The long wavelength range temperature variations of the mid-UV acetylene absorption coefficient

In the reductive atmospheres of the giant planets and Titan, acetylene is k nown to be the major unsaturated hydrocarbon. It is of great importance to determine precisely and to model its abundance profile in order to be able to fully understand the chemistry of these environments. To achieve this ta sk one needs the knowledge of the absorption coefficient in IR and UV which are complementary wavelength ranges for studying atmospheres. The mid-UV a bsorption coefficient is of special importance when trying to model the pho to-dissociation of C2H2 because of the rise of the solar flux above 200 nm. We have previously shown that the most recent data on acetylene cross secti ons had to be taken with caution because of the presence of acetone bands i n the published spectra. Moreover, absolute absorption coefficient of C2H2 is poorly known above 200 nm. Consequently, we have measured C2H2 absorptio n coefficient in the 185-235 nm range, at 295 and 173 K. We present the obt ained results, putting a special emphasis on their temperature dependence. Then, we discuss the implications of those results on theoretical photochem istry modeling and on future observations of methane rich atmospheres. (C) 2000 Elsevier Science Ltd. All rights reserved.