ABSTRACT The controlled synthesis of aluminium oxide (Al2O3) thin films by metal–organic chemical vapour deposition (MOCVD) critically depends on the gas‐phase decomposition behaviour of metal–organic precursors. Aluminium acetylacetonate (Al(acac)3) is widely used owing to its low toxicity and thermal stability; however, its gas‐phase chemistry in the presence of reactive carrier gases such as water vapour and oxygen remains incompletely understood. Here, we present an in‐situ mass spectrometric investigation of the gas‐phase decomposition of Al(acac)3 using vacuum ultraviolet (VUV) synchrotron radiation coupled with double imaging photoelectron photoion coincidence spectroscopy (i2PEPICO). Temperature‐dependent spectra were recorded in humid (Ar+H2O) and oxygen‐containing carrier gas environments, enabling identification of key decomposition products and intermediates. In the presence of water vapour, Al(acac)3 decomposes at lower temperatures than under inert conditions, consistent with a water‐mediated weakening of the Al–acetylacetonate bond. This pathway enhances the formation of acetylacetone and acetone, while suppressing characteristic intermediates. In contrast, oxygen exhibits a fundamentally different behaviour: the precursor signal remains nearly constant over a broad temperature range. Only above 700 K does oxygen promote alternative gas‐phase decomposition routes. These results highlight the distinct, non‐equivalent roles of water vapour and oxygen in MOCVD and their impact on deposition kinetics.

We generated the aromatic resonance-stabilized vinylcyclopentadienyl radical, C5H4-CH=CH2, in the gas phase via flash pyrolysis of meta-vinylanisole, CH3O-C6H4-CH=CH2. Double-imaging photoelectron photoion coincidence spectroscopy was used to measure the photoion mass-selected threshold photoelectron spectrum of the vinylcyclopentadienyl radical. Spectral assignments were enabled by independent, high-level ab initio coupled cluster calculations of adiabatic ionization energies originating from the ground electronic state of the doublet neutral, X̃ 2A″, to both the ground state of the cation, X̃+ 1A', and the lowest triplet state, ã+ 3A'. The experimentally determined band origin energies are found to be (7.81 ± 0.02) eV (X̃+ 1A' ← X̃ 2A″) and (8.08 ± 0.02) eV (ã+ 3A' ← X̃ 2A″), in agreement with the respective calculated values of (7.83 ± 0.01) eV and (8.09 ± 0.01) eV. The combined experimental and theoretical characterization presented here provides a foundation to identify important resonance-stabilized radicals in complex gas-phase environments.

Catalytic pyrolysis of lignin, the most abundant natural aromatic polymer, offers a route to obtain value-added products with a low carbon footprint. In such a process, the lignin structure undergoes decomposition through an intricate network of reaction routes. Despite the use of model compounds to gain insights into the decomposition pathways, the formation mechanism of coke and its role in critically affecting catalyst performance remain poorly understood. Herein, we use operando electron paramagnetic resonance (EPR) spectroscopy together with ex situ pulsed EPR experiments and density functional theory (DFT) calculations to understand coke formation in catalytic pyrolysis of phenol over HFAU and HZSM-5 zeolites. Our results pinpoint that coke formation is heavily influenced by zeolite topology. The large cages in HFAU facilitate the initial formation of linear configurations that grow to extended structures, whereas the narrower channels in HZSM-5 promote the formation of more linear structures. These results provide comprehensive mechanistic insights into coke formation and growth that are relevant for the development of lignin valorization strategies and for the general phenomenon of coke formation in zeolites and beyond.

