Probing molecular conformations with electron momentum spectroscopy: The case of n-butane

High-resolution (e,2e) measurements of the valence electronic structure and momentum-space electron density distributions of n-butane have been exhaus tively reanalyzed in order to cope with the presence of two stable structur es in the gas phase, namely the all-staggered and gauche conformers. The me asurements are compared to a series of Boltzmann-weighted simulations based on the momentum-space form of Kohn-Sham (B3LYP) orbital densities, and to ionization spectra obtained from high-level [ADC(3)] one-particle Green's F unction calculations. Indubitable improvements in the quality of the simula ted (e,2e) ionization spectra and electron momentum profiles are seen when the contributions of the gauche form of n-butane are included. Both the one -electron binding energies and momentum distributions consistently image th e distortions and topological changes that molecular orbitals undergo due t o torsion of the carbon backbone, and thereby exhibit variations which can be traced experimentally. With regard to the intimate relation of (e,2e) cr oss sections with orbital densities, electron momentum spectroscopy can the refore be viewed as a very powerful, but up to now largely unexploited, con formational probe. The study also emphasizes the influence of thermal agita tion in photoionization experiments of all kind.