Bálint Sztáray

Imaging photoelectron photoion coincidence spectroscopy with velocity focusing electron optics

An imaging photoelectron photoion coincidence spectrometer at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source is presented and a few initial measurements are reported. Monochromatic synchrotron VUV radiation ionizes the cooled or thermal gas-phase sample. Photoelectrons are velocity focused, with better than 1 meV resolution for threshold electrons, and also act as start signal for the ion time-of-flight analysis. The ions are accelerated in a relatively low, 40-80 V cm(-1) field, which enables the direct measurement of rate constants in the 10(3)-10(7) s(-1) range. All electron and ion events are recorded in a triggerless multiple-start/multiple-stop setup, which makes it possible to carry out coincidence experiments at >100 kHz event frequencies. As examples, the threshold photoelectron spectrum of the argon dimer and the breakdown diagrams for hydrogen atom loss in room temperature methane and the chlorine atom loss in cold chlorobenzene are shown and discussed.

Modeling unimolecular reactions in photoelectron photoion coincidence experiments.

A computer program has been developed to model and analyze the data from photoelectron photoion coincidence (PEPICO) spectroscopy experiments. This code has been used during the past 12 years to extract thermochemical and kinetics information for almost a hundred systems, and the results have been published in over forty papers. It models the dissociative photoionization process in the threshold PEPICO experiment by calculating the thermal energy distribution of the neutral molecule, the energy distribution of the molecular ion as a function of the photon energy, and the resolution of the experiment. Parallel or consecutive dissociation paths of the molecular ion and also of the resulting fragment ions are modeled to reproduce the experimental breakdown curves and time-of-flight distributions. The latter are used to extract the experimental dissociation rates. For slow dissociations, either the quasi-exponential fragment peak shapes or, when the mass resolution is insufficient to model the peak shapes explicitly, the center of mass of the peaks can be used to obtain the rate constants. The internal energy distribution of the fragment ions is calculated from the densities of states using the microcanonical formalism to describe consecutive dissociations. Dissociation rates can be calculated by the RRKM, SSACM or VTST rate theories, and can include tunneling effects, as well. Isomerization of the dissociating ions can also be considered using analytical formulae for the dissociation rates either from the original or the isomer ions. The program can optimize the various input parameters to find a good fit to the experimental data, using the downhill simplex algorithm.

On the protonation of water

Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy on isolated water molecules and water dimers establishes a new route to determining the water proton affinity (PA) with unprecedented accuracy. A floating thermochemical cycle constructed from the OH+ and H3O+ appearance energies and three other spectroscopic values establishes the water PA as 683.22 ± 0.25 kJ mol−1 at 0 K, which converts to 688.81 ± 0.25 kJ mol−1 at room temperature. The experimental results are corroborated by a hierarchy of coupled-cluster calculations up to pentuple excitations and septuple-ζ basis set. Combined with diagonal Born–Oppenheimer and Dirac–Coulomb–Gaunt relativistic corrections, they provide the best theoretical estimate for both the hydronium ions geometry and a water PA of 683.5 ± 0.4 kJ mol−1 and 689.1 ± 0.4 kJ mol−1 at 0 K and 298.15 K, respectively.

Bonding in a Borylene Complex Investigated by Photoionization and Dissociative Photoionization

The borylene complex [(OC)(5)Cr=B=N(SiMe(3))(2)] has been investigated by using threshold photoelectron-photoion coincidence spectroscopy with synchrotron radiation. The ionization energy of the parent complex and the 0u2005K appearance energies of the sequential CO loss channels have been determined. The derived bond-dissociation energies are used to discuss bonding and energetics in this compound.