Understanding the catalytic pyrolysis mechanism of lignin is essential for developing efficient biomass valorization strategies. How does the bond structure influence reactivity, product selectivity, and side reactions? By using photoionization mass spectrometry (PIMS) with synchrotron vacuum ultraviolet radiation, intermediates as well as products are identified, and pyrolysis mechanisms are unveiled. The dimers studied here feature different functionalization at the β‐O‐4 linkage, connecting aromatic rings in 2‐phenylethyl phenyl ether (PPE), 2‐phenoxy‐1‐phenylethanone (Ppone), and 2‐phenoxy‐1‐phenylethanol (Ppol). Major products during the catalytic pyrolysis include phenol, styrene, phenylacetylene, and benzene; however, their abundances vary significantly due to differently pronounced mechanisms. Dehydration as the primary reaction dominates in Ppol and Ppone, yielding phenylacetylene and phenol or phenanthrenols, respectively, which are potent coke precursors responsible for catalyst deactivation. Moreover, Ppone is the only model compound producing bibenzyl. Retro‐ene and Maccoll eliminations are proposed as major pathways in PPE, while their contribution is reduced in Ppol and Ppone due to the outcompeting dehydration. Our mechanistic analysis shows that the decomposition pathways of β‐O‐4 linkages are sensitive to their substitution pattern. This dependence may be exploited by choosing lignins from specific processes, since these yield characteristic substitution motifs and therefore influence the product distributions.

Carbenes are reactive species found across gas-phase environments, from combustion to planetary atmospheres and interstellar space. Their reactions with radicals represent a compelling path to increasing chemical complexity, in which the formation of the first aromatic ring is a foundational step. To date, no selective gas-phase bottom-up route to the smallest nitrogen-bearing aromatic ring, pyrrole, is known. We investigated the reaction of the simplest aminocarbene, aminomethylene, with the prototypical resonance stabilized propargyl radical. Photoelectron photoion coincidence spectroscopy and semiautomated electronic structure calculations reveal a barrierless, addition-elimination mechanism producing pyrrole + H. The reaction path depends on the orientation of propargyl during the association, in which the allenyl resonance form (H2C═C═CH•) of propargyl leads to pyrrole formation. This selective pathway highlights the promise of radical chemistry to fill important gaps in chemical reaction networks.

We present threshold photoelectron spectra of several reactive selenium-containing intermediates generated by pyrolysis of dimethyldiselenide, (CH 3 ) 2 Se 2 .

The NCN radical is a key intermediate in prompt-NO formation. For flame simulations that involve detailed NCN radical chemistry such as the prompt-NO switch reaction NCN + H, it is vital to have both accurate kinetic and thermodynamic data on NCN available. This study employed synchrotron-based imaging photoelectron photoion coincidence (iPEPICO) spectroscopy on NCN3 and NCN to determine a consistent and accurate enthalpy of formation for NCN. The obtained value of ΔfH0K∘(NCN) =450.5±3.5 kJ/mol (ΔfH298K∘(NCN) =451.0±3.5 kJ/mol) provides experimental evidence for the recently recommended, similarly low theoretical values. The determination is based on thermodynamic cycles with input from the first reported threshold photoelectron spectrum (TPES) of the NCN radical, indicating an adiabatic ionization energy of IENCN=12.608±0.027eV, complemented by the appearance energy of NCN+ from dissociative ionization of NCN3, AENCN+=12.060±0.015eV. We also report the adiabatic ionization energy of NCN3, IENCN3=10.995±0.010eV along with Franck–Condon spectral modeling.Novelty and significance statement: The enthalpy of formation of the biradical NCN has been a subject of debate until recently. Over the years, theoretical calculations, kinetic models and experiments have yielded very different values, ranging from 445 - 473 kJ/mol. However, most recent high-level theoretical calculations seem to converge to a consensus value of about 451 kJ/mol, but an independent experimental verification of this value is still lacking to clarify the discrepancy between two previously reported spectroscopic measurements of 452 kJ/mol and 466 kJ/mol. We took advantage of the synchrotron-based imaging photoelectron photoion coincidence spectroscopy setup at the Swiss Light Source to record the first threshold photoelectron spectrum of NCN, where NCN radicals were generated by a flash pyrolysis of NCN3. By combining the derived adiabatic ionization energy with the appearance energy of NCN+ from the dissociative photoionization of NCN3, we were able to verify the lower enthalpy of formation value.

