Alexander K. Lemmens

Ionization of hydrogen-bonded molecular clusters can initiate reaction pathways that are inaccessible in isolated molecules, yet the role of local solvation in directing this chemistry remains poorly understood. Here, we employ weakly bound CO2–H2O clusters as experimentally tractable model systems to investigate how microsolvation influences ionization-driven molecular growth and bond formation. Using synchrotron-based vacuum ultraviolet photoionization mass spectrometry over the 10–14 eV photon-energy range, we investigate mixed CO2–H2O clusters and observe new oxygen-bearing products formed through molecular growth and bond reorganization. We detect m/z 78, assigned to [H2CO4]+, consistent with peroxide bond formation within mixed CO2–H2O clusters. We further identify an indirect route to H2O2 through cluster rearrangement and evaporation, evidenced by m/z 112 and assigned to [(H2O2)2(CO2)]+. Product branching depends strongly on cluster composition, with water-rich environments favoring peroxide-forming chemistry. Additional products, including formic acid and carbonic acid, reveal extensive ionization-induced molecular growth within hydrogen-bonded clusters. Density functional theory at the double-hybrid level reveals roaming-like intermediates, charge localization, and low-barrier rearrangements unique to the mixed cluster environment. Together, the experimental and theoretical results identify nanoscale CO–H2O aggregates as chemically active reactors.

Liquids exposed to vacuum through micrometer-scale apertures undergo rapid nonequilibrium phase evolution driven by flash evaporation, evaporative cooling, and transient solidification. Here we investigate the self-sustained freeze-sublime oscillatory behavior of pure acetone and acetone-water mixtures effusing through a 3 μm aperture into vacuum using single-photon ionization time-of-flight mass spectrometry. Periodic vapor emission events are observed on time scales of tens of seconds and are attributed to a recurring freeze-sublime cycle in which evaporative cooling induces transient solidification within the microchannel, temporarily suppressing flow until sublimation reopens the aperture. The temporal decay of acetone emission is analyzed using a Hertz-Knudsen evaporation framework, allowing estimation of the transient temperature profile and the characteristic dimensions of the frozen region. Addition of water substantially modifies the oscillatory dynamics, reducing the cycle frequency and producing delayed water emission relative to acetone, consistent with previously observed compositional segregation within the frozen plug. Heat-transfer simulations support the hypothesis of the formation of highly localized cooling zones near the liquid-vacuum interface capable of initiating transient solidification. These results establish micron-scale liquid-vacuum interfaces as nonequilibrium systems governed by evaporation, heat transport, and phase transitions, with implications for microfluidics, vacuum-interface chemistry, and outgassing processes in astrophysical environments.

Lignin pyrolysis generates oxygenated aromatics that can serve as renewable biofuel precursors. Anisole and substituted anisoles are widely used as model compounds to probe processes related to their production and use. We present a comparative study of anisole and 4-vinylanisole thermal decomposition using vacuum-ultraviolet photoionization mass spectrometry in a silicon carbide microreactor from 300 to 1300 K. For both molecular species, the initial decomposition pathway is O — CH 3 bond cleavage. The vinyl substituent lowers the bond dissociation energy, shifting decomposition to lower temperatures for 4-vinylanisole. Anisole pyrolysis produces hydrogen-atom, methyl, propargyl, phenoxy, and cyclopentadienyl radicals, phenol, cyclopentadiene, cyclopentadienyl dimer, methylcyclopentadiene, CO , acetylene, and benzene via methylcyclopentadiene. The products of 4-vinylanisole pyrolysis include hydrogen-atom, methyl, propargyl, fulvenallenyl, vinylphenoxy, vinylcyclopentadienyl, and cyclopentadienyl radicals, vinylphenol, vinylcyclopentadiene, dimethylfulvene, fulvenallene, vinylacetylene, CO , and acetylene, but no benzene. Vinyl substitution thus facilitates resonance-stabilized radical (RSR) formation at temperatures lower than the cracking temperature of anisole. Whereas 4-vinylanisole decomposes at a lower temperature than anisole, both species produce significant RSR products that facilitate aromatic growth and gas-to-condensed-phase hydrocarbon conversion, in contrast to their methyl-substituted analog. These new insights into RSR formation are applicable to lignin pyrolysis and may be useful for limiting char or tar formation during biofuel production and for developing predictive models of emissions from smoldering and propagating flaming wildfires.

The ultrafast dynamics of azulene have been debated for decades, with reported picosecond decay constants variously attributed to intramolecular vibrational redistribution (IVR), internal conversion, or rotational dephasing. Using polarization- and femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy with a nanosecond delay window, we disentangle this long-standing inconsistency and show that the early 2–5 ps decay component arises entirely from the rotational dephasing of an excited-state wavepacket. Identical time constants extracted from the decay of the parallel signal and the rise of the perpendicular signal across multiple vibronic origins provide an unambiguous rotational anisotropy signature, eliminating the need for IVR-based interpretations. Extending the measurement window to 1.3 ns reveals well-structured J-type and C-type rotational coherence revivals in S2 azulene on top of the well-documented fluorescence decay, demonstrating that both the short- and long-time dynamics contain information about the coherent rotational dynamics. These results show that azulene, and by extension polycyclic aromatic hydrocarbons, can sustain structured rotational coherence deep into the nanosecond regime, positioning PAHs as model systems for quantum-coherent wavepacket dynamics and providing a framework for probing coherence, decoherence, and rotational control in electronically rich molecular systems.

