Photoionisation von stickstoffhaltigen reaktiven Molekülen mit Synchrotronstrahlung

The photoionization of several nitrogen-containing reactive intermediates relevant in combustion processes was investigated in the gas phase employing VUV synchrotron radiation. The intermediates were either freshly prepared and stored under cryogenic temperatures during the experiment or generated in situ by vacuum flash pyrolysis of suitable precursor molecules. The iPEPICO (imaging photoelectron photoion coincidence) setups of the VUV beamlines at the Swiss Light Source and Synchrotron SOLEIL were then used to record mass-selected threshold photoelectron (TPE) spectra. TPE spectra reveal the ionization energy and vibrational structure in the cationic states can often be resolved, which enables to distinguish different isomers. Accurate ionization energies for the radicals carbonyl amidogen, pyrrolyl, and 3-picolyl, and for the closed shell molecules isocyanic acid and cyanovinylacetylene were obtained. The analysis of the dissociative photoionization of the pyrolysis precursors enables in some cases to retrieve thermochemical data. Beyond, the absolute photoionization cross section of the cyclic carbene cyclopropenylidene was determined, NEXAFS and normal Auger spectra of isocyanic acid were recorded and analyzed at the O1s, N1s, and C1s edges, and the dissociative photoionization and pyrolysis of 1,4-di-tert-butyl-1,4-azaborinine was studied.Die Photoionisiation von stickstoffhaltigen reaktiven Intermediaten, die in Verbrennungsprozessen vorkommen, wurde in der Gasphase mit VUV Synchrotronstrahlung untersucht. Die Intermediate wurden entweder unmittelbar vor dem Experiment hergestellt und während des Experiments bei sehr niedrigen Temperaturen gehalten oder in situ durch Vakuum Flash Pyrolyse eines geeigneten Vorläufermoleküls erzeugt. Massenselektive Schwellenphotoelektronen(TPE)-Spektren wurden an den iPEPICO (imaging photoion photoelectron coincidence) Setups der VUV Strahllinien der Swiss Light Source und des Synchrotrons SOLEIL aufgenommen. Die Ionisierungsenergie kann in TPE-Spektren bestimmt werden und eine Auflösung von Schwingungsstruktur im Kation ist in vielen Fällen möglich, wodurch verschiedene Isomere unterschieden werden können. Verlässliche Ionisierungsenergien konnten für die Radikale Carbonylamidogen, Pyrrolyl und 3-Picolyl sowie für die geschlossenschaligen Moleküle Isocyansäure und Cyanovinylacetylen erhalten werden. Die Analyse der dissoziativen Photoionisation der Pyrolysevorläufer eröffnet in manchen Fällen Zugang zu thermochemischen Daten. Darüber hinaus wurde der absolute Photoionisationsquerschnitt des cyclischen Carbens Cyclopropenyliden bestimmt, wurden die NEXAFS und nicht-resonanten Auger Spektren von Isocyansäure an der O1s, N1s und C1s Kante aufgenommen und analysiert und die dissoziative Photoionisation und Pyrolyse von 1,4-di-tert-butyl-1,4-azaborinin untersucht

Decomposition of Picolyl Radicals at High Temperature: A Mass Selective Threshold Photoelectron Spectroscopy Study

The reaction products of the picolyl radicals at high temperature were characterized by mass‐selective threshold photoelectron spectroscopy in the gas phase. Aminomethylpyridines were pyrolyzed to initially produce picolyl radicals (m /z =92). At higher temperatures further thermal reaction products are generated in the pyrolysis reactor. All compounds were identified by mass‐selected threshold photoelectron spectroscopy and several hitherto unexplored reactive molecules were characterized. The mechanism for several dissociation pathways was outlined in computations. The spectrum of m /z =91, resulting from hydrogen loss of picolyl, shows four isomers, two ethynyl pyrroles with adiabatic ionization energies (IE$_{ad}$) of 7.99 eV (2‐ethynyl‐1H ‐pyrrole) and 8.12 eV (3‐ethynyl‐1H ‐pyrrole), and two cyclopentadiene carbonitriles with IE′s of 9.14 eV (cyclopenta‐1,3‐diene‐1‐carbonitrile) and 9.25 eV (cyclopenta‐1,4‐diene‐1‐carbonitrile). A second consecutive hydrogen loss forms the cyanocyclopentadienyl radical with IE′s of 9.07 eV (T$_0$) and 9.21 eV (S$_1$). This compound dissociates further to acetylene and the cyanopropynyl radical (IE=9.35 eV). Furthermore, the cyclopentadienyl radical, penta‐1,3‐diyne, cyclopentadiene and propargyl were identified in the spectra. Computations indicate that dissociation of picolyl proceeds initially via a resonance‐stabilized seven‐membered ring

