The distinction of direct and pulsed-field ionized zero kinetic energy photoelectrons in electron/ion coincidence experiments

From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF3SF5) by Double Velocity Map Imaging

Trifluoromethylsulfur pentafluoride (CF₃SF₅) was valence threshold photoionized in a double imaging photoelectron photoion coincidence spectrometer using vacuum ultraviolet synchrotron radiation. In the 12.5-16.4 eV photon energy range, CF₃⁺, SF₅⁺, and SF₃⁺ cations were observed in both room temperature (RT) and molecular beam (MB) experiments. Their fractional abundances exhibited differences beyond the sample temperature. Kinetic energy analysis of the fragment ions confirmed the difference in the dissociative photoionization mechanism. In the RT experiment, the CF₃⁺ kinetic energies were extrapolated to a 11.84 ± 0.15 eV threshold, which was used in an ion cycle to determine the enthalpy of formation of CF₃SF₅ as Δ_fH°_298K(CF₃SF₅) = -1593 ± 16 kJ mol^-1. We also updated the enthalpy of formation of the sulfur pentafluoride radical as Δ_fH°_298K(SF₅) = -854 ± 7 kJ mol^-1 and discuss the discrepancy between the CF₃ ionization energy based on the Active Thermochemical Tables and the value anchored to the CF ionization energy. A computed reaction enthalpy network optimization resulted in Δ_fH°_298K(CF₃SF₅) = -1608 ± 20 kJ mol^-1. Both values for Δ_fH°_298K(CF₃SF₅) agree with previous ab initio ones in contrast to the original, experimental determination. SF₃⁺ is formed by F-transfer processes both in the RT and MB experiments. Although the same peaks were observed in both experiments, the lower SF₃⁺ onset energy and the more slowly rising CF₃⁺ kinetic energy release in the MB experiment revealed clustering and intracluster F-transfer reactions upon ionization. The monomer and dimer cation potential energy surfaces were explored to rationalize the observations. In the dimer cation, the observer CF₃SF₅ catalyzes fluorine transfer and promotes CF₄ formation, which ultimately leads to the SF₃⁺ fragment ion.

Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO)

The history and evolution of molecular threshold photoelectron spectroscopy and threshold photoelectron photoion coincidence spectroscopy (TPEPICO) over the last fifty years are reviewed.

Bond-dissociation energies of cations--pushing the limits to quantum state resolution

Currently available concepts for measuring the bond-dissociation energy of cations D(o) are reviewed. Starting from the traditional approach of directly measuring the threshold energy for the appearance of fragment ions, attention is directed towards indirect measurements, where threshold energies are obtained by extrapolation of, for example, rate constants k(E) or kinetic energy release (KER) data to the threshold of interest. More recent high precision techniques again utilize direct measurements, for example, of the disappearance energy of the parent ion. Most precise data are obtained from quantum state resolved measurements of the dissociation energy, where the threshold energy is bracketed by the existing quantum states above and below the threshold. Ultimately the limits can even be pushed beyond the bracketing limit, by investigating steps on the lineshape of homogeneously broadened single rotational transitions.

A threshold photoelectron-photoion coincidence spectrometer with double velocity imaging using synchrotron radiation

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 (2)P(1/2) ionization of argon. The measured mass resolving power for the present TPEPICO imaging spectrometer is above 900 of M/DeltaM. 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 DeltaE/E.