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H Elimination and Metastable Lifetimes in the UV Photoexcitation Of H elimination and metastable lifetimes in the UV SPECIAL FEATURE photoexcitation of diacetylene R. Silva†, W. K. Gichuhi†, C. Huang†, M. B. Doyle†, V. V. Kislov‡, A. M. Mebel‡, and A. G. Suits†§ †Department of Chemistry, Wayne State University, Detroit, MI 48202; and ‡Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199 Edited by F. Fleming Crim, University of Wisconsin, Madison, WI, and approved May 12, 2008 (received for review February 11, 2008) We present an experimental investigation of the UV photochem- ever, at the time, the published thermochemical thresholds were istry of diacetylene under collisionless conditions. The H loss in error, and it is now known that the threshold for H elimination channel is studied using DC slice ion imaging with two-color is 5.77 eV (215 nm), significantly higher than the energy assumed reduced-Doppler detection at 243 nm and 212 nm. The photochem- by Glicker and Okabe. The diacetylene dissociation quantum istry is further studied deep in the vacuum UV, that is, at Lyman- yield was ascribed to reactivity of a long-lived metastable form, ,alpha (121.6 nm). Translational energy distributions for the H ؉ C4H universally assumed to be the lowest triplet state. Subsequently product arising from dissociation of C4H2 after excitation at 243, Zwier and coworkers (10, 13) extensively investigated the UV 212, and 121.6 nm show an isotropic angular distribution and photoinduced chemistry of diacetylene through reactions in a characteristic translational energy profile suggesting statistical ceramic nozzle with a VUV probe of the products downstream. 1 dissociation from the ground state or possibly from a low-lying After excitation of the ⌬u excited state, secondary reactions triplet state. From these distributions, a two-photon dissociation were found to lead to the formation of various larger hydrocar- process is inferred at 243 nm and 212 nm, whereas at 121.6 nm, a bons (12, 14, 15). The laser-based studies, principally at 231 and one-photon dissociation process prevails. The results are inter- 243 nm, also found no evidence for radical products proceeding preted with the aid of ab initio calculations on the reaction from primary photodissociation of diacetylene (1, 11, 16–18). pathways and statistical calculations of the dissociation rates and Although triplet diacetylene reactions invoked to account for the product branching. In a second series of experiments, nanosecond observed chemistry are now often incorporated in models of time-resolved phototionization measurements yield a direct de- Titan’s atmosphere, with an assumed metastable lifetime of 1 ms termination of the lifetime of metastable triplet diacetylene under or more, to clarify their role, the triplet lifetime must be collisionless conditions, as well as its dependence on excitation measured directly and as a function of excitation energy (19). We energy. The observed submicrosecond lifetimes suggest that reac- present such measurements here. tions of metastable diacetylene are likely to be less important in Several other theoretical models and experiments have exam- Titan’s atmosphere than previously believed. ined the secondary photochemistry of C4H2 (2, 20, 21). However, for a clear understanding of the role of diacetylene in Titan’s ion imaging ͉ photochemistry ͉ Titan CHEMISTRY atmosphere it is essential to have a better knowledge of its primary photochemistry (product branching and energy depen- aturn’s moon, Titan, is the only solar system body besides dence), in addition to the electronic decay pathways and rates. SEarth and Venus with a dense atmosphere (1, 2). It is widely In this article, we report the experimental results for primary considered as a natural laboratory on the planetary scale in C4H2 photodissociation and metastable lifetimes under colli- understanding the prebiotic chemistry on proto-Earth. Diacety- sionless conditions. The experiments are supported by a series of lene is believed to play a key role in the formation of polyynes ab initio and Rice–Ramsperger–Kassel–Marcus (RRKM) calcu- and polycyclic aromatic hydrocarbons (PAHs) that partially lations to assist in interpretation of the results (22). comprise the haze layer in Titan’s upper atmosphere (2–4). It is well established that the formation of diacetylene is initiated by Results and Discussion photodissociation of acetylene below 217 nm (2, 5–8) according To facilitate the following discussion, computed stationary to the following reaction mechanism: points and dissociation asymptotes for ground-state diacetylene C H ϩ hv ¡ C H ϩ H͑␭ Ͻ 217 nm͒ are shown in Fig. 1. In the first series of experiments, DC-sliced 2 2 2 ion images of H atoms from diacetylene photodissociation at ϩ ¡ ϩ C2H C2H2 C4H2 H three different wavelengths were recorded (Fig. 2). Background