaged by TANG Xiaofeng)