Threshold Photoionization of Fluorenyl, Benzhydryl, Diphenylmethylene, and Their Dimers

Two π-conjugated radicals, fluorenyl (C₁₃H₉) and benzhydryl (C₁₃H₁₁), as well as the carbene diphenylmethylene (C₁₃H₁₀) were studied by imaging photoelectron–photoion coincidence spectroscopy using VUV synchrotron radiation. The reactive intermediates were generated by flash pyrolysis from 9-bromofluorene and α-aminodiphenylmethane (adpm), respectively. Adiabatic ionization energies (IE_ad) for all three species were extracted. Values of 7.01 ± 0.02 eV for fluorenyl and 6.7 ± 0.1 eV for benzhydryl are reported. For the triplet diphenylmethylene, an IE_ad of 6.8 ± 0.1 eV is found. The dissociative photoionization of 9-bromofluorene, the precursor for fluorenyl, was also studied and modeled with an SSACM approach, yielding an appearance energy AE_0K(C₁₃H₉⁺/C₁₃H₉Br) of 9.4 eV. All experimental values are in very good agreement with computations. For fluorenyl, the IE_ad agrees well with earlier values, while for the benzhydryl radical, we report a value that is more than 0.6 eV lower than the one previously reported. The geometry change upon ionization is small for all three species. Although individual vibrational bands cannot be resolved, some vibrational transitions in the threshold photoelectron spectrum of fluorenyl are tentatively assigned based on a Franck–Condon simulation. In addition, the dimerization products of fluorenyl and the benzhydryl radical were detected. Ionization energies of (7.69 ± 0.04) and (8.11 ± 0.04) eV were determined for C₂₆H₁₈ and C₂₆H₂₂, respectively. On the basis of the ionization energies, we identified both molecules to be the direct dimerization products, formed in the pyrolysis without further rearrangement. Both dimers might be expected to play a role in soot formation because the radical monomers do appear in flames

Decomposition of Diazomeldrum’s Acid: A Threshold Photoelectron Spectroscopy Study

Derivatives of meldrum’s acid are known precursors for a number of reactive intermediates. Therefore, we investigate diazomeldrum’s acid (DMA) and its pyrolysis products by photoionization using vacuum ultraviolet (VUV) synchrotron radiation. The threshold photoelectron spectrum of DMA yields an ionization energy (IE) of 9.68 eV. Several channels for dissociative photoionization are observed. The first one is associated with loss of CH₃, leading to a daughter ion with <i>m</i>/<i>z</i> = 155. Its appearance energy AE_0K was determined to be 10.65 eV by fitting the experimental data using statistical theory. A second parallel channel leads to <i>m</i>/<i>z</i> = 69, corresponding to N₂CHCO, with an AE_0K of 10.72 eV. Several other channels open up at higher energy, among them the formation of acetone cation, a channel expected to be the result of a Wolff-rearrangement (WR) in the cation. When diazomeldrum’s acid is heated in a pyrolysis reactor, three thermal decomposition pathways are observed. The major one is well-known and yields acetone, N₂ and CO as consequence of the WR. However, two further channels were identified: The formation of 2-diazoethenone, NNCCO, together with acetone and CO₂ as the second channel and E-formylketene (OCCHCHCO), propyne, N₂ and O₂ as a third one. 2-Diazoethenone and E-formylketene were identified based on their threshold photoelectron spectra and accurate ionization energies could be determined. Ionization energies for several isomers of both molecules were also computed. One of the key findings of this study is that acetone is observed upon decomposition of DMA in the neutral as well as in the ion and both point to a Wolff rearrangement to occur. However, the ion is subject to other decomposition channels favored at lower internal energies

