Those who develop the models to account for the observed properties of the planetary atmospheres are faced with much the same challenge as the combustion community. Major constituents of the atmosphere can be determined from remote observation, but some of the most intriguing and potentially important properties of these atmospheres arise from the cumulative effects of many minor constituents whose concentrations are difficult to determine, and whose formation and destruction pathways are much less well-defined. Titan, one of the moons of Saturn, has an atmosphere that is reminiscent in some respects to that postulated for pre-biotic earth. It is the only other planet or moon in our solar system with a predominantly nitrogen atmosphere; however, unlike earth, it has a high hydrocarbon content (~3% methane), and almost no oxygen. The chemical composition of Titan’s atmosphere is driven predominantly by photochemical processes, which has produced a rich mixture of complex hydrocarbons. A portion of one of the recent photochemical models of Titan is shown to the right. The Cassini satellite mission is currently in orbit around Saturn, and has made several close fly-by’s of Titan. In December, 2004, the Huygens probe was released from Cassini, and dropped through the atmosphere of Titan to its surface, recording as it dropped the altitude-dependent concentrations of the atmosphere’s constituents with a sensitivity almost 1000 times greater than that from remote observation.

The third major thrust of our research for the past several years has been to carry out laboratory photochemical studies in support of this mission. Using the molecular beam time-of-flight mass spectrometer shown to the right and an array of lasers tunable from the infrared to the vacuum ultraviolet, we have particularly concentrated on photochemical processes on the large-molecule end of current photochemical models.

Much of our early work involved studies of diacetylene, C₄H₂, the first member of the poly-ynes, which is a known constituent of Titan’s atmosphere, and is extremely photochemically reactive. Diacetylene plays a role not unlike that of ozone in our atmosphere, absorbing ultraviolet radiation at longer wavelengths than other abundant constituents, and acting as a precursor to the formation of larger hydrocarbon species. Much of the photochemistry of C₄H₂ was ill-characterized prior to our work, because its efficient photopolymerization led to bulk polymer. By co-expanding C₄H₂ and a reaction partner into a short reaction tube attached to the end of a pulsed valve, photochemical reaction could be initiated with an ultraviolet laser pulse that intersected the mixture while it was traversing the reaction tube. Reaction was carried out at temperatures relevant to Titan’s atmosphere (~70-100 K) due to the cooling that accompanied expansion into the reaction tube. Furthermore, because the reaction mixture traversed the tube in about 20 μsec, the photochemical reactions were shut off when the reaction mixture expanded into vacuum. This enabled the study of the first photochemical steps. The reaction mixture expands into the ion source region of the time-of-flight mass spectrometer, where photoionization occurs. Vacuum ultraviolet radiation is used to gently photoionize the products with a single photon (118 nm light = 10.5 eV), thereby determining the mass-to-charge ratios of the photochemical products. In many cases, information beyond the mass-to-charge ratio is needed for structural characterization. This is achieved using resonant two-photon ionization, followed by our infrared-ultraviolet double resonance schemes as needed. Diacetylene photochemistry in the ultraviolet occurs out of metastable triplet states. Reactions of triplet C₄H₂ with unsaturated hydrocarbons lead to a rich array of poly-yne, enyne, free radical and aromatic products. The reaction of C₄H₂* with 1,3-butadiene produces benzene and phenylacetylene as major products. We have since then been exploring a range of other photochemical reactions that could lead to aromatics, and have successfully found several in the reactions of diacetylene, vinylacetylene (C₄H₄) and 1,3-butadiene (C₄H₆).

Finally, we have recently initiated a series of studies comparing the photochemistry of reaction mixtures with the corresponding discharge-driven chemistry carried out under the same conditions. We have outfitted a similar reaction tube with concentric discharge plates between which a pulsed discharge can be turned on and off on fast timescales. Our goal is to not only learn more about the chemistry associated with lightning in Titan’s atmosphere (which has been observed by the Cassini mission), but also to put some quantitative data behind what has traditionally been an area lacking such characterization, and hence ripe for speculation regarding their results. The early discharge experiments by Urey and Miller, which caused such controversy over the formation of amino acids through spark discharge of pre-biotic gas phase mixtures, and the later discharge experiments from Sagan on ‘tholins’ did little to characterize anything beyond the final solid products formed after long exposure to the discharge. Our experiment characterizes the early-time products formed in a 10 μsec exposure to a discharge with well-defined properties. Of course, the chemistry can still be quite complicated, but we now have characterized not only the primary neutral dissociation products but a broad swath of the larger molecules formed in discharges of vinylacetylene and 1,3-butadiene, concentrating particularly on the rich array of aromatic and free radical products formed. Here R2PI spectroscopy is crucial to proper identification of the products, since a myriad of neutral products share the same molecular formula. We are currently studying the spectroscopy and excited state photophysics of a series of hydronaphthalene radicals using these methods.