The threshold photoelectron spectrum, dissociative photoionization, and pyrolysis of aziridine, a nitrogen-bearing hydrocarbon of c-C2H4NH composition, were studied by double-imaging photoelectron photoion coincidence spectroscopy at the Swiss Light Source. The aziridine adiabatic ionization energy (AIE) was measured at (9.30 ± 0.03) eV. The full ground-state band of the threshold photoelectron spectrum, exhibiting strong anharmonicity, was simulated using the Thawed Gaussian Approximation (TGA) method. At higher photon energies, dissociative photoionization products corresponding to hydrogen loss and methyl loss were identified with respective appearance energies of (10.59 ± 0.05) eV and (10.53 ± 0.05) eV obtained by fitting a statistical model to the dissociative ionization data. Aziridine pyrolysis was also studied at a temperature of 1360 K. Isomerization plays an important role with multiple C2H5N isomers contributing to the thermal decomposition of neutral aziridine. Ethenamine, methyl radical, and radicals of H2CN• composition were detected as pyrolysis products. The AIE for ethenamine was measured at (8.16 ± 0.03) eV. Isomerization to (E/Z)-ethanimine and N-methylmethanimine was also implicated in the pyrolysis data.

The performance of technical zeolite catalysts in industrial processes such as methanol-to-hydrocarbon (MTH) conversion is strongly influenced by their binder component, the mechanism of action of which remains incompletely understood. Through a combination of catalytic testing and analysis of surface and gas-phase intermediates using infrared and photoelectron photoion coincidence spectroscopies, we demonstrate that commonly used binder materials, alumina, silica, and kaolin, catalyze methanol transformations that affect the MTH conversion over zeolites. While alumina promotes deep dehydrogenation to carbon monoxide and hydrogen, silica and kaolin favor hydrogen transfer, yielding methane and formaldehyde. Furthermore, the formation of C2+ hydrocarbons was detected over all binders, suggesting that induction of the MTH reaction does not necessarily require micropore confinement and that it may involve Lewis acid sites. Importantly, the formaldehyde evolution activity of binders correlates with their impact on the coking propensity of a ZSM-5 catalyst and alteration of product distribution, indicating that the hydrogen transfer activity of binders has a substantial contribution to the binder effects in the MTH conversion. The findings reveal that binders are not mere spectators in the MTH conversion and that besides solid binder-zeolite reaction, the catalytic activity of binders has an impact on the performance of technical catalysts.

The incorporation of heteroatoms into the framework of polycyclic aromatic hydrocarbons (PAHs), in particular of nitrogen to yield polycyclic aromatic nitrogen heterocycles (PANHs), has been proposed for both astronomical and combustion environments, but no suitable precursors and pathways have been found. Analogous pathways to PAH formation are kinetically or energetically inhibited in the presence of a nitrogen heteroatom. We report on the reaction of phenylnitrene (3PhN, c-C6H5N) with resonance-stabilized propargyl radicals (C3H3) and find that the association reaction bifurcates depending on the orientation of the attacking propargyl radical and yields multiple isomeric products. Among them, we identify the condensed-ring quinoline and conclude that nitrenes are viable candidates to drive the formation of PANHs.

Dechlorination channels and pathways to olefins and aromatics in the catalytic pyrolysis of the polyvinylchloride (PVC) model compound 1,3-dichlorobutane are revealed using operando photoelectron photoion coincidence (PEPICO) spectroscopy. Experimental and computational results agree that the primary pathway involves double dehydrochlorination producing 1,3-butadiene and HCl. Minor radical channels are evidenced by the detection of chloromethyl, methyl, and propargyl radicals in thermal decomposition, while chlorine radicals are absent. HZSM-5 zeolites lower the reaction temperature and facilitate 1,3-butadiene association reactions producing C5-C12 olefins. Further reaction steps, detected experimentally and in part isomer-selectively, mimic previously postulated cross-linking pathways to aromatics in PVC catalytic pyrolysis. This study identifies CC coupling as well as Diels-Alder dimerization of butadiene to yield polymethylated cyclopentadienes. These are central precursors to aromatics, for example, benzene, toluene, and xylenes (BTX). Ring expansion and contraction as well as transmethylation reactions are found to be dominant routes to aromatic products. The mechanisms during thermocatalytic conversion of PVC are applicable to other plastics and resemble the chemistry upon methanol- and methylchloride-to-hydrocarbon and aromatics conversion, which will inspire new strategies to enhance selectivity towards aromatics and mitigate coke formation.

