The primary reactions of the lowest energy triplet states of diacetylene (C4H2*) with 1,3-butadiene (C4H6) in a helium buffer are characterized with a molecular beam pump−probe technique. Triplet diacetylene is prepared in the early portions of a molecular expansion by laser excitation of the 2 061 0 band of the 1Δu ← X1Σ+ g transition in C4H2 at 231.5 nm, which rapidly interconverts to high vibrational levels of the lowest energy triplet surfaces. The subsequent reactions with C4H6 are allowed to proceed for 20 μs while the expansion traverses a short ceramic reaction tube or slit channel. Primary products are observed by quenching secondary processes as molecular collisions cease outside the tube. The major photochemical products C6H6 and C8H6 are detected in a linear time-of-flight mass spectrometer using both vacuum ultraviolet photoionization and resonant two-photon ionization (R2PI). R2PI spectra of the C6H6 and C8H6 products unambiguously identify them as benzene and phenylacetylene, respectively. Based on deuterium substitution experiments, a mechanism for these ring-forming reactions is proposed. The potential importance of these reactions for forming aromatics in sooting flames and planetary atmospheres is discussed.

The primary reaction products formed when metastable diacetylene (C 4 H 2 *) reacts with ground-state diacetylene (C 4 H 2 ), ethylene (C 2 H 4 ), and propene (CH 3 -CHdCH 2 ) have been spectroscopically characterized using a laser pump-probe scheme. Reaction is initiated in a constrained expansion which limits reaction times to about 20 µs. The molecular structures of the C 6 H 2 product formed in the C 4 H 2 * + C 4 H 2 reaction is identified from its R2PI spectrum and two-photon ionization threshold (9.49 ( 0.01 eV) as triacetylene (H-CtCCtC-CtC-H). The analogous scans of the C 8 H 2 product show a clear two-photon ionization onset at 9.09 ( 0.02 eV, but little resolved vibronic structure above this threshold, consistent with its assignment as tetraacetylene. The R2PI spectra of the C 6 H 4 product formed in the reactions of C 4 H 2 * with C 2 H 4 and CH 3 -CHdCH 2 lead to an identification of this product as 1-hexene-3,5-diyne (CH 2 dCH-CtC-CtCH). The C 7 H 6 product formed in the reaction of C 4 H 2 * with CH 3 -CHdCH 2 possesses an R2PI spectrum close to that of 1-hexene-3,5-diyne, supporting its identification as a methylated derivative with the same ene-diyne structure. Two C 7 H 6 isomers meet this criterion, 5-heptene-1,3-diyne (CH 3 -CHdCH 2 -CtC-CtC-H) and 2-methyl-1-hexene-3,5-diyne (H 2 CdC(CH 3 )-CtC-CtC-H), between which we cannot distinguish. Implications for the mechanisms for these C 4 H 2 * reactions are discussed.