Musahid Ahmed

Nonalternant polycyclic aromatic hydrocarbons (PAHs) incorporating five-membered rings are fundamental intermediates linking planar aromatics to curved carbon nanostructures including fullerenes and nanotubes, yet their formation mechanisms are not fully understood. The detection of fluoranthene (C16H10) in samples from the near-Earth carbonaceous asteroid 162,173 Ryugu and 101,955 Bennu highlights the need for chemically viable pathways under both combustion and astrochemical conditions. Here, we demonstrate that fluoranthene forms efficiently commencing with barrierless recombination of resonance-stabilized indenyl (C9H7•) and benzyl (C7H7•) radicals. Microreactor experiments combined with electronic structure calculations reveal rapid carbon–carbon bond formation, cyclization, and dehydrogenation to the nonalternant framework. This mechanism represents an alternative to stepwise acetylene addition and enables early incorporation of pentagonal motifs during molecular growth. These findings establish radical–radical recombination as a versatile route to nonalternant PAHs, providing a unified framework for their formation in high-temperature combustion systems and carbon-rich astrophysical environments like circumstellar envelopes of carbon stars and planetary nebulae as their descendants.

Crystals formed in aqueous solutions often encode chemical memories of their growth environment. However, the molecular pathway of element partitioning that links solution chemistry to solid composition remains elusive when growth proceeds through nonclassical intermediates. In this study, we investigated calcite growth mechanisms in the presence of aqueous manganese (Mn2+) under chemostat conditions using a custom titration method. At near-neutral pH, where bicarbonate ions dominate, we show that Mn2+ triggers the formation of bicarbonate-rich, ion-paired liquid condensates that accumulate at the calcite–water interface and control Mn incorporation into the solid phase. Time-resolved in situ X-ray scattering and Raman spectroscopy, combined with cryogenic transmission electron microscopy and molecular dynamics simulations, reveal a pathway in which these condensates deprotonate and dehydrate into Mn-bearing amorphous carbonate that crystallizes into polycrystalline calcite–rhodochrosite interfacial solid solutions with progressively increasing Mn content. These results identify an interfacial, bicarbonate-rich liquid condensate as a mechanistic link between crystallization pathway and element partitioning, suggesting that nanoscale interfacial fluid organization can dominate selective ion uptake during crystallization in multicomponent aqueous environments that are relevant to geoscience, environmental science, chemistry, or materials science.

X-ray spectroscopy and quantum simulations establish a direct probe of the thermal and structural evolution of rapidly evaporating aerosols through temperature-dependent oxygen K-edge NEXAFS.

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.

Polycyclic aromatic hydrocarbons (PAHs) represent key molecular building blocks of carbonaceous nanoparticles and have been identified in cold molecular clouds (TMC‐1), meteorites (Murchison, Allende), and asteroids (Bennu, Ryugu). However, the understanding of their formation and molecular mass growth processes in these extreme environments has remained elusive, especially with the emergence of resonantly‐stabilized free radical (RSFR)‐mediated pathways. Here, we report a combined experimental and computational study on the self‐recombination of the aromatic and resonantly‐stabilized 1‐indenyl radical (C ₉ H ₇ ^• ) isomer‐selectively forming the 18π‐Hückel PAH chrysene (C ₁₈ H ₁₂ ) in an overall endoergic reaction. These features account for the circumstellar origin of chrysene in pristine samples of the carbonaceous asteroid Ryugu and of the Murchison meteorite. At the microscopic level, the ring expansion involving cyclopentadienyl moieties to two six‐membered aromatic rings via pivotal spiroaromatic intermediates affords a versatile, RSFR‐initiated mass growth channel essentially leading to graphene‐type PAHs and eventually two‐dimensional nanostructures in high temperature circumstellar envelopes and in combustion processes.

Polycyclic aromatic hydrocarbons (PAHs) represent key molecular building blocks of carbonaceous nanoparticles and have been identified in cold molecular clouds (TMC‐1), meteorites (Murchison, Allende), and asteroids (Bennu, Ryugu). However, the understanding of their formation and molecular mass growth processes in these extreme environments has remained elusive, especially with the emergence of resonantly‐stabilized free radical (RSFR)‐mediated pathways. Here, we report a combined experimental and computational study on the self‐recombination of the aromatic and resonantly‐stabilized 1‐indenyl radical (C ₉ H ₇ ^• ) isomer‐selectively forming the 18π‐Hückel PAH chrysene (C ₁₈ H ₁₂ ) in an overall endoergic reaction. These features account for the circumstellar origin of chrysene in pristine samples of the carbonaceous asteroid Ryugu and of the Murchison meteorite. At the microscopic level, the ring expansion involving cyclopentadienyl moieties to two six‐membered aromatic rings via pivotal spiroaromatic intermediates affords a versatile, RSFR‐initiated mass growth channel essentially leading to graphene‐type PAHs and eventually two‐dimensional nanostructures in high temperature circumstellar envelopes and in combustion processes.

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.