Barrierless proton transfer across weak CH⋯O hydrogen bonds in dimethyl ether dimer

We present a combined computational and threshold photoelectron photoion coincidence study of two isotopologues of dimethyl ether, (DME - h6)n and (DME - d6)n n = 1 and 2, in the 9-14 eV photon energy range. Multiple isomers of neutral dimethyl ether dimer were considered, all of which may be present, and exhibited varying C-H⋯O interactions. Results from electronic structure calculations predict that all of them undergo barrierless proton transfer upon photoionization to the ground electronic state of the cation. In fact, all neutral isomers were found to relax to the same radical cation structure. The lowest energy dissociative photoionization channel of the dimer leads to CH3OHCH3 (+) by the loss of CH2OCH3 with a 0 K appearance energy of 9.71 ± 0.03 eV and 9.73 ± 0.03 eV for (DME - h6)2 and deuterated (DME - d6)2, respectively. The ground state threshold photoelectron spectrum band of the dimethyl ether dimer is broad and exhibits no vibrational structure. Dimerization results in a 350 meV decrease of the valence band appearance energy, a 140 meV decrease of the band maximum, thus an almost twofold increase in the ground state band width, compared with DME - d6 monomer.

Tunneling in H loss from energy selected ethanol ions

The H/D loss and CH(3)/CD(3) loss reactions from energy selected ethanol isotopologue ions C(2)H(5)OH(+), C(2)D(5)OD(+), CD(3)CH(2)OH(+), and CH(3)CD(2)OH(+) have been studied by imaging threshold photoelectron photoion coincidence (iPEPICO) spectroscopy. In the lowest energy dissociation channel, the α-carbon loses a hydrogen or a deuterium atom. Asymmetry in the daughter ion time-of-flight (TOF) peaks, an ab initio study of the reaction rates, and shifts in the phenomenological onsets between isotopologues revealed that H/D loss is slow at its onset. Tunneling through a reverse barrier along the reaction coordinate was found to play a significant role. Modeling the data with an Eckart barrier suggests that H loss from light ethanol ions proceeds via a reverse barrier of 151 meV, which agrees very well with the ab initio result of 155 meV. The higher energy methyl loss channel appears at its thermochemical threshold, but the branching ratios for methyl and H loss as a function of the ion internal energy are not entirely consistent with statistical theory. The methyl-loss signal cannot completely outcompete the hydrogen atom loss process. The shape of the photoelectron spectrum as well as our calculations indicate that the lowest energy ethanol ion structure lies considerably below the reported IE of 10.48 eV. Franck-Condon factors are favorable for ionization to a metastable ion state, which can rearrange to a more stable equilibrium structure. Combining theoretical results with previous experimental ones yields a revised ethanol adiabatic ionization energy of 10.37 eV. This applies to all isotopologues, as the isotope effect on the ionization energy is not more than a few meV.

Threshold photoelectron spectrum of the benzyl radical

We measure threshold photoelectron spectra of the benzyl radical, which show transitions to at least three electronic states of the benzylium cation: 1A1, 3B2, and 1B2, with possible contributions from transitions to 3A1. The main features in the vibrationally resolved threshold photoelectron spectrum between 7.1 and 10.5 eV are assigned with the aid of Franck–Condon simulations to these four electronic states of benzylium. We measure the adiabatic ionisation energy of the benzyl radical to be 7.252(5) eV and observe a well-resolved vibrational progression in the lowest triplet state, 3B2, from which we obtain a measured singlet–triplet splitting of 1.928(7) eV in benzylium.

CRF-PEPICO: Double velocity map imaging photoelectron photoion coincidence spectroscopy for reaction kinetics studies

Photoelectron photoion coincidence (PEPICO) spectroscopy could become a powerful tool for the time-resolved study of multi-channel gas phase chemical reactions. Toward this goal, we have designed and tested electron and ion optics that form the core of a new PEPICO spectrometer, utilizing simultaneous velocity map imaging for both cations and electrons, while also achieving good cation mass resolution through space focusing. These optics are combined with a side-sampled, slow-flow chemical reactor for photolytic initiation of gas-phase chemical reactions. Together with a recent advance that dramatically increases the dynamic range in PEPICO spectroscopy [D. L. Osborn et al., J. Chem. Phys. 145, 164202 (2016)], the design described here demonstrates a complete prototype spectrometer and reactor interface to carry out time-resolved experiments. Combining dual velocity map imaging with cation space focusing yields tightly focused photoion images for translationally cold neutrals, while offering good mass resolution for thermal samples as well. The flexible optics design incorporates linear electric fields in the ionization region, surrounded by dual curved electric fields for velocity map imaging of ions and electrons. Furthermore, the design allows for a long extraction stage, which makes this the first PEPICO experiment to combine ion imaging with the unimolecular dissociation rate constant measurements of cations to detect and account for kinetic shifts. Four examples are shown to illustrate some capabilities of this new design. We recorded the threshold photoelectron spectrum of the propargyl and the iodomethyl radicals. While the former agrees well with a literature threshold photoelectron spectrum, we have succeeded in resolving the previously unobserved vibrational structure in the latter. We have also measured the bimolecular rate constant of the CH2I + O2 reaction and observed its product, the smallest Criegee intermediate, CH2OO. Finally, the second dissociative photoionization step of iodocyclohexane ions, the loss of ethylene from the cyclohexyl cation, is slow at threshold, as illustrated by the asymmetric threshold photoionization time-of-flight distributions.

