We report here the recent upgrade of the SAPHIRS permanent photoionization end-station at the DESIRS vacuum ultraviolet beamline of synchrotron SOLEIL, whose performances have been enhanced by installing an additional double-skimmer differential chamber. The smaller molecular beam profile obtained at the interaction region has increased the mass resolution of the double imaging photoelectron photoion coincidence (i²PEPICO) spectrometer, DELICIOUS III, installed in the photoionization chamber of the SAPHIRS endstation, by a factor of two, to M/ΔM ∼ 1700 (FWHM). The electron kinetic energy resolution offered by the velocity map imaging (VMI) part of the spectrometer has been improved down to 2.8% (ΔE/E) as we show on the N₂ photoionization case in the double skimmer configuration. As a representative example of the overall state-of-the-art i²PEPICO performances, experimental results of the dissociation of state-selected O₂⁺ (B${}^2{\sum _{\mathrm{g}}}^{-}$ , v⁺ = 0-6) molecular ions performed at the fixed photon energy of hν = 21.1 eV are presented.

Gas-phase methyl radicals have been long proposed as the key intermediate in catalytic oxidative coupling of methane, but the direct experimental evidence still lacks. Here, employing synchrotron VUV photoionization mass spectroscopy, we have directly observed the formation of gas-phase methyl radicals during oxidative coupling of methane catalyzed by Li/MgO catalysts. The concentration of gas-phase methyl radicals correlates well with the yield of ethylene and ethane products. These results lead to an enhanced fundamental understanding of oxidative coupling of methane that will facilitate the exploration of new catalysts with improved performance.

A novel threshold photoelectron-photoion coincidence (TPEPICO) imaging spectrometer at the U14-A beamline of the Hefei National Synchrotron Radiation Laboratory is presented. A set of open electron and ion lenses are utilized to map velocity imaging of photoelectrons and photoions simultaneously, in which a repelling electric field using an extra lens is applied to magnify images of photoelectrons instead of traditional accelerating electric field in order to suppress the contribution of energetic electrons in the threshold photoelectron spectroscopy (TPES) and the mass-selected TPEPICO spectroscopy. The typical energy resolution of TPES is measured to be 9 meV (full width at half maximum), as shown on the P21/2 ionization of argon. The measured mass resolving power for the present TPEPICO imaging spectrometer is above 900 of M/ΔM. Subsequently as a benchmark, oxygen molecule is photoionized by monochromatic synchrotron radiation at 20.298 eV and dissociates to an oxygen atomic ion and a neutral oxygen atom, and the translation energy distribution of oxygen atomic ion is measured by the time-sliced imaging based on mass-selected TPEPICO experiment. The kinetic energy resolution of the present ion velocity imaging is better than 3% of ΔE/E.

We present a microwave discharge flow tube coupled with a double imaging electron/ion coincidence device and vacuum ultraviolet (VUV) synchrotron radiation. The system has been applied to the study of the photoelectron spectroscopy of the well-known radicals OH and OD. The coincidence imaging scheme provides a high selectivity and yields the spectra of the pure radicals, removing the ever-present contributions from excess reactants, background, or secondary products, and therefore obviating the need for a prior knowledge of all possible byproducts. The photoelectron spectra encompassing the X³Σ⁻ ground state of the OH⁺ and OD⁺ cations have been extracted and the vibrational constants compared satisfactorily to existing literature values. Future advantages of this approach include measurement of high resolution VUV spectroscopy of radicals, their absolute photoionization cross section, and species/isomer identification in chemical reactions as a function of time.

“Imaging” biomass conversion: pyrolysis is the first reaction involved in all thermal processes for biofuels and green chemicals production. Synchrotron light ionisation and mass spectrometry is used for the first time to investigate biomass pyrolysis. The soft and tunable ionisation source coupled with ab initio calculations reveals chemical mechanisms and new major intermediate species. This methodology could be extended to the thermal and catalytic conversion of all other materials. Primary volatile products are analysed online as a function of photon energy, biomass composition (cellulose, xylan, lignin), reactor temperature and time of conversion. Hydroxyacetaldehyde was detected at very minor yields for cellulose pyrolysis confirming that it is a secondary product. The effect of cellulose structure and ash content on primary tar formation was also studied. The mechanism of levoglucosan dissociative photoionisation is depicted. A new major intermediate product which could be a precursor of furanone-based species from cellulose is evidenced thanks to the soft ionisation and MSMS structural analysis of ions. Different lignin markers and evolutions upon time of conversion are shown for miscanthus and oak pyrolysis.