Unimolecular isomerisation of 1,5-hexadiyne observed by threshold photoelectron photoion coincidence spectroscopy.

The unimolecular isomerisation of the prompt propargyl + propargyl "head-to-head" adduct, 1,5-hexadiyne, to fulvene and benzene by the 3,4-dimethylenecyclobut-1-ene (DMCB) intermediate (all C6H6) was studied in the high-pressure limit by threshold photoelectron (TPE) spectroscopy. TPE spectra (TPES) were recorded with photoelectron photoion coincidence spectroscopy using synchrotron vacuum ultraviolet radiation. Reference TPES, obtained using pure compounds or judiciously extracted from the pyrolysis data, served as basis functions for pyrolysis quantification. From these spectra, we measured a revised fulvene ionisation energy of 8.401 ± 0.005 eV. Temperature-dependent pyrolysis spectra were decomposed using these basis functions. The basis function coefficients were converted to product yields relying on assumed integral threshold photoionisation cross sections obtained by three, partially mutually exclusive sets of assumptions. Thus, the product yields of DMCB, fulvene, and benzene have been established, as well as their uncertainty. The derived mole fractions are consistent with modeling based on the C6H6 potential and RRKM master equation model of Miller and Klippenstein [J. Phys. Chem. A, 2003, 107, 7783]. Although our results are fully consistent with the parallel isomerisation pathways to benzene and fulvene found by Miller and Klippenstein, we observe the onset of fulvene at a lower temperature than that of the onset of benzene, in agreement with the master equation model but in contrast to the previous experiments of Stein et al. [Proc. Combust. Inst., 1990, 23, 85]. This work promotes the use of photoion mass-selected threshold photoelectron spectroscopy as a rapid, sensitive, isomer-selective, and quantitative detection tool among the panoply of established analytical techniques.

Coincidence ion pair production (cipp) spectroscopy of diiodine.

Coincidence ion pair production (I+ + I-) (cipp) spectra of I2 were recorded in a double imaging coincidence experiment in the one-photon excitation region of 71 600-74 000 cm-1. The I+ + I- coincidence signal shows vibrational band head structure corresponding to iodine molecule Rydberg states crossing over to ion-pair (I+I-) potential curves above the dissociation limit. The band origin (ν0), vibrational wavenumber (ωe) and anharmonicity constants (ωexe) were determined for the identified Rydberg states. The analysis revealed a number of previously unidentified states and a reassignment of others following a discrepancy in previous assignments. Since the ion pair production threshold is well established, the electric field-dependent spectral intensities were used to derive the cutoff energy in the transitions to the rotational levels of the 7pσ(1/2) (v' = 3) state.

High-Resolution Double Velocity Map Imaging Photoelectron Photoion Coincidence Spectrometer for Gas-Phase Reaction Kinetics.

We present a new photoelectron photoion coincidence (PEPICO) spectrometer that combines high mass resolution of cations with independently adjustable velocity map imaging of both cations and electrons. We photoionize atoms and molecules using fixed-frequency vacuum ultraviolet radiation. Mass-resolved photoelectron spectra associated with each cation's mass-to-charge ratio can be obtained by inversion of the photoelectron image. The mass-resolved photoelectron spectra enable kinetic time-resolved probing of chemical reactions with isomeric resolution using fixed-frequency radiation sources amenable to small laboratory settings. The instrument accommodates a variety of sample delivery sources to explore a broad range of physical chemistry. To demonstrate the time-resolved capabilities of the instrument, we study the 193 nm photodissociation of SO2 via the C̃(1B2) ← X̃(1A1) transition. In addition to the well-documented O(3Pj) + SO(3Σ-) channel, we observe direct evidence for a small yield of S(3Pj) + O2(3Σg-) as a primary photodissociation product channel, which may impact sulfur mass-independent fractionation chemistry.

A plethora of isomerization processes and hydrogen scrambling in the fragmentation of the methanol dimer cation: a PEPICO study.

