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Tomas Baer - One of the best experts on this subject based on the ideXlab platform.

  • Threshold Photoelectron−Photoion Coincidence Spectroscopy: Dissociation of the 1-Chloroadamantane Ion and the Heat of Formation of the 1-Adamantyl Cation
    Journal of Physical Chemistry A, 2020
    Co-Authors: Yue Li, Tomas Baer
    Abstract:

    Threshold photoelectron−photoion Coincidence spectroscopy was used to investigate the dissociation of the 1-chloroadamantane ion (C10H15Cl+). The product of Cl loss reaction, 1-C10H15+, was the only fragment ion at experimental photon energies below 13.0 eV. Through the simulation of the metastable time-of-flight distributions of the 1-C10H15+ ion and the breakdown diagram, the 0 K appearance energy of 1-C10H15+ was determined to be 10.10 ± 0.02 eV. Using this result and the IE value of 1-C10H15Cl, 9.30 ± 0.10 eV, the dissociation energy of the Cl loss reaction was determined to be 0.80 ± 0.10 eV. Using the known heats of formation of C10H15Cl and Cl, the 298 and 0 K heats of formation of the 1-C10H15+ ion were obtained to be 679.2 ± 3.2 and 731.9 ± 3.2 kJ/mol, respectively.

  • Ionic dissociation dynamics and energetics of hexamethyldigermanium, (CH3)6Ge2, by threshold Photoelectron-Photoion Coincidence spectroscopy
    Chemical Physics Letters, 2017
    Co-Authors: Juan Z. Dávalos, Tomas Baer, Carlos Blancas
    Abstract:

    Abstract Threshold Photoelectron-Photoion Coincidence Spectroscopy (TPEPICO) has been used to study the ionic dissociation dynamics and energetics of Me 6 Ge 2 (Me = CH 3 ). Ions are energy-selected and the 0 K dissociation onsets for methyl and Me 3 Ge loss are obtained from the breakdown diagram and mass spectra distributions, both of them analyzed and modeled with statistical SSACM theory and quantum chemical (DFT, MP2) calculations. An updated value for the enthalpy of formation, Δ f H m 0 (g) of Me 6 Ge 2 , is used to derive Δ f H m 0 ( Me 3 Ge ,g) = 70.8 ± 8.9 kJ·mol −1 . The bond dissociation enthalpies, BDE (Ge X), of Me 3 Ge X (X = Me, GeMe 3 , Cl and Br) were also derived in this study.

  • Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion Coincidence (TPEPICO).
    Physical Chemistry Chemical Physics, 2017
    Co-Authors: Tomas Baer, Richard P. Tuckett
    Abstract:

    The history and evolution of molecular threshold photoelectron spectroscopy and threshold photoelectron photoion Coincidence spectroscopy (TPEPICO) over the last fifty years are reviewed. Emphasis is placed on instrumentation and the extraction of dynamical information about energy selected ion dissociation, not on the detailed spectroscopy of certain molecules. Three important advances have expanded greatly the power of the technique, and permitted its implementation on modern synchrotron radiation beamlines. The use of velocity focusing of threshold electrons onto an imaging detector in the 1990s simultaneously improved the sensitivity and electron energy resolution, and also facilitated the subtraction of hot electron background in both threshold electron spectroscopy and TPEPICO studies. The development of multi-start multi-stop collection detectors for both electrons and ions in the 2000s permitted the use of the full intensity of modern synchrotron radiation thereby greatly improving the signal-to-noise ratio. Finally, recent developments involving imaging electrons in a range of energies as well as ions onto separate position-sensitive detectors has further improved the collection sensitivity so that low density samples found in a variety of studies can be investigated. As a result, photoelectron photoion Coincidence spectroscopy is now well positioned to address a range of challenging problems that include the quantitative determination of compositions of isomer mixtures, and the detection and spectroscopy of free radicals produced in pyrolysis or discharge sources as well as in combustion studies.

