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 C 4H2 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.ion imaging ͉ photochemistry ͉ Titan S aturn's moon, Titan, is the only solar system body besides Earth and Venus with a dense atmosphere (1, 2). It is widely considered as a natural laboratory on the planetary scale in understanding the prebiotic chemistry on proto-Earth. Diacetylene is believed to play a key role in the formation of polyynes and polycyclic aromatic hydrocarbons (PAHs) that partially comprise the haze layer in Titan's upper atmosphere (2-4). It is well established that the formation of diacetylene is initiated by photodissociation of acetylene below 217 nm (2, 5-8) according to the following reaction mechanism:The importance ascribed to diacetylene arises in part because it absorbs light at longer wavelengths, where the solar flux is higher, than any other major constituents of Titan's atmosphere; moreover, experimental results suggest it is still photochemically reactive even well below the threshold for dissociation (9-12). Understanding the dynamics of diacetylene photoexcitation is thus key to revealing the factors driving the chemistry of Titan's atmosphere.To date, no experiments on the photochemistry of diacetylene have been performed under collisionless conditions. In a pioneering study, Glicker and Okabe (9) determined a quantum yield of 2.0 Ϯ 0.5 for diacetylene photodissociation in the wavelength region of 147-254 nm. Between 184 and 254 nm, no free-radical products were detected and polymeric material was found to coat the inside of the reaction cell. The upper limit for the quantum yield of C 4 H formation was then determined to be only 0.06 at 228 nm based on experimental uncertainty. However, at the time, t...