signals were subtracted from the raw images and total center- The importance ascribed to diacetylene arises in part because it of-mass translational energy distributions were derived from the absorbs light at longer wavelengths, where the solar flux is refined data (Fig. 3). The images all show isotropic angular higher, than any other major constituents of Titan’s atmosphere; distributions. The distributions at all three wavelengths studied moreover, experimental results suggest it is still photochemically (243 nm, 212 nm, and 121.6 nm) have peaks Ϸ0.45 eV and decay reactive even well below the threshold for dissociation (9–12). to higher recoil energies, extending to 3–4 eV for the 243 and Understanding the dynamics of diacetylene photoexcitation is 121.6 nm results and to Ϸ5 eV for the 212 nm dissociation. The thus key to revealing the factors driving the chemistry of Titan’s isotropic angular distributions and structureless translational atmosphere. energy distributions peaking at low energy are typical of the To date, no experiments on the photochemistry of diacetylene have been performed under collisionless conditions. In a pio- neering study, Glicker and Okabe (9) determined a quantum Author contributions: A.M.M. and A.G.S. designed research; R.S., W.K.G., C.H., M.B.D., yield of 2.0 Ϯ 0.5 for diacetylene photodissociation in the V.V.K., and A.M.M. performed research; and R.S., W.K.G., M.B.D., A.M.M., and A.G.S. wrote wavelength region of 147–254 nm. Between 184 and 254 nm, no the paper. free-radical products were detected and polymeric material was The authors declare no conflict of interest. found to coat the inside of the reaction cell. The upper limit for This article is a PNAS Direct Submission. § the quantum yield of C4H formation was then determined to be To whom correspondence should be addressed. E-mail: [email protected]. only 0.06 at 228 nm based on experimental uncertainty. How- © 2008 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0801180105 PNAS ͉ September 2, 2008 ͉ vol. 105 ͉ no. 35 ͉ 12713–12718 Downloaded by guest on September 28, 2021 Fig. 1. Profile of the ground-state potential energy surface of diacetylene calculated at the CCSD(T)/CBSϩ ZPE(B3LYP/6-311G**) level of theory. statistical, barrierless hydrogen elimination process (23) and species. In the introduction we mentioned the importance suggest dissociation on the ground electronic state or possibly ascribed to metastable diacetylene in Titan’s atmosphere. If from the lowest triplet. As shown in Fig. 1, the C–H bond in reactive C4H2* is very long-lived, its contribution to the chem- diacetylene is very strong, with a dissociation energy of 133 istry in Titan’s stratosphere will clearly be much greater than if kcal/mol. The threshold wavelength for single-photon dissocia- intersystem crossing (ISC) takes it to the unreactive ground state tion of diacetylene is thus Ϸ215 nm, whereas at 243 nm the before it has an opportunity to encounter a suitable reaction single-photon energy is only 118 kcal/mol. Single-photon disso- partner (e.g., some other unsaturated hydrocarbon.) ciation at 243 nm is clearly not possible. However, if C4H2 To examine these issues, first, we consider the possible excited absorbs two photons at 243 nm, dissociation to C4H ϩ His states involved. Vila et al. (22) have calculated energies and possible with a total excess energy of 103 kcal/mol. Such a geometries for a range of low-lying excited states of diacetylene process is consistent with the translational energy distribution in by using CASSCF and CASMP2 methods and we draw on their Fig. 3, which extends nearly to this limit. results for this discussion. If we consider first the linear geometry We can directly compare this result with that obtained at the of the ground state, the initial excitation is to the second singlet same two-photon energy by considering dissociation (and probe) state. This is the only low-lying excited state that is linear. at 121.6 nm. The kinetic energy distribution recorded at this Internal conversion (IC) may then populate the first singlet state, wavelength is also shown in Fig. 3 and is nearly superimposable or ISC may take the system to one of several triplet states. IC in on that obtained at 243 nm. We thus conclude that 243 nm the triplet manifold will then result in formation of the lowest production of H atom from diacetylene likely results from triplet (T1), generally regarded as the identity of the long-lived absorption of two photons, but at 121.6 nm, it is a single-photon metastable species. We should note that these other excited dissociation. states split into cis and trans and even nonplanar isomers fairly We next consider 212 nm, which is several kilocalories per close in energy, but with significant associated relaxation energy mole above the threshold for H loss in diacetylene, the lowest in some cases.
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