Breaking through the false coincidence barrier in electron–ion coincidence experiments

Photoelectron Photoion Coincidence (PEPICO) spectroscopy holds the promise of a universal, isomer-selective, and sensitive analytical technique for time-resolved quantitative analysis of bimolecular chemical reactions. Unfortunately, its low dynamic range of ∼103 has largely precluded its use for this purpose, where a dynamic range of at least 105 is generally required. This limitation is due to the false coincidence background common to all coincidence experiments, especially at high count rates. Electron/ion pairs emanating from separate ionization events but arriving within the ion time of flight (TOF) range of interest constitute the false coincidence background. Although this background has uniform intensity at every m/z value, the Poisson scatter in the false coincidence background obscures small signals. In this paper, temporal ion deflection coupled with a position-sensitive ion detector enables suppression of the false coincidence background, increasing the dynamic range in the PEPICO TOF mass spectrum by 2-3 orders of magnitude. The ions experience a time-dependent electric deflection field at a well-defined fraction of their time of flight. This deflection defines an m/z- and ionization-time dependent ion impact position for true coincidences, whereas false coincidences appear randomly outside this region and can be efficiently suppressed. When cold argon clusters are ionized, false coincidence suppression allows us to observe species up to Ar9+, whereas Ar4+ is the largest observable cluster under traditional operation. This advance provides mass-selected photoelectron spectra for fast, high sensitivity quantitative analysis of reacting systems.

Metal-cyclopentadienyl bond energies in metallocene cations measured using threshold collision-induced dissociation mass spectrometry.

Metal-cyclopentadienyl bond dissociation energies (BDEs) were measured for seven metallocene ions (Cp(2)M(+), Cp = η(5)-cyclopentadienyl = c-C(5)H(5), M = Ti, V, Cr, Mn, Fe, Co, Ni) using threshold collision-induced dissociation (TCID) performed in a guided ion beam tandem mass spectrometer. For all seven room temperature metallocene ions, the dominant dissociation pathway is simple Cp loss from the metal. Traces of other fragment ions were also detected, such as C(10)H(10)(+), C(10)H(8)(+), C(8)H(8)(+), C(3)H(3)(+), H(2)M(+), C(3)H(3)M(+), C(6)H(6)M(+), and C(7)H(6)M(+), depending on the metal center. Statistical modeling of the Cp-loss TCID experimental data, including consideration of energy distributions, multiple collisions, and kinetic shifts, allow the extraction of 0 K [CpM(+)- Cp] BDEs. These are found to be 4.85 ± 0.15, 4.02 ± 0.14, 4.22 ± 0.13, 3.51 ± 0.12, 4.26 ± 0.15, 4.57 ± 0.15, and 3.37 ± 0.12 eV for Cp(2)Ti(+), Cp(2)V(+), Cp(2)Cr(+), Cp(2)Mn(+), Cp(2)Fe(+), Cp(2)Co(+), and Cp(2)Ni(+), respectively. The measured BDE trend is largely in line with arguments based on a simple molecular orbital picture, with the exception of the anomalous case of titanocene, most likely attributable to its bent structure. The new results presented here are compared to previous literature values and are found to provide a more complete and accurate set of thermochemistry.