H elimination and metastable lifetimes in the UV photoexcitation of diacetylene

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 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...

Photodissociation dynamics of C4H2 at 164.41 nm: Competitive dissociation pathways

“…The characteristic translational energy profile, with the peak at about 2000 cm −1 and extending to the energetic limit, is quite similar to that observed in 121.6 photolysis of C 4 H 2 reported by Suits and co-workers. 8 This suggests the statistical component should come from the dissociation process on the ground state following IC from initially excited Rydberg state to the ground state S 0 . Such dissociation time is long enough to allow for complete internal energy randomization.…”

“…However, the recently observed submicron-second lifetimes suggested that reactions of metastable C 4 H 2 are likely to be less important in Titan's atmosphere than previously believed. 8 The second spectrum region, between 115 and 175 nm, shows a succession of several strong absorption features, which belong to the R and R Rydberg series. For example, an intense peak observed at 164.4 nm has been assigned by Smith et al to the R 1 u ← 1 g + transition.…”

“…8 Spectroscopic studies of C 4 H 2 are abundant in the UV and VUV region. [9][10][11] The spectrum between 195 and 225 nm shows structured vibrational progressions, which is assigned to a 1 u ← 1 g + forbidden electronic transition.…”

Photoinduced C–H bond fission in prototypical organic molecules and radicals

“…Reactions involving metastable C4H2* molecules formed by such photoexcitation were touted as a possible route to forming the larger polyynes and polycyclic aromatic hydrocarbons that contribute to the haze that cloaks Titan, 30 but such a view was challenged following later measurements that showed that these C4H2* states have sub-s lifetimes. 31 The TKER spectra of the H + C4H products formed when exciting C4H2 at shorter wavelengths (in the range 127.5    164.4 nm) chosen to match with resolved Rydberg features in the parent absorption spectrum show peaks sitting on a continuous background. 32,33 Such structure reflects the formation of excited C4H(A 2 ) state radicals, with specific vibrational excitation in the CCC bend and CC stretch modes.…”

Chemical reaction dynamics

“…Again, the interaction of multiple potential energy surfaces, including the formation of conical intersections, is an essential feature of the system. A study of the photodissociation of diacetylene uses several excitation wavelengths, including one deep in the UV region, and observes the recoiling hydrogen atoms with ion imaging (12).…”