The photoionization of naphthalene (N)-carbon dioxide (CO₂) clusters was studied using tunable vacuum ultraviolet (VUV) radiation from a synchrotron in the photon range of 8.0 to 13.7 eV, in combination with time-of-flight mass spectrometry. Clusters of monomer, dimer, and trimer naphthalene with CO₂ (N(CO₂)_0-6, N2(CO₂)_0-3, N3) were observed. The lowest-energy conformers were obtained via a conformer search, followed by geometry optimizations at the ωB97X-V2/aug-cc-pVTZ (monomer) and ωB97X-V2/aug-cc-pVDZ (dimer) levels of theory. Carbon dioxide was found to preferentially cluster on top of the naphthalene molecule (in an out-of-plane configuration). From the mass spectra, photoionization intensity curves (PICs) were constructed, and appearance energies (AEs) were determined. No substantial trend in AE was observed with increasing size of the naphthalene-carbon dioxide clusters; rather, AE oscillations around the value for pure naphthalene were observed. These AE oscillations are also observed in a recently studied naphthalene-water cluster system, though in this system a slight downward trend in AE for pure naphthalene clusters (N1/2/3/4) was observed. The differences between the two systems are attributed to differing interaction strengths. Understanding these differences may aid in determining how photoprocessing can proceed differently depending on the dominant matrix component in interstellar ices.

This feature focuses on bridging isolated water and bulk water studies. It assembles experiments and theory on water clusters, mainly probed by vacuum ultraviolet (VUV) radiation, and summarizes what the effects are of intermolecular interactions on both the spectroscopy and the VUV-induced processes in water. In particular, it highlights studies of heterogeneous water clusters─those incorporating other molecular species such as naphthalene, methane, formic acid, and glycerol─which serve as model systems to investigate the fundamental roles of water in hydrogen bonding networks, proton transfer, and astrochemical processes. These mixed clusters provide a platform to revisit the stability of protonated water clusters and compile observations of ionization-induced structural rearrangements and fragmentation, especially in systems involving hydroxyl-rich cosolvents. This feature then explores energy transfer mechanisms in molecular clusters, following VUV photoexcitation. It concludes with potential future directions. First, we investigate excited-state dynamics in molecular clusters through direct probing, complementing the synchrotron studies discussed here. Second, exploring gas-phase molecule evaporation from confined spaces and interfaces using advanced spectroscopic techniques sheds light on these ubiquitous, yet currently debated, molecular processes.

Conical intersections (CIs), or diabolic points, play a critical role in fast electronic energy relaxation in aromatic molecules and their aggregates. While it is recognized that vibronic coupling drives the dynamics through CIs, there is a paucity of experimental information on the effect of vibrations on relaxation in molecular aggregates. Here, we describe experiments on the ultrafast electronic energy relaxation in the isolated anthracene dimer—an archetype of a π-stacked aromatic system relevant to intermolecular photon-driven processes such as those in optoelectronics. The combination of a molecular beam with femtosecond, tunable laser spectroscopy allows for both the necessary spectral and time resolution to investigate the relaxation of different vibrational levels in the excited state of the anthracene dimer. In general, our experimental observations corroborate predictions of the lifetimes of initial excited states in aromatic dimers. More specifically, we evaluate our results using time-dependent density functional theory, which identifies potential electronic relaxation pathways via CIs and supports multiple phenomenological interpretations for the observed lifetimes of different vibrational states in the electronically excited state manifold.

Thiourea-based organocatalysts offer a wide variety of benefits over conventional metal-based catalysts; among other things, they are cheap, harmless, and highly enantioselective. At the core of their catalytic activity is the ability to simultaneously coordinate reactants through hydrogen bonds and lower energy barriers. Detailed insight into the structure and conformational heterogeneity of the catalyst together with the coordination of the reactants is thus key to understanding the mechanisms by which organocatalysts perform their function. Here, using the Takemoto organocatalyst and 1-(2-nitroethyl)naphthalene as a prototypical catalyst-reactant system involved in Michael addition reactions, we show that mid-infrared spectroscopy covering the wide spectral range from 650 to 3500 cm^-1 combined with molecular beam experiments and density functional theory calculations is a powerful approach to meet these challenges. We determine the precise geometry adopted by the catalyst-nitroolefin complex, thereby providing direct information about the hydrogen bonds involved. Moreover, we solve a long-standing problem of the preferred conformation of the bare catalyst by providing an unambiguous determination of its structure. The general applicability of our approach holds great promise for extending it to elucidate other relevant complex reactions involving (organo)catalysts.