Pyrolysis of 3‑Methoxypyridine. Detection and Characterization of the Pyrrolyl Radical by Threshold Photoelectron Spectroscopy

Pyrolysis of 3-methoxypyridine in a heated pyrolysis reactor was found to be an efficient way to generate the pyrrolyl radical, <i>c</i>-C₄H₄N, in the gas phase. The threshold photoelectron (TPE) spectrum of this radical was recorded using vacuum ultraviolet synchrotron radiation. The spectrum revealed a singlet ground state at 9.11 ± 0.02 eV (X̃^+ 1A) and an excited triplet state (ã^+ 3A) at 9.43 ± 0.05 eV. Vibrational structure was observed for both cationic states and could be assigned to ring deformation modes. Furthermore, (<i>E</i>)- and (<i>Z</i>)-1-cyanoallyl radicals were found to contribute to the TPE spectrum below 8.9 eV. In addition, we have identified two parallel decomposition channels of the pyrrolyl radical, yielding either hydrogen cyanide and propargyl radical or acetylene and cyanomethyl radical. The reaction energy profiles have also been calculated for these reactions. In addition, the dissociative photoionization of the precursor 3-methoxypyridine is reported

Angularly resolved photoionization dynamics in atoms and molecules combining temporally and spectrally resolved experiments at ATTOLab and Synchrotron SOLEIL

International audienceWe report results for XUV-IR two-photon ionization of Ar, Ne, NO, and O2, where an XUV attosecond pulse train is superimposed with a synchronized IR pulse, obtained at the ATTOLab laser facility using electron–ion coincidence 3D momentum spectroscopy. Temporally resolved photoelectron angular distributions providing angle-resolved time-delays for np ionization of Ar and Ne, achieved by reconstruction of attosecond beating by interference of two-photon transitions through a unified formalism (Joseph et al. in J Phys B At Mol Opt Phys 53:184007, 2020), are summarized. For inner valence XUV-IR dissociative photoionization of NO and O2 molecules, we report electron–ion kinetic energy correlation diagrams and disentangle the dissociative photoionization processes relying on parallel XUV experiments at Synchrotron SOLEIL. For ionization into the NO+(c3Π) ionic state, extending the formalism developed for single-photon ionization, we focus on photoelectron angular distributions averaged on the delay between the XUV and the IR field in the field frame, molecular frame, and electron frame of reference

Angularly resolved photoionization dynamics in atoms and molecules combining temporally and spectrally resolved experiments at ATTOLab and Synchrotron SOLEIL

We report results for XUV-IR two-photon ionization of Ar, Ne, NO, and O2, where an XUV attosecond pulse train is superimposed with a synchronized IR pulse, obtained at the ATTOLab laser facility using electron–ion coincidence 3D momentum spectroscopy. Temporally resolved photoelectron angular distributions providing angle-resolved time-delays for np ionization of Ar and Ne, achieved by reconstruction of attosecond beating by interference of two-photon transitions through a unified formalism (Joseph et al. in J Phys B At Mol Opt Phys 53:184007, 2020), are summarized. For inner valence XUV-IR dissociative photoionization of NO and O2 molecules, we report electron–ion kinetic energy correlation diagrams and disentangle the dissociative photoionization processes relying on parallel XUV experiments at Synchrotron SOLEIL. For ionization into the NO+(c3Π) ionic state, extending the formalism developed for single-photon ionization, we focus on photoelectron angular distributions averaged on the delay between the XUV and the IR field in the field frame, molecular frame, and electron frame of reference

Threshold photoelectron spectroscopy of unstable N-containing compounds: Resolution of ΔK subbands in HNCO+ and vibrational resolution in NCO+

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Valence shell threshold photoelectron spectroscopy of C 3 H x ( x = 0–3)

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