Tomas Baer

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.

Suppression of hot electrons in threshold photoelectron photoion coincidence spectroscopy using velocity focusing optics

Velocity focusing of electrons is combined with photoelectron photoion coincidence (PEPICO) spectroscopy to achieve a true threshold PEPICO signal without contributions from energetic electrons. Ions are generated by a continuous vacuum ultraviolet light source. Electrons, extracted by a field of 20 V/cm, pass through a 13 cm drift region and are dispersed in space on a multichannel plate detector by velocity focusing optics. The ions are extracted in the opposite direction by the same electric field, further accelerated by a second field, and collected after passing through a 30 cm drift region. Ions are measured in coincidence with electrons collected from the central 3.2 mm electrode as well as a ring electrode (inner and outer diameters of 5.6 and 8.1 mm). The central ring electrode contains mostly true threshold electrons along with a background of “hot” electrons, whereas the outer ring electrode collects only hot electrons. By subtracting the latter from the former, true threshold photoelectron pho...

Photoionization resonance studies with a steradiancy analyzer. II. The photoionization of CH3I

Abstract A simple, but extremely effective analyzer for resonance electrons in photoionization resonance spectroscopy is presented and demonstrated with a spectrum of CH 3 I + and NO + . The simplicity of design together with its inherently high resolution and flux makes it particularly useful for these high resolution studies of molecular ions.

The dissociation dynamics of internal energy selected C6H6+ a)

Absolute fragmentation rates and average energy released in the gas phase dissociation of 2,4 and 1,5 hexadiyne have been measured by photoion–photoelectron coincidence. The decay rates over the metastable energy range, as measured by the asymmetric time‐of‐flight distributions of C4H4+ and C3H3+ fragments are identical, and equal to those measured by others for benzene. This indicates that over this energy range all three C6H6+ isomers rearrange to a common ion structure prior to dissociation. Calculated rates based on the statistical theory suggest that the C6H6+ ions fragment to all product channels by competitive pathways and that the existence of isolated states or noncommunicating isomeric structures is highly unlikely. Above this energy range, the branching ratios to the C3H3+ and C4H4+ fragments indicate that the 1,5 hexadiyne ion no longer isomerizes prior to dissociation.

Photoionization study of the ionization potentials and fragmentation paths of the chlorinated methanes and carbon tetrabromide

The photoionization yields of CH3Cl, CH2Cl2, CHCl3, CCl4, and CBr4 have been measured in the 10–13.5 eV energy range. Ionization potentials and fragment appearance potentials derived from a detailed study of the hot band contribution at 300°K have been obtained. Although CCl4+ is apparently unstable, we have found that CBr4+ is stable. Fragmentation of the chlorinated and brominated methanes in the energy range studied appears to be limited to halogen loss, even though hydrogen loss is in certain cases energetically favored. An argument based on the symmetry of the fragments and the transition state producing these fragments is proposed to account for this observation.

Absolute unimolecular decay rates of energy selected C4H6+ metastable ions

The rate constants for the unimolecular decomposition of energy selected C4H6+ ions have been measured using the recently developed technique of zero kinetic energy (ZKE) photoelectron–photoion coincidence spectroscopy in conjunction with time‐of‐flight (TOF) mass spectrometry. The C4H6+ ions were prepared from 1,3‐butadiene, 1,2‐butadiene, cyclobutene, 2‐butyne, and 1‐butyne by photoionization. Absolute rate constants for unimolecular decay were determined from the TOF distributions of mass selected C3H3+ daughter ions. For parent metastables prepared with energies between 0.05 and 0.5 eV above the appearance potentials for the lowest fragmentation process, decomposition to C3H3+ and CH3, total unimolecular decay lifetimes were in the range 0.5–4 μsec. The experimental TOF decay curves were found to be exponential, with rate constants increasing monotonically with increasing excitation energy. These measurements also indicate that parent C4H6+ metastables rearrange to a common structure regardless of the...

Kinetic energy release distributions for the dissociation of internal energy selected CH3I+ and CD3I+ ions

Product translational energy distributions have been measured from an analysis of symmetrically broadened time‐of‐flight peaks of CH3+ and CD3+ ions in photoion–photoelectron coincidence spectra of CH3I and CD3I, respectively. The reactions were investigated at various precursor ion internal energies up to 1.7 eV above the dissociation threshold. We found that the experimentally obtained kinetic energy release distributions can be fitted accurately by simple exponential functions over the energy range considered. The experimental results are compared to predictions of the statistical theory of unimolecular reactions. An angular momentum conserving version of the quasiequilibrium theory, formulated in terms of the Langevin collision cross section model for the reverse association reaction, predicts kinetic energy release distributions which agree very well with the experimental results at precursor ion energies up to 0.65 eV above the dissociation onset. At approximately this energy, the average kinetic en...

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.

Data acquisition schemes for continuous two-particle time-of-flight coincidence experiments

Three data acquisition schemes for two-particle coincidence experiments with a continuous source are discussed. The single-start/single-stop technique, implemented with a time-to-pulse-height converter, results in a complicated spectrum and breaks down severely at high count rates. The single-start/multiple-stop setup, based on a time-to-digital converter and the first choice in todays similar coincidence experiments, performs significantly better at high count rates, but its performance is still hampered if the time-of-flight range is large, and the false coincidence background is variable if the event frequency and the collection efficiency of the starts are both high. A straightforward, multistart/multistop setup is proposed for coincidence experiments. By collecting all detector data, it ensures the highest signal-to-noise ratio, constant background, and fast data acquisition and can now be easily constructed with commercially available time-to-digital converters. Analytical and numerically evaluated formulas are derived to characterize the performance of each setup in a variety of environments. Computer simulated spectra are presented to illustrate the analytically predicted features of the various raw time-of-flight distributions obtained with each technique.

Heats of formation of C6H5?, C6H5+, and C6H5NO by threshold photoelectron photoion coincidence and active thermochemical tables analysis

Threshold photoelectron photoion coincidence has been used to prepare selected internal energy distributions of nitrosobenzene ions [C(6)H(5)NO(+)]. Dissociation to C(6)H(5)(+) + NO products was measured over a range of internal energies and rate constants from 10(3) to 10(7) s(-1) and fitted with the statistical theory of unimolecular decay. A 0 K dissociative photoionization onset energy of 10.607 ± 0.020 eV was derived by using the simplified statistical adiabatic channel model. The thermochemical network of Active Thermochemical Tables (ATcT) was expanded to include phenyl and phenylium, as well as nitrosobenzene. The current ATcT heats of formation of these three species at 0 K (298.15 K) are 350.6 (337.3) ± 0.6, 1148.7 (1136.8) ± 1.0, and 215.6 (198.6) ± 1.5 kJ mol(-1), respectively. The resulting adiabatic ionization energy of phenyl is 8.272 ± 0.010 eV. The new ATcT thermochemistry for phenyl entails a 0 K (298.15 K) C-H bond dissociation enthalpy of benzene of 465.9 (472.1) ± 0.6 kJ mol(-1). Several related thermochemical quantities from ATcT, including the current enthalpies of formation of benzene, monohalobenzenes, and their ions, as well as interim ATcT values for the constituent atoms, are also given.