Unraveling reaction mechanisms of aromatic and resonance-stabilized radicals is critical to understanding molecular mass growth processes to polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanoparticles in distinct astrophysical environments (molecular clouds, circumstellar envelopes) and combustion systems. Using photoelectron photoion coincidence spectroscopy (PEPICO), we explored the gas-phase reaction of the methyl radical (CH3•) with the aromatic and resonance-stabilized fluorenyl radical (C13H9•) under high-temperature conditions in a chemical microreactor. Anthracene and phenanthrene were detected isomer-selectively using photoionization efficiency (PIE) curves and mass-selected threshold photoelectron (ms-TPE) spectra. While phenanthrene is produced through a radical-radical recombination of the carbon-centered radicals, anthracene may plausibly be formed through an unconventional radical addition to a low spin-density fluorenyl carbon. These pathways result in five-membered ring expansion-a critical mechanism crucial to PAH mass growth converting bent PAHs into planar nanostructures.

Ketene intermediates lead to branching and lower phenol selectivities in the catalytic pyrolysis of lignin model compounds, which makes understanding their formation mechanism key to enable targeted process optimization. While gas-phase pyrolysis of methoxy- and hydroxy-substituted benzaldehydes favors fulvenone ketene formation, it is unclear if the same reaction pathways dominate in the presence of Brønsted acid sites. Thus, we tested if HZSM-5 produces fulvenone utilizing operando photoelectron photoion coincidence spectroscopy. Hydroxybenzaldehydes undergo acid-catalyzed decarbonylation, via oxonium mediated hydrogen transfer reactions, to phenol instead of dehydrogenation to fulvenone. The catalytic pyrolysis of anisaldehydes is initiated by demethylation and decarbonylation to yield anisole or hydroxybenzaldehydes and does not produce ketene either. Subsequently, decarbonylation and demethylation, respectively, lead to phenol and methylated derivatives due to abundant surface methyl groups over HZSM-5. Comparative analysis of the catalytic pyrolysis pathways of methoxyphenols and anisaldehydes, reveals that the chemistry of individual functional groups outcompetes the interactions of the vicinal substituents (ortho effect) in anis- and salicylaldehydes, resulting in the suppression of fulvenone ketene. We discuss how the high reactivity of aldehyde functionalities by decarbonylation may be leveraged to increase selectivities to value-added products.

The resonance-stabilized cyclopentadienyl (C5H5) and propargyl (C3H3) radicals are important precursors for polycyclic aromatic hydrocarbons (PAHs) and thus play a significant role in molecular-weight growth and soot formation processes under combustion conditions. In this work, we describe an experimental and theoretical investigation of the C5H5 + C3H3 reaction. Experimentally, we studied this reaction in a resistively heated microtubular SiC reactor at a controlled temperature of ∼1150 K and a pressure of 10-20 mbar. The reactants C5H5 and C3H3 were pyrolytically generated from anisole (C6H5OCH3) and propargyl bromide (C3H3Br). We identified the reactants and the C8H8 products isomer-selectively utilizing photoion mass-selected threshold photoelectron spectroscopy (ms-TPES). The experimentally observed predominant formation of dihydropentalenes over the ring-enlargement reaction to styrene is consistent with our theoretical predictions of the kinetics on the newly calculated C8H8 potential energy surface. This work highlights dihydropentalenes as reactants in molecular-weight growth reactions and as potential building blocks in versatile routes for the formation of curved PAHs.

The photophysics and photochemistry of isolated phenanthridine have been investigated by time-resolved UV pump/X-ray probe spectroscopy at the SwissFEL free-electron laser combined with computations. Phenanthridine serves as the example for a polycyclic aromatic nitrogen-containing hydrocarbon (PANH), a class of molecules of considerable interest in material science and astrochemistry. It was excited at 268 nm into the bright 2ππ* state. The dynamics was subsequently probed by time-resolved X-ray photoelectron (TR-XPS) and X-ray absorption (TR-XAS) spectroscopy at the nitrogen 1s edge. Two time constants of τ 1 ≈ 0.3 ps and τ 2 ≈ 3 ps were determined. The excited-state dynamics was simulated using the trajectory surface hopping method and computed TR-XAS to support the band assignments. The study reveals a sequential decay to the electronic ground state via internal conversion. Spectra recorded over longer delay times indicate a dissociation on a time scale of several hundred picoseconds.

