Investigation of the valence electronic structure of n-butane using (e, 2e) spectroscopy

The first complete (e, 2e) measurements of the valence ionization spectrum (6–56 eV) of n-butane (C4H10) are reported for two relative azimuth angles. Measurements have been carried out using a high-resolution (ΔE=0.9 eV, Δp=0.1 a.u.) spectrometer, using an impact energy of 1200 eV and symmetric non-coplanar kinematics. The results confirm a complete breakdown of the orbital picture of ionization of the innermost C2s level, as predicted recently by a third-order ADC Greenʹs function calculation. They also point out a strong angular dependence of the relative intensities from which orbital symmetries can easily be traced.