Direct Detection of C ₄ H ₂ Photochemical Products: Possible Routes to Complex Hydrocarbons in Planetary Atmospheres

Abstract

The photochemistry of diacetylene (C4H2), the largest hydrocarbon to be unambiguously identified in planetary atmospheres, is of considerable importance to understanding the mechanisms by which complex molecules are formed in the solar system. In this work, the primary products of C4H2's ultraviolet photochemistry were determined in a two-laser pump-probe scheme in which the products of C4H2 photoexcitation are detected by vacuum ultraviolet photoionization in a time-of-flight mass spectrometer. Three larger hydrocarbon primary products were observed with good yield in the C4H2* + C4H2 reaction: C6H2, C812, and C8H3. Neither C6H2 nor C8H3 is anticipated by current photochemical models of the atmospheres of Titan, Uranus, Neptune, Pluto, and Triton. The free hydrogen atoms that are released during the formation of the C8H3 and C8H2 products also may partially offset the role of C4H2 in catalysing the recombination of free hydrogen atoms in the planetary atmospheres.

H elimination and metastable lifetimes in the UV photoexcitation of diacetylene

“…In attempting to incorporate metastable reactions into atmospheric models, a lifetime of 1 ms has generally been used (10,12,28), and this has been cited as the lifetime of triplet acetylene determined indirectly by Klemperer and coworkers (29,30). Recent experiments have more directly examined the triplet lifetime in acetylene after ISC from specific rovibrational levels of S 1 and obtained a value on the order of 80-100 s that depends on excitation energy (31), just as we have seen here for diacetylene.…”

“…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)(3)(4). It is well established that the formation of diacetylene is initiated by photodissociation of acetylene below 217 nm (2,(5)(6)(7)(8) according to the following reaction mechanism: C 2 H 2 ϩ hv ¡ C 2 H ϩ H͑ Ͻ 217 nm͒ C 2 H ϩ C 2 H 2 ¡ C 4 H 2 ϩ H 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)(10)(11)(12). Understanding the dynamics of diacetylene photoexcitation is thus key to revealing the factors driving the chemistry of Titan's atmosphere.…”

“…The diacetylene dissociation quantum yield was ascribed to reactivity of a long-lived metastable form, universally assumed to be the lowest triplet state. Subsequently, Zwier and coworkers (10,13) extensively investigated the UV photoinduced chemistry of diacetylene through reactions in a ceramic nozzle with a VUV probe of the products downstream. After excitation of the 1 ⌬ u excited state, secondary reactions were found to lead to the formation of various larger hydrocarbons (12,14,15).…”

Ultraviolet Photochemistry of Diacetylene with Alkynes and Alkenes: Spectroscopic Characterization of the Products

“…In fact, our earlier work 3,4 showed that when N 2 was used as buffer gas, C 8 H 3 was formed in greater yield at the expense of C 8 H 2 formation, consistent with reaction occurring from a lower-energy C 4 H 2 * distribution in the presence of N 2 .…”

Theoretical investigation of the infrared spectrum of small polyynes

“…[20][21][22][23][24][25] They are of particular interest for astronomers, because they are believed to act as the ultraviolet (UV) shield in hydrocarbon-rich atmospheres, [26][27][28][29] and in the formation and destruction of polycyclic aromatic hydrocarbons (PAHs), 18,30-33 a major reservoir of carbon in the universe. In astronomical environments, the formation of long chain polyynes from acetylene is believed to occur through polymerization reactions, 19,34,35 HC 2n H + C 2 H -HC 2n+2 H + H,…”