  • Threshold Photoelectron Photoion Coincidence (TPEPICO) Studies. The Road to ± 0.1 kJ/mol Thermochemistry
    2013
    Co-Authors: Tomas Baer
    Abstract:

    The threshold photoelectron photoion Coincidence (TPEPICO) technique is utilized to investigate the dissociation dynamics and thermochemistry of energy selected medium to large organic molecular ions. The reactions include parallel and consecutive steps that are modeled with the statistical theory in order to extract dissociation onsets for multiple dissociation paths. These studies are carried out with the aid of molecular orbital calculations of both ions and the transition states connecting the ion structure to their products. The results of these investigations yield accurate heats of formation of ions, free radicals, and stable molecules. In addition, they provide information about the potential energy surface that governs the dissociation process. Isomerization reactions prior to dissociation are readily inferred from the TPEPICO data.

  • Dissociation Dynamics and Thermochemistry of Tin Species, (CH3)4Sn and (CH3)6Sn2, by Threshold Photoelectron−Photoion Coincidence Spectroscopy
    Journal of Physical Chemistry A, 2010
    Co-Authors: Juan Z. Dávalos, Nicholas S. Shuman, Rebeca Herrero, Tomas Baer
    Abstract:

    The dissociative photoionization of tetramethyltin (Me4Sn) and hexamethylditin (Me6Sn2) has been investigated by threshold photoelectron−photoion Coincidence (TPEPICO). Ions are energy-selected, and their 0 K dissociation onsets are measured by monitoring the mass spectra as a function of ion internal energy. Me4Sn+ dissociates rapidly by methyl loss, with a 0 K onset of E0 = 9.382 ± 0.020 eV. The hexamethylditin ion dissociates slowly on the time scale of the experiment (i.e., during the 40 μs flight time to the detector) so that dissociation rate constants are measured as a function of the ion energy. RRKM and the simplified statistical adiabatic channel model (SSACM) are used to extrapolate the measured rate constants for methyl and Me3Sn• loss to their 0 K dissociation onsets, which were found to be 8.986 ± 0.050 and 9.153 ± 0.075 eV, respectively. Updated values for the heats of formation of the neutral Me4Sn and Me6Sn2 are used to derive the following 298.15 K gas-phase standard heats of formation, ...

Richard P. Tuckett - One of the best experts on this subject based on the ideXlab platform.

  • Fragmentation of the valence electronic states of PSCl3+ studied by threshold photoelectron–photoion Coincidence spectroscopy
    Chemical Physics, 2020
    Co-Authors: Chris R Howle, Richard P. Tuckett, R. Y. L. Chim, Andrew E.r Malins
    Abstract:

    The threshold photoelectron spectrum and threshold photoelectron photoion Coincidence spectrum of PSCl\(_3\) are reported in the range 9.5-22.5 eV. Tunable ionising radiation with a resolution of 0.3 nm is provided from a synchrotron source coupled to a vacuum-UV monochromator. The Coincidence spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. The ground state of PSCl\(_3^+\) is bound with respect to dissociation, whereas the five lowest excited electronic states dissociate by P-Cl bond cleavage to PSCl\(_2^+\). An upper limit of 629 ± 30 kJ mol\(^{-1}\) is determined for the enthalpy of formation of PSCl\(_2^+\) at 298K. For energies above ca. 15 eV, fragment ions corresponding to multiple bond cleavages are observed; PSCl\(^+\) and PCl\(_2^+\), PS\(^+\) and PCl\(^+\). Because of uncertainties in thermochemistry, it is possible only for PS\(^+\) to determine the neutrals that form with this fragment cation. Translational kinetic energy releases have been measured into PSCl\(_2^+\) + Cl at the energies of the Franck-Condon maxima of the valence states of PSCl\(_3^+\), and the results compared with models assuming statistical and impulsive dissociation. The kinetic energy data suggest that the six lowest excited states of PSCl\(_3^+\) decay by a mechanism intermediate between statistical dissociation from the ground state of PSCl\(_3^+\) following rapid internal conversion and impulsive dissociation from the initially-excited electronic state

  • Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion Coincidence (TPEPICO).
    Physical Chemistry Chemical Physics, 2017
    Co-Authors: Tomas Baer, Richard P. Tuckett
    Abstract:

