Penning ionization electron spectroscopy of n-alkane ultrathin films. Molecular orbitals and orientation of molecules

Abstract

Penning ionization electron spectra (PIES) were measured for evaporated ultrathin films (4 (monolayer) to 200 {angstrom} thick) of long-chain n-alkanes. The electron distributions as well as the energies of three types of MOs (pseudo-{pi}, {sigma}{sub 2p}, and {sigma}{sub 2s}), obtained by ab initio MO calculations, could be surveyed experimentally by PIES; the agreement between the calculated and experimental results was fairly good. The valence electronic structure of long-chain n-alkanes is characterized by the high density of states of the pseudo-{pi} and {sigma}{sub 2s} MOs, which were predominantly detected for monolayer films made up of flat-lying molecules. For crystalline films comprised of molecules standing upright, MOs with large distribution on the terminal hydrogen atom were exclusively detected. These data serve for the PIES characterization of various molecular aggregates of alkane derivatives, including Langmuir-Blodgett films.