The valence photoionization of light and deuterated methanol dimers was studied by imaging photoelectron photoion coincidence spectroscopy in the 10.00-10.35 eV photon energy range. Methanol clusters were generated in a rich methanol beam in nitrogen after expansion into vacuum. They generally photoionize dissociatively to protonated methanol cluster cations, (CH3OH)nH+. However, the stable dimer parent ion (CH3OH)2+ is readily detected below the dissociation threshold to yield the dominant CH3OH2+ fragment ion. In addition to protonated methanol, we could also detect the water- and methyl-loss fragment ions of the methanol dimer cation for the first time. These newly revealed fragmentation channels are slow and cannot compete with protonated methanol cation formation at higher internal energies. In fact, the water- and methyl-loss fragment ions appear together and disappear at a ca. 150 meV higher energy in the breakdown diagram. Experiments with selectively deuterated methanol samples showed H scrambling involving two hydroxyl and one methyl hydrogens prior to protonated methanol formation. These insights guided the potential energy surface exploration to rationalize the dissociative photoionization mechanism. The potential energy surface was further validated by a statistical model including isotope effects to fit the experiment for the light and the perdeuterated methanol dimers simultaneously. The (CH3OH)2+ parent ion dissociates via five parallel channels at low internal energies. The loss of both CH2OH and CH3O neutral fragments leads to protonated methanol. However, the latter, direct dissociation channel is energetically forbidden at low energies. Instead, an isomerization transition state is followed by proton transfer from a methyl group, which leads to the CH3(H)OH+⋯CH2OH ion, the precursor to the CH2OH-, H2O-, and CH3-loss fragments after further isomerization steps, in part by a roaming mechanism. Water loss yields the ethanol cation, and two paths are proposed to account for m/z 49 fragment ions after CH3 loss. The roaming pathways are quickly outcompeted by hydrogen bond breaking to yield CH3OH2+, which explains the dominance of the protonated methanol fragment ion in the mass spectrum.

Photoionization of Two Potential Biofuel Additives: γ-Valerolactone and Methyl Butyrate.

The photoionization of two potential biofuel additives, γ-valerolactone (GVL, C5H8O2) and methyl butyrate (MB, C5H10O2) has been studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) at the VUV beamline of the Swiss Light Source (SLS). The vibrational fine structure in the photoelectron spectrum is compared with a Franck-Condon simulation for the electronic ground-state band of the GVL cation. In the lowest energy dissociative photoionization channel of GVL, CO2 is lost, resulting in a 1-butene fragment ion with a 0 K appearance energy of E0 = 10.35 ± 0.01 eV. A newly calculated 1-butene ionization energy of 9.595 ± 0.015 eV establishes the reverse barrier height to CO2 loss as 66.6 ± 4.3 kJ mol-1. Methyl butyrate cations undergo McLafferty rearrangement, which explains the missing ion signal at the computed adiabatic ionization energy of 9.25 eV. After H transfer, ethylene is lost in the lowest energy dissociation channel to yield the methyl acetate enol ion at E0 = 10.24 ± 0.04 eV. This value connects the energetics of methyl butyrate with that of methyl acetate enol ion, which is established at ΔfHo0K[CH2C(OH)OCH3+] = 502 ± 6 kJ mol-1. Parallel to ethylene loss, methyl loss is also observed from the enol tautomer of the parent ion. Both samples exhibit low-energy nonstatistical dissociative ionization channels. In GVL, the methyl-loss abundance rises quickly but levels off suddenly in the energy range of the first electronically excited states, indicating nonstatistical competition between CH3 and CO2 loss. In MB, the major parallel dissociation channel is the loss of a methoxy radical. Calculations indicate that McLafferty rearrangement is inhibited on the excited-state surface. Indeed, breakdown curve modeling of this and a sequential CO-loss channel confirms a second statistical regime in dissociative photoionization, decoupled from ethylene loss.

Valence Photoionization and Autoionization of the Formyl Radical.

