The temperature dependent absorption cross sections of C4H2 at mid ultraviolet wavelengths

We have measured the mid UV (195–265 nm) absorption coefficients of diacetylene, C4H2, over the temperature range 193–293 K. An impurity involved in the synthesis of C4H2, namely C4H3Cl, can strongly influence the measured absorption coefficients. This impurity has a mid UV absorption approximately 300 times stronger than C4H2 and we demonstrate that its presence, at low levels, in previously published spectra has strongly influenced the published coefficients and their temperature dependence. We have obtained an ultra-pure sample of C4H2 by repeated distillation and show here its clean spectrum and temperature dependence. We confirm previous band assignments, but our superior resolution (0.02 nm) has shown the presence of other, previously undetected bands. These new results have wide implications for the modelling of Titans atmosphere and the analysis of observational data.