    The history and evolution of molecular threshold photoelectron spectroscopy and threshold photoelectron photoion Coincidence spectroscopy (TPEPICO) over the last fifty years are reviewed. Emphasis is placed on instrumentation and the extraction of dynamical information about energy selected ion dissociation, not on the detailed spectroscopy of certain molecules. Three important advances have expanded greatly the power of the technique, and permitted its implementation on modern synchrotron radiation beamlines. The use of velocity focusing of threshold electrons onto an imaging detector in the 1990s simultaneously improved the sensitivity and electron energy resolution, and also facilitated the subtraction of hot electron background in both threshold electron spectroscopy and TPEPICO studies. The development of multi-start multi-stop collection detectors for both electrons and ions in the 2000s permitted the use of the full intensity of modern synchrotron radiation thereby greatly improving the signal-to-noise ratio. Finally, recent developments involving imaging electrons in a range of energies as well as ions onto separate position-sensitive detectors has further improved the collection sensitivity so that low density samples found in a variety of studies can be investigated. As a result, photoelectron photoion Coincidence spectroscopy is now well positioned to address a range of challenging problems that include the quantitative determination of compositions of isomer mixtures, and the detection and spectroscopy of free radicals produced in pyrolysis or discharge sources as well as in combustion studies.

  • Photoelectron photoion Coincidence study of the fragmentation of valence states of CHF2-CH3+ in the range 12-25 eV
    International Journal of Modern Physics B, 2009
    Co-Authors: Weidong Zhou, R. Y. L. Chim, D. J. Collins, Richard P. Tuckett
    Abstract:

    Using vacuum ultraviolet radiation from a synchrotron source, threshold photoelectron photoion Coincidence (TPEPICO) spectroscopy has been used to study the dissociative photoionisation of CHF2–CH3 in the photon energy from 11.8 eV to 24.8 eV. Combining with ab initio molecular orbital calculation, fragmentation process and fragmental mechanism of the valence states of parent ion are discussed. For single bond cleavage process, a statistical mechanism might govern the C–C bond cleavage, while the kinetic energy release derived from channel CHF2–CH3+ → CH3–CHF++ F + e- shows evidence for impulsive mechanism. A two-step increase of signal CF2–CH3+/CHF2–CH2+ was observed. The two-step is due to the formation of ion CF2–CH3+/CHF2–CH2+ through two different dissociation channels. Both computed molecular orbital character and related G2 dissociation energy support the assumption. The ionisation threshold of 12.02 ± 0.04 eV measured from the TPES and the appearance energies (AEs) identified from the ion yield cu...

  • Threshold photoelectron photoion Coincidence spectroscopy of trichloroethene and tetrachloroethene
    Molecular Physics, 2008
    Co-Authors: M. A. Parkes, Richard P. Tuckett, Matthew J. Simpson, A.e.r. Malins
    Abstract:

    The threshold photoelectron, the threshold photoelectron photoion Coincidence and ion breakdown spectra of trichloroethene and tetrachloroethene have been recorded from 9–22 eV. Comparisons with the equivalent data for the three dichloroethene molecules and theoretical calculations highlight the nature of the orbitals involved during photoionisation in this energy range. The ground electronic state of ( ) is bound, with excited valence states dissociating to ( ) and C2HCl+ ( ). Appearance energies suggest that C2HCl+ forms from by loss of two chlorine atoms, whereas forms from by loss of a Cl2 molecule. The translational kinetic energy release into ( ) + Cl is determined as a function of energy. In both cases, the fraction of the available energy released into translational energy of the two products decreases as the photon energy increases.

  • Threshold photoelectron–photoion Coincidence study of the fragmentation of valence states of CH2F–CH3+
    New Journal of Physics, 2007
    Co-Authors: Weidong Zhou, R. Y. L. Chim, D. J. Collins, Richard P. Tuckett
    Abstract:

    We have carried out a comprehensive study of the decay dynamics of the valence electronic states of CH2F–CH3+, by using threshold photoelectron–photoion Coincidence (TPEPICO) spectroscopy and a tunable vacuum ultraviolet synchrotron radiation source. Threshold photoelectron spectroscopy (TPES) has been recorded for the first time in the photon energy range of ~12–24 eV. Ion yield curves and breakdown diagrams have been determined in the energy range of ~ 11.5–18 eV. The dissociation process and fragmental mechanism of the valence states of the parent ion are discussed. The mean translational kinetic energy releases for fragmentation of CH2F–CH3+, via a single bond cleavage have been measured and compared with the predictions of statistical and impulsive mechanisms. For dissociation CH2F–CH3+→CHF–CH3+ + H+e−, a non-statistical decay is assumed, and there seems to be a transition to statistical behaviour with increasing photon energy. For cleavage of the C–C bond to form CH2F+, there seems again to be a transition to statistical decay. The ionization threshold measured from the TPES and the appearance energies (AEs) identified from the ion yield curves are presented. An upper limit for the ionization threshold of CH2F–CH3 (11.66 ± 0.03 eV) and the enthalpy of formation of CH2F–CH3+ at 298 K (679 ± 3 kJ mol−1) are determined.

Bálint Sztáray - One of the best experts on this subject based on the ideXlab platform.

  • Radical Thermometers, Thermochemistry, and Photoelectron Spectra: A Photoelectron Photoion Coincidence Spectroscopy Study of the Methyl Peroxy Radical.
    Journal of Physical Chemistry Letters, 2018
    Co-Authors: Krisztina Voronova, Patrick Hemberger, Andras Bodi, Thomas Gerber, David L. Osborn, Krisztián G. Torma, Kent M. Ervin, Bálint Sztáray
    Abstract:

    We investigated the simplest alkylperoxy radical, CH3OO, formed by reacting photolytically generated CH3 radicals with O2, using the new combustion reactions followed by photoelectron photoion Coincidence (CRF-PEPICO) apparatus at the Swiss Light Source. Modeling the experimental photoion mass-selected threshold photoelectron spectrum using Franck–Condon simulations including transitions to triplet and singlet cationic states yielded the adiabatic ionization energy of 10.265 ± 0.025 eV. Dissociative photoionization of CH3OO generates the CH3+ fragment ion at the appearance energy of 11.164 ± 0.010 eV. Combining these two values with ΔfH0K°(CH3) yields ΔfH0K°(CH3OO) = 22.06 ± 0.97 kJ mol–1, reducing the uncertainty of the previously determined value by a factor of 5. Statistical simulation of the CH3OO breakdown diagram provides a molecular thermometer of the free radical’s internal temperature, which we measured to be 330 ± 30 K.

  • CRF-PEPICO: Double velocity map imaging photoelectron photoion Coincidence spectroscopy for reaction kinetics studies.
    Journal of Chemical Physics, 2017
    Co-Authors: Bálint Sztáray, Patrick Hemberger, Andras Bodi, Thomas Gerber, Krisztina Voronova, Krisztián G. Torma, Kyle J. Covert, David L. Osborn
    Abstract:

    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 res...

  • Mass-Resolved Isomer-Selective Chemical Analysis with Imaging Photoelectron Photoion Coincidence Spectroscopy
    Journal of Physical Chemistry Letters, 2013
    Co-Authors: Andras Bodi, Patrick Hemberger, David L. Osborn, Bálint Sztáray
    Abstract:

    Photoionization mass spectrometry is a powerful method for time- and space-resolved chemical analysis of reactants, intermediates, and products. Tunable synchrotron light enables isomer separation based on unique photoionization spectra; however, mixtures of three or more isomers can be difficult to resolve by this method. In this work we demonstrate, by measuring the photoelectron spectrum corresponding to each cation m/z ratio, that imaging photoelectron photoion Coincidence spectroscopy provides a more detailed spectral fingerprint than photoionization spectra. This method offers increased selectivity for analyzing gas-phase mixtures of many components. Mixtures of two C4H6 and four C5H8 unsaturated hydrocarbons were analyzed, and the components could easily be identified based on mass-selected threshold photoelectron spectra (scanning the photon energy) or photoelectron velocity map images (PEVMIs) at a single or a few fixed photon energies. The PEVMI approach is more highly multiplexed, trading somew...

