Chemical Physics 175 (1993) 427-446 North-Holland Diagonal 2ph-TDA Green’s function simulation of the valence X-ray photoelectron spectra of n-alkane compounds: a theoretical search for conformational signatures MichaH Deleuze, Joseph DeIhaIIe Luboratoire de Chimie ThkoriqueAppliquke, Fact&% Universitaires Notre-Dame de la Paix, 6 I rue de Bruxelles, B-5000 Namur, Belgium and Barry T. Pickup Department of Chemistry, the University, Sheffield S3 7HF, UK Received 22 February 1993; in final form 12 May 1993 With the aim of improving and validating theoretical quantum mechanical tools for tracking confonnational 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 STP3G and 3-21G basis sets, and se&energy expansions derived at several levels of decoupling approximation: second-order, shifted Born collision (SBC) and diagonal two-particle-hole Tamm-Dankoff approximation (Zph-TDA). The latter scheme is shown to be accurate enough to generate reliable relative line positions and convoluted photoionixation 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-enetgy 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 C9Hmmodeling the topmost layer of polyeth- ylene in crystalline form, enabling the identification of fold structuresat the extreme surface of lamellar crystals of polyethylene. 1. Introduction X-ray photoelectron spectroscopy (XPS or ESCA ) allows the study of the electron energy levels in atoms and molecules in gas, liquid and solid phases. Con- sidering the growing importance in highly demand- ing fields (electronics, catalysis, biotechnology, space) of polymer thin films deposited on various substrates (metal, glasses, etc. ), materials for which classical structural analysis techniques such as IR or NMR are usually inapplicable, the XPS method pro- vides a particularly appropriate tool to probe the sur- faces and interfaces of technologically useful organic materials. Most studies have been devoted to the core- level shifts which supply information on the chemi- cal composition and, in favourable situations, on the primary molecular structure. The valence region has received much less attention, probably because the many XPS valence lines of low intensity to be iden- tified in a small energy interval, typically 50 eV, are more complicated to analyse. Combined with quan- tum mechanical calculations, invaluable direct and indirect information on structural aspects (substitu- tion, conformation) and the chemical bonding of the constitutive compounds can however be obtained from their valence XPS spectra [ 11. In the purpose of finding conformational signatures, recent studies have been conducted at the uncorrelated Koopmans level of approximation on models of polyethylene [ 2 1, syndiotactic propylene [ 3 1, polyoxymethylene [ 41, and polyacrylonitrile [ 5 1. The simulated spec- tra exhibit variations of both band positions and photoionization intensities among the selected con- formations, encouraging efforts to implement corre- 0301-0104/93/S 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.