We have used 308 nm photolysis of acetaldehyde to measure a photoionization spectrum of the formyl (HCO) radical between 8 and 11.5 eV using an 11 meV FWHM photoionization energy resolution. We have confirmed that the formyl radical is the carrier of the spectrum by generating an identical spectrum of the HCO product in the Cl + H2CO reaction. The spectrum of HCO and its deuterated isotopologue (DCO) have several resolved autoionizing resonances above the Franck-Condon envelope, which we assign to autoionization after initial excitation into neutral 3sσ and 3p Rydberg states converging to the first triplet excited state of HCO+(ã 3A'). The quantum defects for these states are δ3sσ = 1.06 ± 0.02 and δ3p = 0.821 ± 0.019. We report absolute photoionization cross-section measurements of σHCOPI(9.907 eV) = 4.5 ± 0.9 Mb, σHCOPI(10.007 eV) = 4.8 ± 1.0 Mb, σHCOPI(10.107 eV) = 6.0 ± 1.2 Mb, σHCOPI(10.107 eV) = 5.7 ± 1.2 Mb, and σHCOPI(10.304 eV) = 10.6 ± 2.2 Mb relative to the photoionization cross section of the methyl radical. The absolute cross-section measurements are a factor of ∼1.5 larger than those determined in past studies, although the presence of strong autoionizing features supports a dependence on photoionization energy resolution. We propose that the semiempirical model of Xu and Pratt for estimation of free radical photoionization cross sections is more accurate when applied with a reference species containing the same atoms as the free radical rather than isoelectronic species with different atoms.

Resolving the F2 bond energy discrepancy using coincidence ion pair production (cipp) spectroscopy.

Coincidence ion pair production (cipp) spectra of F2 were recorded on the DELICIOUS III coincidence spectrometer in the one-photon excitation region of 125 975-126 210 cm-1. The F+ + F- signal shows a rotational band head structure, corresponding to F2 Rydberg states crossing over to the ion pair production surface. Spectral simulation and quantum defect analysis allowed the characterization of five new molecular Rydberg states (F2**): one Π and four Σ states. The lowest-energy Rydberg state spectrum observed (T0 = 125 999 cm-1) lacked some of the predicted rotational structure, which allowed an accurate determination of the ion pair production threshold of 15.62294± 0.00043 eV. Using the well-known atomic fluorine ionization energy and electron affinity, this number leads to a ground state F-F dissociation energy of 1.60129± 0.00044 eV. Photoelectron photoion coincidence (PEPICO) experiments were also carried out on F2 and the dissociative photoionization threshold to F+ + F was determined as 19.0242 ± 0.0006 eV. Using the atomic fluorine ionization energy, this can be converted to an F2 dissociation energy of 1.60132± 0.00062 eV, further confirming the cipp-derived value above. Because the two experiments were independently energy-calibrated, they can be averaged to 1.60130± 0.00036 eV and this value can be used to derive the fluorine atom's 0 K heat of formation as 77.251± 0.017 kJ mol-1. This latter is in excellent agreement with the latest Active Thermochemical Table (ATcT) value but improves its accuracy by almost a factor of three.

Dissociative Photoionization of Methyl Vinyl Ketone-Thermochemical Anchors and a Drifting Methyl Group.

The dissociative photoionization of methyl vinyl ketone (MVK), an important intermediate in the atmospheric oxidation of isoprene, has been studied by photoelectron photoion coincidence spectroscopy. In the photon energy range of 9.5-13.8 eV, four main fragment ions were detected at m/z 55, 43, 42, and 27 aside from the parent ion at m/z 70. The m/z 55 fragment ion (C2H3CO+) is formed from ionized MVK by direct methyl loss, while breaking the C-C bond on the other side of the carbonyl group results in the acetyl cation (CH3CO+, m/z 43) and the vinyl radical. The m/z 42 fragment ion is formed via a CO-loss from the molecular ion after a methyl shift. The lightest fragment ion, the vinyl cation (C2H3+ at m/z 27), is produced in two different reactions: acetyl radical loss from the molecular ion and CO-loss from C2H3CO+. Their contributions to the m/z 27 signal are quantified based on the acetyl and vinyl fragment thermochemical anchors and quantum chemical calculations. Based on the experimentally derived appearance energy of the m/z 43 fragment ion, a new, experimentally derived heat of formation is proposed herein for gaseous methyl vinyl ketone (ΔfH0K = -94.3 ± 4.8 kJ mol-1; ΔfH298K = -110.5 ± 4.8 kJ mol-1), together with cationic heats of formation and bond dissociation energies.

Dissociative photoionization of 1,3-dioxolane: We need six channels to fit the elephant.

