Carbon K-shell excitation spectra of linear and branched alkanes

The electron energy loss spectra of ethane, propane, n-butane, n-pentane, n-hexane, isobutane, isopentane and neopentane in the region of carbon K-shell excitation have been recorded under dipole-dominated conditions (2.8 ke V impact energy, small angle). The spectra are dominated by transitions to unoccupied valence π∗(CH2, CH3) and σ∗(C-C) levels. Additional weak features are assigned to Rydberg transitions. The position of the main continuum feature in each spectrum is consistent with the predictions of an empirical relationship with bond length. Systematic variations of spectral intensities are observed which support our assignments. The dominant feature in the K-shell spectrum of ethane, which was previously assigned to C 1s → 3p Rydberg transitions, is reassigned to excitation to a 3p/π∗(CH3 ), mixed Rydberg/valence orbital (of antibonding σ-∗(C-H) character), in comparison to the other alkane spectra. An improved calibration value of 290.74(5) eV for the energy of the C 1s → π∗ transition in CO2 is also obtained.