Photodissociation Pathways of Vinylacetylene (C ₄ H ₄ ) and Implications for the Interstellar Hydrocarbon Chemistry
- 1University of Hawai’i at Manoa
- 2Florida International University
- 3Sapienza Università di Roma
Abstract
Vinylacetylene (C ₄ H ₄ ; CH ₂ =CH–C≡CH) represents a key molecular building block in the hydrogen abstraction–vinylacetylene addition pathway leading barrierlessly to polycyclic aromatic hydrocarbons (PAHs) via benzene ring annulation. Although the formation of vinylacetylene in cold molecular clouds is well understood and proceeds via the barrierless and exoergic neutral–neutral reaction of ethynyl radicals (C ₂ H) with ethylene (C ₂ H ₄ ), its photodissociation channels have been poorly constrained. Here, we investigate the photodissociation dynamics of vinylacetylene at 193 nm under collision-free conditions using a supersonic molecular beam coupled with a rotatable mass-spectrometric detector. Time-of-flight spectra were recorded for m / z = 51, 50, 26, and 25 and analyzed through forward-convolution fitting. Three primary dissociation pathways are identified: H loss yielding C ₄ H ₃ radicals, H ₂ loss forming C ₄ H ₂ isomers, and C–C bond cleavage producing C ₂ H ₂ /CCH ₂ fragments, with experimental branching ratios of C ₄ H ₃ :C ₄ H ₂ :C ₂ H ₂ /CCH ₂ = (74 ± 2):(12 ± 2):(14 ± 1). For the dominant H loss channel, the high-energy cutoff of the translational energy distribution is consistent with formation of trans -/ cis -1-buten-3-yne-1-yl (CH=CH–C≡CH), whereas RRKM calculations predict that the resonance-stabilized 1-buten-3-yne-2-yl radical (H ₂ C=C=C=CH) is the dominant statistical product. These findings can be reconciled if the latter is formed with substantial rovibrational excitation, reducing the energy released into translation. The molecular hydrogen elimination channel is assigned mainly to the formation of butatrienylidene (H ₂ C=C=C=C:), while C–C bond-cleavage is consistent with the formation of acetylene plus vinylidene. These findings provide isomer-specific laboratory constraints on the photodissociation of vinylacetylene and help refine astrochemical models of the hydrocarbon evolution and PAH growth in low-temperature cold molecular clouds like TMC-1.
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BibTeX
@article{metya2026photodissociation,
title = {{Photodissociation Pathways of Vinylacetylene (C ₄ H ₄ ) and Implications for the Interstellar Hydrocarbon Chemistry}},
author = {Surajit Metya and Daniel González and Alexander N. Morozov and Alexander M. Mebel and Domenico Stranges and Ralf I. Kaiser},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aea41c},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea41c},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Photodissociation Pathways of Vinylacetylene (C ₄ H ₄ ) and Implications for the Interstellar Hydrocarbon Chemistry
AU - Surajit Metya
AU - Daniel González
AU - Alexander N. Morozov
AU - Alexander M. Mebel
AU - Domenico Stranges
AU - Ralf I. Kaiser
PY - 2026
DA - 2026/10/12
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aea41c
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aea41c
AB - Vinylacetylene (C ₄ H ₄ ; CH ₂ =CH–C≡CH) represents a key molecular building block in the hydrogen abstraction–vinylacetylene addition pathway leading barrierlessly to polycyclic aromatic hydrocarbons (PAHs) via benzene ring annulation. Although the formation of vinylacetylene in cold molecular clouds is well understood and proceeds via the barrierless and exoergic neutral–neutral reaction of ethynyl radicals (C ₂ H) with ethylene (C ₂ H ₄ ), its photodissociation channels have been poorly constrained. Here, we investigate the photodissociation dynamics of vinylacetylene at 193 nm under collision-free conditions using a supersonic molecular beam coupled with a rotatable mass-spectrometric detector. Time-of-flight spectra were recorded for m / z = 51, 50, 26, and 25 and analyzed through forward-convolution fitting. Three primary dissociation pathways are identified: H loss yielding C ₄ H ₃ radicals, H ₂ loss forming C ₄ H ₂ isomers, and C–C bond cleavage producing C ₂ H ₂ /CCH ₂ fragments, with experimental branching ratios of C ₄ H ₃ :C ₄ H ₂ :C ₂ H ₂ /CCH ₂ = (74 ± 2):(12 ± 2):(14 ± 1). For the dominant H loss channel, the high-energy cutoff of the translational energy distribution is consistent with formation of trans -/ cis -1-buten-3-yne-1-yl (CH=CH–C≡CH), whereas RRKM calculations predict that the resonance-stabilized 1-buten-3-yne-2-yl radical (H ₂ C=C=C=CH) is the dominant statistical product. These findings can be reconciled if the latter is formed with substantial rovibrational excitation, reducing the energy released into translation. The molecular hydrogen elimination channel is assigned mainly to the formation of butatrienylidene (H ₂ C=C=C=C:), while C–C bond-cleavage is consistent with the formation of acetylene plus vinylidene. These findings provide isomer-specific laboratory constraints on the photodissociation of vinylacetylene and help refine astrochemical models of the hydrocarbon evolution and PAH growth in low-temperature cold molecular clouds like TMC-1.
SN - 0004-637X
SN - 1538-4357
ER -