Vacuum ultraviolet absorbance of alkanes: an experimental and theoretical investigation

The electronic absorption spectra of alkanes are known to be broad and lack discrete band structures. Previous studies have suggested their HOMO-LUMO energy gaps could be used to understand the absorbance edges of these spectra. With the advent of a new benchtop vacuum ultraviolet (VUV) spectroscopic absorption detector, it is now possible to collect VUV absorption spectra (from 120 to 240 nm) for an extended range of structures under an inert gas phase environment more conveniently. The previously reported hypothesis was revisited and its limits were explored using a combination of new extended experimental VUV spectral data and theoretical quantum mechanics calculations. It was pointed out from this study that the first strong excitations of alkanes are not always the HOMO-LUMO transition. As a result, the HOMO-LUMO energy gap of alkanes should not be used directly as a universal and reliable parameter to understand their experimental absorbance edges. VUV spectral data for a larger variety of alkanes structures were reported and the relation between these structures and spectra was discussed.