PROBING THE MOLECULAR PRIMARY AND SECONDARY STRUCTURES OF SATURATED-HYDROCARBONS BY X-RAY PHOTOIONIZATION SPECTROSCOPY

The dependence of X-ray photoionization spectra on molecular structure characteristics is assessed by comparing Al K alpha photoelectron results obtained in the gas phase for n-pentane, n-hexane, cyclopentane, and cyclohexane to many-body Green's function theoretical simulations coping with the leading correlation and relaxation effects. Both the experimental spectra and their simulations exhibit obvious signatures for changes in the molecular configuration and conformation. The comparison of theory with experiment corroborates nicely disordering effects in gaseous samples of the n-alkane compounds. Cyclohexane, on the contrary, is found to exist essentially in its chair conformation. It gives evidence for a weak but significant breakdown of the molecular picture above an approximate threshold of 24 eV in the inner valence region of medium-size saturated hydrocarbons, in particular cycloalkanes. Vibronic coupling effects are also found to affect strongly the outermost ionization photoelectron band of cyclopentane. As a direct application, inner- and outer-valence XPS results are used to probe the secondary structure of macromolecular chains at the extreme surface of lamellar microcrystals of polyethylene. The identified leading fold-stacking sequences are found to match the patterns of Reneker and Geil.