The dissociative photoionization of 1,3-dioxolane was studied by photoelectron photoion coincidence (PEPICO) spectroscopy in the photon energy range of 9.5-13.5 eV. Our statistical thermodynamics model shows that a total of six dissociation channels are involved in the formation of three fragment ions, namely, C3 H5 O2 + (m/z 73), C2 H5 O+ (m/z 45), and C2 H4 O+ (m/z 44), with two channels contributing to the formation of each. By comparing the results of ab initio quantum chemical calculations to the experimentally derived appearance energies of the fragment ions, the most likely mechanisms for these unimolecular dissociation reactions are proposed, including a description of the relevant parts of the potential energy surface.

The Vagabond Fluorine Atom: Dissociative Photoionization of trans-1,3,3,3-Tetrafluoropropene.

The dissociative photoionization of trans-1,3,3,3-tetrafluoropropene (HFO-1234ze) was investigated by imaging photoelectron photoion coincidence (PEPICO) spectroscopy. From the threshold photoelectron spectrum (TPES), an adiabatic ionization energy of 10.91 ± 0.05 eV is determined and reported for the first time. Over a 4 eV wide range, internal-energy selected trans-1,3,3,3-tetrafluoropropene cations decay by three parallel dissociative photoionization channels, which were modeled using statistical theory. The 0 K appearance energies of CF2CHCF2 (H-loss, m/z 113), CFHCHCF2 (F-loss, m/z 95), and CH2═CF2 (CF2-loss, m/z 64) fragment ions were determined to be 12.247 ± 0.030, 12.66 ± 0.10, and 12.80 ± 0.05 eV, respectively. From the last, the heat of formation of neutral trans-1,3,3,3-tetrafluoropropene was determined to be -779.9 ± 9.7 kJ/mol. While the lowest-energy fluorine loss occurs directly, the first H-loss and CF2-loss channels involve both a fluorine- and a hydrogen-migration prior to dissociation. At higher internal energies, several other rearrangement pathways open up, which involve fluorine and hydrogen transfer and, through fluorine loss, lead to the formation of several additional isomeric allylic fragment ions.

Vacuum ultraviolet photodynamics of the methyl peroxy radical studied by double imaging photoelectron photoion coincidences.

The vacuum ultraviolet photoionization of the methyl peroxy radical, CH3O2, and unimolecular dissociation of internal energy selected CH3O2 + cations were investigated in the 9.7-12.0 eV energy range by synchrotron-based double imaging photoelectron photoion coincidence. A microwave discharge flow tube was employed to produce CH3O2 via the reaction of methyl radicals (CH3) with oxygen gas. After identifying and separating the different sources of CH3 + from photoionization of CH3 or dissociative photoionization of CH3O2, the high resolution slow photoelectron spectrum (SPES) of CH3O2 was obtained, exhibiting two broad bands superimposed with a complex vibrational structure. The first band of the SPES is attributed to the X3A″ and a1A' overlapped electronic states of CH3O2 + and the second is assigned to the b1A' electronic state with the help of theoretical calculations. The adiabatic ionization energy of CH3O2 is derived as 10.215 ± 0.015 eV, in good agreement with high-accuracy theoretical data from the literature. The vertical ionization energy of the b1A' electronic state is measured to be 11.5 eV and this state fully dissociates into CH3 + and O2 fragments. The 0 K adiabatic appearance energy (AE0K) of the CH3 + fragment ion is determined to be 11.15 ± 0.02 eV.

To Boldly Look Where No One Has Looked Before: Identifying the Primary Photoproducts of Acetylacetone.

