DIAGONAL 2PH-TDA GREEN-FUNCTION SIMULATION OF THE VALENCE X-RAY PHOTOELECTRON-SPECTRA OF N-ALKANE COMPOUNDS - A THEORETICAL SEARCH FOR CONFORMATIONAL SIGNATURES

With the aim of improving and validating theoretical quantum mechanical tools for tracking conformational signatures in the XPS spectra of n-alkane compounds, the XPS spectra of model structures are simulated by means of direct Green's function schemes and compared to available experimental records obtained in the gas and solid phases. Calculations are performed using the minimal STO-3G and 3-21G basis sets, and self-energy expansions derived at several levels of decoupling approximation: second-order, shifted Born collision (SBC) and diagonal two-particle-hole Tamm-Dankoff approximation (2ph-TDA). The latter scheme is shown to be accurate enough to generate reliable relative line positions and convoluted photoionization intensities in the inner valence region, which is of fundamental importance for the identification of secondary molecular structures from XPS spectra. Preventing overcounting of self-energy diagrams containing a diagonal ladder part is crucial in that respect. As a by-product, this method of simulation is applied to some particular conformations of C9H20 modeling the topmost layer of polyethylene in crystalline form, enabling the identification of fold structures at the extreme surface of lamellar crystals of polyethylene.