We report vibrationally resolved threshold photoelectron spectra and ionization energies of the AlCH3 (7.89 ± 0.04 eV), GaCH3 (7.90 ± 0.02 eV), and GaOH (9.68 ± 0.02 eV). All molecules play a crucial role in the metal-organic vapor phase epitaxy synthesis of GaN and GaAlN thin films. To generate these reactive species, trimethylaluminum and trimethylgallium were pyrolyzed and their pyrolysis products were characterized by threshold photoelectron spectroscopy, utilizing the double imaging photoelectron photoion coincidence spectroscopy setup at the VUV beamline at the Paul Scherrer Institute. Vibrations were assigned in Franck-Condon simulations based on their computed geometries.

Radical-radical reactions play a crucial role in the molecular-weight growth that leads to the formation of polycyclic aromatic hydrocarbons (PAHs) and ultimately soot. In this study, we experimentally investigated the reaction between C6H5 (phenyl) and C3H3 (propargyl) at pressures around 30 Torr and combustion-relevant temperatures (∼1200 K). The reactants were generated through flash pyrolysis of nitrosobenzene and propargyl bromide in a resistively heated SiC tube. We identified the reaction intermediates and products using mass-selected threshold photoelectron spectroscopy (ms-TPES) with the photoelectron-photoion coincidence (PEPICO) instrument at the vacuum-ultraviolet (VUV) beamline of the Swiss Light Source at the Paul Scherrer Institute. Our findings indicate that C6H5 associates with C3H3 to form C9H8, which partially decomposes via hydrogen loss to yield C9H7 radicals. The C6H5 + C3H3 reaction is complex and yields more than just the most stable indene isomer. The experimental threshold photoelectron spectrum provides clear spectroscopic evidence of five isomers: indene, phenylallene, 1-phenyl-1-propyne, 1-phenyl-3-propyne, and cycloprop-2-en-1-ylbenzene. The inclusion of a sixth isomer, 3aH-indene, provides an even better fit to the experimental spectrum, although its presence should not be considered conclusive. All of these species correspond to minima on the known C9H8 potential energy surface [Selby et al., J. Phys. Chem. A, 2023, 127(11), 2577-2590 and Morozov et al., Phys. Chem. Chem. Phys. 2020, 22(13), 6868-6880]. Many of these isomers are not included in kinetic mechanisms that seek to describe the chemical pathways leading to PAHs.

We report a combined experimental and theoretical investigation into the fragmentation dynamics of the HNCS+ ion formed by single photon ionization, with relevance to its possible occurrence in astrochemical environments. Using valence electron–ion coincidence spectroscopy at He Iα (21.22 eV) and He IIα (40.81 eV) photon energies, along with complementary threshold photoelectron–photoion coincidence data, we identify electronic state specific dissociation channels involved in the molecular breakup. Electron spectra correlated with individual fragment ions form the basis of a breakdown diagram and, in conjunction with the valence photoelectron spectrum of HNCS, lead to the observation of an energy-dependent predissociation process. High-level calculated dissociation limits and cuts through the six-dimensional potential energy surfaces of the electronic states of the HNCS+ cation are compared to the experimental data. This makes it possible to suggest the unimolecular fragmentation pathways undergone by HNCS upon ionization and provides new kinetic information on HNCS+ ions, which will contribute to the refinement of astrochemical reaction network models. In particular, sulfur-containing species such as HNCS are increasingly recognized as potential tracers, for instance, in the atmospheres of M-dwarf exoplanets, where sulfur chemistry may play a key role in defining planetary habitability.

We report a study on the photoionization of the C6H6 isomer 3,4-dimethylenecyclobutene, DMCB. The molecule is an intermediate in the formation of benzene from the propargyl radical self-reaction, a suggested first step in the formation of polycyclic aromatic hydrocarbons and soot. From a threshold photoelectron spectrum we determine an adiabatic ionization energy of 8.75 eV. The geometry change upon ionization is associated with considerable vibrational activity, which is assigned to a symmetric in-plane bending mode of the =CH2 groups. A breakdown diagram shows that the dissociative photoionization resembles the one observed for benzene. Computations reveal that DMCB cation isomerizes to the benzene cation and dissociates from there.