  • From iron pentacarbonyl to the iron ion by imaging photoelectron photoion Coincidence.
    Journal of Physical Chemistry A, 2013
    Co-Authors: Eileen M. Russell, Bálint Sztáray, James P. Kercher, Elvis Cudjoe, Michael E. Mastromatteo, Andras Bodi
    Abstract:

    The dissociation dynamics of internal energy selected iron pentacarbonyl cations, Fe(CO)5+, have been investigated using the imaging photoelectron photoion Coincidence (iPEPICO) spectrometer at the Swiss Light Source. The molecular ion loses all five carbonyl ligands in sequential dissociations in the 8.5–20 eV photon energy range. The Fe(CO)5+ parent ion is metastable at the onset of the first dissociation reaction on the time scale of the experiment. The slightly asymmetric and broad daughter ion time-of-flight distributions indicate parent ion lifetimes in the microsecond range, and are used to obtain an experimental dissociation rate curve. Further carbonyl losses were found to be fast at threshold. The fractional parent and daughter ion abundances as a function of the photon energy, that is, breakdown diagram, as well as the dissociation rates for the first CO loss were modeled using the statistical Rice–Ramsperger–Kassel–Marcus (RRKM) and statistical adiabatic channel model (SSACM) theories. The exc...

  • Binding Energies and Isomerization in Metallocene Ions from Threshold Photoelectron Photoion Coincidence Spectroscopy
    Journal of the American Chemical Society, 2010
    Co-Authors: Ágnes Révész, Tomas Baer, László Szepes, Bálint Sztáray
    Abstract:

    Metallocene ions (Cp2M+, M = Cr, Co, Ni) were studied by threshold photoelectron photoion Coincidence spectroscopy (TPEPICO) to investigate the mechanism, energetics, and kinetics of the ionic dissociation processes. The examined energy-selected Cp2M+ ions fragment by losing the neutral cyclopentadienyl ligand. In addition, CH and C2H2 losses appear as minor channels, while the cobaltocene ion also loses an H atom. A possible isomerization pathway has also been observed for Cp2Ni+, yielding a complex with pentafulvalene (C10H8) with a loss of H2. In order to determine the 0 K appearance energies for the CpM+ fragment ions, the asymmetric time-of-flight peak shapes and the breakdown diagrams of the energy-selected metallocene ions were modeled by both the rigid activated complex (RAC) Rice−Ramsperger−Kassel−Marcus (RRKM) theory and the simplified statistical adiabatic channel model (SSACM). The following appearance energies were obtained with SSACM, which is more reliable for loose transition states: 10.57...

Andras Bodi - One of the best experts on this subject based on the ideXlab platform.

  • Swiss Light Source VUV beamline, Imaging photoelectron photoion Coincidence spectroscopy
    2020
    Co-Authors: Andras Bodi, Melanie Johnson, Thomas Gerber
    Abstract:

    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 ambient temperature gas-phase sample. Photoelectrons are velocity focused, with a resolution better than 1 meV in order to detect threshold electrons. The electron detection also starts the counter for the time-offlight analysis of the associated ion. The ions are accelerated in a relatively low, 40 – 80 V/cm field, which enables the direct measurement of rate constants in the 10 3 – 10 7 Hz range. All electron and ion events are recorded in a triggerless multiple-start/multiple-stop protocol enabling Coincidence measurements at 100 kHz event frequencies.

  • New analytical tools for advanced mechanistic studies in catalysis: photoionization and photoelectron photoion Coincidence spectroscopy
    Catalysis Science & Technology, 2020
    Co-Authors: Patrick Hemberger, Javier Pérez-ramírez, Jeroen A. Van Bokhoven, Andras Bodi
    Abstract:

    What are the reaction mechanisms responsible for the selective product formation in catalysis? How can we identify the reactive intermediates steering the reaction towards the desired reaction pathway? In this mini review, we explore novel in situ analysis techniques, such as photoionization and photoelectron photoion Coincidence spectroscopy, to detect gas-phase reactive intermediates (radicals, carbenes, and ketenes) isomer-selectively. Mass spectrometry with tunable vacuum ultraviolet (VUV) synchrotron radiation is the first experimental approach to detect elusive species sensitively and selectively in catalytic oxidation reactions or the Fischer–Tropsch process. Further, we introduce a second analytical dimension, which utilizes imaging to enhance radical detection capabilities and provides strategies to address fragmentation in C–H activation of alkanes. Thereafter, we present photoion mass-selected threshold photoelectron spectroscopy as the third analytical dimension revealing spectroscopic fingerprints to assign the elusive intermediates unequivocally and isomer-selectively in lignin catalytic fast pyrolysis. Last, limitations and future perspectives are discussed.