We investigate the gas-phase photochemistry of the enolone tautomer of acetylacetone (pentane-2,4-dione) following S2(ππ*) ← S0 excitation at λ = 266 and 248 nm, using three complementary time-resolved spectroscopic methods. Contrary to earlier reports, which claimed to study one-photon excitation of acetylacetone and found OH and CH3 as the only important gas-phase products, we detect 15 unique primary photoproducts and demonstrate that five of them, including OH and CH3, arise solely by multiphoton excitation. We assign the one-photon products to six photochemical channels and show that the most significant pathway is phototautomerization to the diketone form, which is likely an intermediate in several of the other product channels. Furthermore, we measure the equilibrium constant of the tautomerization of the enolone to diketone on S0 from 320 to 600 K and extract Δ H = 4.1 ± 0.3 kcal·mol-1 and Δ S = 6.8 ± 0.5 cal·mol-1·K-1 using a van't Hoff analysis. We correct the C-OH bond dissociation energy in acetylacetone, previously determined as 90 kcal·mol-1 by theory and experiment, to a new value of 121.7 kcal·mol-1. Our experiments and electronic structure calculations provide evidence that some of the product channels, including phototautomerization, occur on S0, while others likely occur on excited triplet surfaces. Although the large oscillator strength of the S2 ← S0 transition results from the (ππ*) excitation of the C═C-C═O backbone, similar to conjugated polyenes, the participation of triplets in the dissociation pathways of acetylacetone appears to have more in common with ketone photochemistry.

Dissociative Photoionization of the C7H8 Isomers Cycloheptatriene and Toluene: Looking at Two Sides of the Same Coin Simultaneously.

The dissociation of energy-selected 1,3,5-cycloheptatriene (CHT) and toluene (Tol) cations was investigated by imaging photoelectron photoion coincidence spectroscopy. In the measured energy ranges of 10.30-11.75 eV for CHT and 11.45-12.55 eV for Tol, only the hydrogen atom loss channels open up, leading to C7H7+ from both molecular ions, which are both metastable at the H-loss threshold. Quantum chemical calculations showed that an interconversion of the molecular ions happens below the dissociation threshold. Therefore, a single statistical model was constructed to describe both systems simultaneously. We determined 0 K appearance energies for the tropylium (Tr+) and benzyl (Bz+) fragment ions from CHT to be 9.520 ± 0.060 and 9.738 ± 0.082 eV and that from Tol to be 10.978 ± 0.063 and 11.196 ± 0.080 eV, respectively. Using the experimentally determined benzyl ion appearance energy, its 0 K heat of formation was calculated to be 937.9 ± 7.7 kJ mol-1. Finally, on the basis of this value and the recently determined benzyl ionization energy, we point out discrepancies concerning the benzyl radical thermochemistry.

Thermochemistry of the smallest QOOH radical from the roaming fragmentation of energy selected methyl hydroperoxide ions.

The dissociative photoionization processes of methyl hydroperoxide (CH3OOH) have been studied by imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy experiments as well as quantum-chemical and statistical rate calculations. Energy selected CH3OOH+ ions dissociate into CH2OOH+, HCO+, CH3+, and H3O+ ions in the 11.4-14.0 eV photon energy range. The lowest-energy dissociation channel is the formation of the cation of the smallest "QOOH" radical, CH2OOH+. An extended RRKM model fitted to the experimental data yields a 0 K appearance energy of 11.647 ± 0.005 eV for the CH2OOH+ ion, and a 74.2 ± 2.6 kJ mol-1 mixed experimental-theoretical 0 K heat of formation for the CH2OOH radical. The proton affinity of the Criegee intermediate, CH2OO, was also obtained from the heat of formation of CH2OOH+ (792.8 ± 0.9 kJ mol-1) to be 847.7 ± 1.1 kJ mol-1, reducing the uncertainty of the previously available computational value by a factor of 4. RRKM modeling of the complex web of possible rearrangement-dissociation processes was used to model the higher-energy fragmentation. Supported by Born-Oppenheimer molecular dynamics simulations, we found that the HCO+ fragment ion is produced through a roaming transition state followed by a low barrier. H3O+ is formed in a consecutive process from the CH2OOH+ fragment ion, while direct C-O fission of the molecular ion leads to the methyl cation.

Photo-tautomerization of acetaldehyde as a photochemical source of formic acid in the troposphere.

