VUV and mid-UV photoabsorption cross sections of thin films of adenine: Application on its photochemistry in the solar system

The photochemistry of an organic compound in extraterrestrial environments is related to its photoabsorption cross sections in the VUV/UV range (<300 nm). Such data, especially in the VUV range (<200 nm) are quite scarce in the literature. This paper presents an experimental setup and associated methodology to measure VUV/UV spectra of thin films of organic molecules. The case of adenine is extensively discussed as an example study. The absorption cross sections spectrum of adenine between 115 and 300 nm is measured from transmission measurements and accurate calculation of the thickness of our samples set thanks to interferometry techniques and infrared spectrometry. From these data, the infrared integrated band strength of solid adenine between 3600 and 1970 cm−1 has also been measured and is equal to 7.9×10–16±4% cm molecule−1. The use of the VUV/UV spectrum to estimate the photolysis rate constant is discussed and compared to direct kinetic measurements available in the literature for low Earth orbit experiments. However the lack of measurements of the photodissociation quantum yield as a function of the wavelength prevents a precise calculation. VUV/UV cross section spectrum is yet a necessity to conduct either proper kinetic studies on optically thin samples (less than 2 nm in the VUV for adenine) or an appropriate modeling for optically thick samples.