  • A pressurized flow reactor combustion experiment interfaced with synchrotron double imaging photoelectron photoion Coincidence spectroscopy
    Review of Scientific Instruments, 2020
    Co-Authors: M. Hoener, Thomas Bierkandt, Patrick Hemberger, Andras Bodi, Dennis Kaczmarek, Tina Kasper
    Abstract:

    : A new pressurized low-temperature combustion experiment has been commissioned at the Swiss Light Source, Paul Scherrer Institute. The experiment uses photoionization with tunable synchrotron radiation and double imaging photoelectron photoion Coincidence (i2PEPICO) detection at the vacuum ultraviolet beamline. The experimental setup is described, including the high-pressure reactor experiment, sampling interface, and reactant delivery system. The CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence) endstation and VUV beamline are briefly elaborated. The novel aspects of the apparatus and the new components are elucidated in detail, such as the fluid supply system to the reactor and the reactor integration into the endstation. We also present a system overview of the experimental setup. The technical details are followed by a description of the experimental procedure used to operate the pressurized flow reactor setup. Finally, first experimental results demonstrating the capability of the setup are provided and analyzed. A major advantage of this new experiment is that the excellent isomer resolution capabilities of the i2PEPICO technique can be transferred to the investigation of reactions at elevated pressures of several bars. This enables the investigation of pressure effects on the reactivity of fuel mixtures and covers more realistic conditions found in technical combustors. The capability to obtain quantitative oxidation data is confirmed, and the main and certain intermediate species are quantified for a selected condition. The results show excellent agreement with a chemical kinetics model and previously published reference measurements performed with a gas chromatography setup.

  • Operando Photoelectron Photoion Coincidence Spectroscopy Unravels Mechanistic Fingerprints of Propane Activation by Catalytic Oxyhalogenation
    Journal of Physical Chemistry Letters, 2020
    Co-Authors: Guido Zichittella, Patrick Hemberger, Fabian Holzmeier, Andras Bodi, Javier Pérez-ramírez
    Abstract:

    Herein, we demonstrate operando photoelectron photoion Coincidence (PEPICO) spectroscopy as a pivotal technique for evidencing unprecedented mechanistic insights by isomer-selective radical detecti...

  • Radical Thermometers, Thermochemistry, and Photoelectron Spectra: A Photoelectron Photoion Coincidence Spectroscopy Study of the Methyl Peroxy Radical.
    Journal of Physical Chemistry Letters, 2018
    Co-Authors: Krisztina Voronova, Patrick Hemberger, Andras Bodi, Thomas Gerber, David L. Osborn, Krisztián G. Torma, Kent M. Ervin, Bálint Sztáray
    Abstract:

    We investigated the simplest alkylperoxy radical, CH3OO, formed by reacting photolytically generated CH3 radicals with O2, using the new combustion reactions followed by photoelectron photoion Coincidence (CRF-PEPICO) apparatus at the Swiss Light Source. Modeling the experimental photoion mass-selected threshold photoelectron spectrum using Franck–Condon simulations including transitions to triplet and singlet cationic states yielded the adiabatic ionization energy of 10.265 ± 0.025 eV. Dissociative photoionization of CH3OO generates the CH3+ fragment ion at the appearance energy of 11.164 ± 0.010 eV. Combining these two values with ΔfH0K°(CH3) yields ΔfH0K°(CH3OO) = 22.06 ± 0.97 kJ mol–1, reducing the uncertainty of the previously determined value by a factor of 5. Statistical simulation of the CH3OO breakdown diagram provides a molecular thermometer of the free radical’s internal temperature, which we measured to be 330 ± 30 K.

Markus Koch - One of the best experts on this subject based on the ideXlab platform.

