Abstract
Chemical processing in the stratospheres of the gas giants is driven by incident vacuum ultraviolet (VUV) light. Ethane is an important constituent in the atmospheres of the gas giants in our solar system. The present work describes translational spectroscopy studies of the VUV photochemistry of ethane using tuneable radiation in the wavelength range 112 ≤ λ ≤ 126 nm from a free electron laser and event-triggered, fast-framing, multi-mass imaging detection methods. Contributions from at least five primary photofragmentation pathways yielding CH2, CH3 and/or H atom products are demonstrated and interpreted in terms of unimolecular decay following rapid non-adiabatic coupling to the ground state potential energy surface. These data serve to highlight parallels with methane photochemistry and limitations in contemporary models of the photoinduced stratospheric chemistry of the gas giants. The work identifies additional photochemical reactions that require incorporation into next generation extraterrestrial atmospheric chemistry models which should help rationalise hitherto unexplained aspects of the atmospheric ethane/acetylene ratios revealed by the Cassini–Huygens fly-by of Jupiter.
Highlights
Understanding, and perhaps one day exploiting, the environment of extraterrestrial bodies is a central objective of planetary science
Chemical processing in the stratospheres of the gas giants is driven by incident vacuum ultraviolet (VUV) light,[4] even in the distant, gas-poor giants Uranus and Neptune.[5]
We further note that the substantial ($60%) branching into C2Hx species following VUV photoexcitation of C2H6 assumed in the current planetary atmospheric photochemistry models derives from indirect measurements made more than half a century ago, and is predicated on an assumption that the decomposition of the internally excited C2H#4 species formed via reaction (1) would mimic that deduced following VUV photoexcitation of strategically deuterated ethene (CH2CD2) molecules.[16,17]
Summary
Understanding, and perhaps one day exploiting, the environment of extraterrestrial bodies is a central objective of planetary science.
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