Core-electron relaxation energies and valence-band formation of linear alkanes studied in the gas phase by means of electron spectroscopy

Core- and valence-electron spectra of the $n$-alkanes have been recorded under high resolution in the gas phase. The measured core-level binding energies decrease with an increasing number of carbon atoms in the molecules. The whole shift ranges over 0.6 eV. These small shifts are ruled by the relaxation energy and are discussed with the aid of various theoretical models. It is found that the transition-potential model together with CNDO/2 (complete neglect of differential overlap) calculations describes satisfactorily the variation of the core-level binding energies. The shakeup spectra of methane, ethane, and propane are presented and tentatively interpreted. The valence-electron structures of methane, ethane, propane, $n$-butane, and $n$-pentane are studied. Results are correlated with ab initio calculations using an extended basis set. The formation of a valence-electron band is discussed. For instance, it appears that $n$-tridecane provides a convenient finite model for the band structure of an infinite linear polymer.