  • Bayesian Analysis of Femtosecond Pump-Probe Photoelectron-Photoion Coincidence Spectra with Fluctuating Laser Intensities
    Entropy, 2019
    Co-Authors: Pascal Heim, Bernhard Thaler, Wolfgang E. Ernst, Markus Koch, Michael Rumetshofer, Sascha Ranftl, Wolfgang Von Der Linden
    Abstract:

    This paper employs Bayesian probability theory for analyzing data generated in femtosecond pump-probe Photoelectron-Photoion Coincidence (PEPICO) experiments. These experiments allow investigating ultrafast dynamical processes in photoexcited molecules. Bayesian probability theory is consistently applied to data analysis problems occurring in these types of experiments such as background subtraction and false Coincidences. We previously demonstrated that the Bayesian formalism has many advantages, amongst which are compensation of false Coincidences, no overestimation of pump-only contributions, significantly increased signal-to-noise ratio, and applicability to any experimental situation and noise statistics. Most importantly, by accounting for false Coincidences, our approach allows running experiments at higher ionization rates, resulting in an appreciable reduction of data acquisition times. In addition to our previous paper, we include fluctuating laser intensities, of which the straightforward implementation highlights yet another advantage of the Bayesian formalism. Our method is thoroughly scrutinized by challenging mock data, where we find a minor impact of laser fluctuations on false Coincidences, yet a noteworthy influence on background subtraction. We apply our algorithm to data obtained in experiments and discuss the impact of laser fluctuations on the data analysis.

  • analysis of femtosecond pump probe photoelectron photoion Coincidence measurements applying bayesian probability theory
    Physical Review A, 2018
    Co-Authors: Michael Rumetshofer, Bernhard Thaler, Pascal Heim, Wolfgang E. Ernst, Markus Koch, Wolfgang Von Der Linden
    Abstract:

    Ultrafast dynamical processes in photoexcited molecules can be observed with pump-probe measurements, in which information about the dynamics is obtained from the transient signal associated with the excited state. Background signals provoked by pump and/or probe pulses alone often obscure these excited state signals. Simple subtraction of pump-only and/or probe-only measurements from the pump-probe measurement, as commonly applied, results in a degradation of the signal-to-noise ratio and, in the case of Coincidence detection, the danger of overrated background subtraction. Coincidence measurements additionally suffer from false Coincidences. Here we present a probabilistic approach based on Bayesian probability theory that overcomes these problems. For a pump-probe experiment with Photoelectron-Photoion Coincidence detection we reconstruct the interesting excited-state spectrum from pump-probe and pump-only measurements. This approach allows to treat background and false Coincidences consistently and on the same footing. We demonstrate that the Bayesian formalism has the following advantages over simple signal subtraction: (i) the signal-to-noise ratio is significantly increased, (ii) the pump-only contribution is not overestimated, (iii) false Coincidences are excluded, (iv) prior knowledge, such as positivity, is consistently incorporated, (v) confidence intervals are provided for the reconstructed spectrum, and (vi) it is applicable to any experimental situation and noise statistics. Most importantly, by accounting for false Coincidences, the Bayesian approach allows to run experiments at higher ionization rates, resulting in a significant reduction of data acquisition times. The application to pump-probe Coincidence measurements on acetone molecules enables novel quantitative interpretations about the molecular decay dynamics and fragmentation behavior.

  • Disentangling Multichannel Photodissociation Dynamics in Acetone by Time-Resolved Photoelectron–Photoion Coincidence Spectroscopy
    Journal of Physical Chemistry A, 2016
    Co-Authors: Paul Maierhofer, Markus Bainschab, Bernhard Thaler, Pascal Heim, Wolfgang E. Ernst, Markus Koch
    Abstract:

    For the investigation of photoinduced dynamics in molecules with time-resolved pump–probe photoionization spectroscopy, it is essential to obtain unequivocal information about the fragmentation behavior induced by the laser pulses. We present time-resolved photoelectron–photoion Coincidence (PEPICO) experiments to investigate the excited-state dynamics of isolated acetone molecules triggered by two-photon (269 nm) excitation. In the complex situation of different relaxation pathways, we unambiguously identify three distinct pump–probe ionization channels. The high selectivity of PEPICO detection allows us to observe the fragmentation behavior and to follow the time evolution of each channel separately. For channels leading to fragment ions, we quantitatively obtain the fragment-to-parent branching ratio and are able to determine experimentally whether dissociation occurs in the neutral molecule or in the parent ion. These results highlight the importance of Coincidence detection for the interpretation of ...