Organic acids play a key role in the troposphere, contributing to atmospheric aqueous-phase chemistry, aerosol formation, and precipitation acidity. Atmospheric models currently account for less than half the observed, globally averaged formic acid loading. Here we report that acetaldehyde photo-tautomerizes to vinyl alcohol under atmospherically relevant pressures of nitrogen, in the actinic wavelength range, λ = 300-330 nm, with measured quantum yields of 2-25%. Recent theoretical kinetics studies show hydroxyl-initiated oxidation of vinyl alcohol produces formic acid. Adding these pathways to an atmospheric chemistry box model (Master Chemical Mechanism) demonstrates increased formic acid concentrations by a factor of ~1.7 in the polluted troposphere and a factor of ~3 under pristine conditions. Incorporating this mechanism into the GEOS-Chem 3D global chemical transport model reveals an estimated 7% contribution to worldwide formic acid production, with up to 60% of the total modeled formic acid production over oceans arising from photo-tautomerization.

Radical Thermometers, Thermochemistry, and Photoelectron Spectra: A Photoelectron Photoion Coincidence Spectroscopy Study of the Methyl Peroxy Radical.

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.

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.

Furfural: The Unimolecular Dissociative Photoionization Mechanism of the Simplest Furanic Aldehyde.

The unimolecular dissociation reactions of energy-selected furfural cations have been studied by imaging photoelectron photoion coincidence spectroscopy at the vacuum-ultraviolet (VUV) beamline of the Swiss Light Source. In the photon energy range of 10.9-14.5 eV, furfural ions decay by numerous fragmentation channels. Modeling the breakdown diagram yielded the 0 K appearance energies of 10.95 ± 0.10, 11.16, and 12.03 eV for the c-C4H3O-CO+ (m/z = 95), c-C4H4O+ (m/z = 68), and c-C3H3+ (m/z = 39) fragment ions, respectively, formed by parallel dissociation channels. An internal conversion from the A″ to the A' electronic state via a conical intersection takes place along the reaction coordinate in the case of the H-loss channel (c-C4H3O-CO+ formation). Quantum chemical calculations and experimental results confirmed a fast conversion to the A' state and that the rate-determining step is a tight transition state on the potential energy surface. Appearance energies were also derived for the sequential dissociation products from the furan cation, c-C4H4O+, for the formation of CH2CO+ (m/z = 42), C3H4+ (m/z = 40), and CHO+ (m/z = 29) at 12.81, 12.80, and 13.34 eV, respectively. Statistical rate theory modeling of the breakdown diagram can also be used to predict the fractional ion abundances and thermal shifts in mass spectrometric pyrolysis studies to help assigning the m/z channels either to ionization of the neutrals or to dissociative ionization processes, with potential use for combustion diagnostics. The cationic geometry optimizations yielded functional-dependent spurious DFT minima and a deviating planar MP2 optimized geometry, which are briefly discussed.

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.

Bifurcated dissociative photoionization mechanism of acetic acid anhydride revealed by imaging photoelectron photoion coincidence spectroscopy.

The fragmentation processes of internal energy selected acetic acid anhydride cations, Ac2O+, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The first dissociation channel leads to the formation of CH3C(O)OCO+ (m/z = 87) by a CH3-loss. The 0 K appearance energy (E0) was determined to be 10.289 ± 0.010 eV, in excellent agreement with the G4-calculated 10.28 eV transition state (TS) energy. Based on the thermochemical onset of CH3C(O)OCO+, a reverse barrier of 40 kJ mol-1 was found. The second dissociation channel leads to the formation of the acetyl cation, CH3CO+ (m/z = 43). The appearance of trace amounts of acetone in the mass spectra, statistical modeling of the branching ratios, and quantum chemical calculations point to the existence of a post-transition-state bifurcation on the potential energy surface and a single TS leading to multiple products. That is, at higher excess energies, the CH3-group may swerve back along an orbiting pathway to form the acetone cation by CO2-loss instead of leaving directly. The acetone cation thus formed is then energetic enough to lose a methyl group and yield the acetyl cation at a phenomenological E0 = 10.316 ± 0.015 eV. The acetyl cation, which dominates the breakdown diagram up to 16 eV photon energy, is also formed by sequential CO2-loss from the CH3C(O)OCO+ intermediate at E0 = 10.53 ± 0.03 eV. The CH3+ (m/z = 15) fragment ion appears above 13 eV photon energy. This species can be produced directly from the parent ion or via two sequential dissociation channels: by acetyl radical loss from the acetone cation or CO-loss from the acetyl cation.