Recoil effects in valence photoemission from simple molecules and clusters

Abstract

We discuss recoil effects from extended molecular orbitals in simple molecular crystals excited by high-energy X-ray photons. Because of Debye-Waller factors the interference effects between photoelectron waves emanating from different atomic sites can be neglected, and the Gelius formula is safely used, which provides us with simple expression of photoemission bands. This formula can explain the recoil energy shift and energy dependence of the intensity of each level. We apply it to the photoemission bands from acetylene and diacetylene molecular crystals in the energy range from 125 to 1200 eV. Hydrogen recoil shift is much larger than carbon, however it is hard to observe recoil effect due to the hydrogen recoil because of its quite small photoionization cross section even in this intermediate energy region. Additionally, we expect that useful information which is obtained from the energy dependence of these bands: the C 2s and 2p components in each molecular orbital can be estimated roughly from the energy dependence. © 2012 The Surface Science Society of Japan.