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Photoelectron Wave Function in Photoionization: Plane Wave or Coulomb Wave?
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Combining synchrotron vacuum-ultraviolet photoionization mass spectrometry and gas chromatography–mass spectrometry for isomer-specific mechanistic analysis with application to the benzyl self-reaction
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Dissociation dynamics of the diamondoid adamantane upon photoionization by XUV femtosecond pulses
This work presents a photodissociation study of the diamondoid adamantane using extreme ultraviolet femtosecond pulses. The fragmentation dynamics of the dication is unraveled by the use of advanced ion and electron spectroscopy giving access to the dissociation channels as well as their energetics. To get insight into the fragmentation dynamics, we use a theoretical approach combining potential energy surface determination, statistical fragmentation methods and molecular dynamics simulations. We demonstrate that the dissociation dynamics of adamantane dications takes place in a two-step process: barrierless cage opening followed by Coulomb repulsion-driven fragmentation.
Elucidating the differences in oxidation of high-performance α- and β- diisobutylene biofuels via Synchrotron photoionization mass spectrometry
Abstract Biofuels are a promising ecologically viable and renewable alternative to petroleum fuels, with the potential to reduce net greenhouse gas emissions. However, biomass sourced fuels are often produced as blends of hydrocarbons and their oxygenates. Such blending complicates the implementation of these fuels in combustion applications. Variations in a biofuel’s composition will dictate combustion properties such as auto ignition temperature, reaction delay time, and reaction pathways. A handful of novel drop-in replacement biofuels for conventional transportation fuels have recently been down selected from a list of over 10,000 potential candidates as part of the U.S. Department of Energy’s (DOE) Co-Optimization of Fuels and Engines (Co-Optima) initiative. Diisobutylene (DIB) is one such high-performing hydrocarbon which can readily be produced from the dehydration and dimerization of isobutanol, produced from the fermentation of biomass-derived sugars. The two most common isomers realized, from this process, are 2,4,4-trimethyl-1-pentene (α-DIB) and 2,4,4-trimethyl-2-pentene (β-DIB). Due to a difference in olefinic bond location, the α- and β- isomer exhibit dramatically different ignition temperatures at constant pressure and equivalence ratio. This may be attributed to different fragmentation pathways enabled by allylic versus vinylic carbons. For optimal implementation of these biofuel candidates, explicit identification of the intermediates formed during the combustion of each of the isomers is needed. To investigate the combustion pathways of these molecules, tunable vacuum ultraviolet (VUV) light (in the range 8.1–11.0 eV) available at the Lawrence Berkeley National Laboratory’s Advanced Light Source (ALS) has been used in conjunction with a jet stirred reactor (JSR) and time-of-flight mass spectrometry to probe intermediates formed. Relative intensity curves for intermediate mass fragments produced during this process were obtained. Several important unique intermediates were identified at the lowest observable combustion temperature with static pressure of 93,325 Pa and for 1.5 s residence time. As this relatively short residence time is just after ignition, this study is targeted at the fuels’ ignition events. Ignition characteristics for both isomers were found to be strongly dependent on the kinetics of C 4 and C 7 fragment production and decomposition, with the tert-butyl radical as a key intermediate species. However, the ignition of α-DIB exhibited larger concentrations of C 4 compounds over C 7 , while the reverse was true for β-DIB. These identified species will allow for enhanced engineering modeling of fuel blending and engine design.
Vibrational autoionization of state-selective jet-cooled methanethiol (CH 3 SH) investigated with infrared + vacuum-ultraviolet photoionization
Vibrational autoionization of Rydberg states provides key information about nonadiabatic processes above an ionization threshold. In this work, we employed time-of-flight mass detection of CH 3 SH + to record vibrational-state selective photo-ionization efficiency (PIE) spectra of jet-cooled methanethiol (CH 3 SH) on exciting CH 3 SH to a specific vibrationally excited state with an infrared (IR) laser, followed by excitation with a tunable laser in the vacuum-ultraviolet (VUV) region for ionization. Autoionizing Rydberg states assigned to the ns, np, nd and nf series are identified. When IR light at 2601 (ν 3 , SH stretching mode) and 2948 cm -1 (ν 2 , CH 3 symmetric stretching mode) was employed, the Rydberg series converged to the respective vibrationally excited (ν 3 and ν 2 ) states of CH 3 SH + . When IR light at 3014 cm -1 (overlapped ν 1 /ν 9 , CH 3 antisymmetric stretching and CH 2 antisymmetric stretching modes) was employed, Rydberg series converging to two vibrationally excited states (ν 1 and ν 9 ) of CH 3 SH + were observed. When IR light at 2867 cm -1 (2ν 10 , overtone of CH 3 deformation mode) and 2892 cm -1 (2ν 4 , overtone of CH 2 scissoring mode) was employed, both Δν = -1 and Δν = -2 ionization transitions were observed; there is evidence for direct ionization from the initial state into the CH 3 SH + (ν 4 + = 1) continuum. In all observed IR-VUV-PIE spectra, the ns and nd series show intensity greater than the other Rydberg series, which is consistent with the fact that the highest-occupied molecular orbital of CH 3 SH is a p-like lone pair orbital on the S atom. Finally, the quantum yields for autoionization of various vibrational excited states are discussed. Values of ν 1 = 3035, ν 2 = 2884, ν 3 = 2514, and ν 9 = 2936 cm -1 for CH 3 SH + derived from the converged limits agree satisfactorily with values observed for Ar-tagged CH 3 SH + at 3026, 2879, 2502, and 2933 cm -1 .
Resonance signatures in the body-frame valence photoionization of CF 4
Here, we present a combined experimental and theoretical investigation of the electron dynamics and body-frame angular dependence of valence photo-single ionization of CF 4 and subsequent dissociation into CF 3 + and F. Ionization from a valence t 2 orbital shows overlapping shape resonances close to threshold that couple to the same total symmetry, leading to striking changes in the photoelectron angular distributions when viewed in the body-frame.
Origin band of the first photoionizing transition of hydrogen isocyanide
Here, the photoelectron spectrum of the X 1 Σ + → X +2 Σ + ionizing transition of hydrogen isocyanide (HNC) is measured for the first time at a fixed photon energy (13 eV).
Unimolecular isomerisation of 1,5-hexadiyne observed by threshold photoelectron photoion coincidence spectroscopy
The unimolecular isomerisation of the propargyl + propargyl “head-to-head” adduct, 1,5-hexadiyne to fulvene and benzene via 3,4-dimethylenecyclobut-1-ene (all C 6 H 6 ) was studied in the high-pressure limit by threshold photoelectron spectroscopy.