H elimination and metastable lifetimes in the UV photoexcitation of diacetylene

R. Silva, W. K. Gichuhi, C. Huang, M. B. Doyle, V. V. Kislov, A. M. Mebel, A. G. Suits

Research output: Contribution to journalArticlepeer-review

20 Scopus citations

Abstract

We present an experimental investigation of the UV photochemistry of diacetylene under collisionless conditions. The H loss channel is studied using DC slice ion imaging with two-color reduced-Doppler detection at 243 nm and 212 nm. The photochemistry is further studied deep in the vacuum UV, that is, at Lymanalpha (121.6 nm). Translational energy distributions for the H + C 4H product arising from dissociation of C4H2 after excitation at 243, 212, and 121.6 nm show an isotropic angular distribution and characteristic translational energy profile suggesting statistical dissociation from the ground state or possibly from a low-lying triplet state. From these distributions, a two-photon dissociation process is inferred at 243 nm and 212 nm, whereas at 121.6 nm, a one-photon dissociation process prevails. The results are interpreted with the aid of ab initio calculations on the reaction pathways and statistical calculations of the dissociation rates and product branching. In a second series of experiments, nanosecond time-resolved phototionization measurements yield a direct determination of the lifetime of metastable triplet diacetylene under collisionless conditions, as well as its dependence on excitation energy. The observed submicrosecond lifetimes suggest that reactions of metastable diacetylene are likely to be less important in Titan's atmosphere than previously believed.

Original languageEnglish (US)
Pages (from-to)12713-12718
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume105
Issue number35
DOIs
StatePublished - Sep 2 2008

Keywords

  • Ion imaging
  • Photochemistry
  • Titan

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