Ab initio calculation of the C1s photoelectron spectrum of C 2 H 2 R. Colle a,b, * , D. Embriaco a , M. Massini b , S. Simonucci c , S. Taioli d a Dipartimento di Chimica Applicata, INSTM, Universit  a di Bologna, Via Saragozza 8, I-40136 Bologna, Italy b Scuola Normale Superiore, I-56100 Pisa, Italy c Dipartimento di Matematica e Fisica, INFM, Universit  a di Camerino, Via Madonna delle Carceri 7, I-62032 Camerino, Italy d Dipartimento di Energetica, Nucleare e Controllo Ambientale, INFM, Universit  a di Bologna, Via Risorgimento 4, 40136 Bologna, Italy Abstract The C1s photoelectron spectrum of C 2 H 2 , measured by Kempgens et al. [Phys. Rev. Lett. 79 (1997) 3617] and characterized by the presence of a core-level splitting, has been calculated and interpreted using an ab initio quantum- mechanical method that allows detailed analysis of photoionization processes and accurate reproduction of spectral profiles. The anisotropy of the electron emission, measured in a hypothetical experiment in which the orientation of the molecules can be established, has been also predicted and analyzed. Ó 2003 Elsevier B.V. All rights reserved. PACS: 31.15.Ar; 33.60.Fy; 33.70.)w; 33.80.Eh Keywords: Molecular photoionization; Core-level splitting; Ab initio scattering wavefunctions 1. Introduction The continuous improvements in synchrotron radiation sources, soft X-ray monochromators and electron analyzers [1–4] have produced in re- cent years considerable progress in K-shell ion- ization spectroscopy of small molecules. The photon- and electron-energy resolutions which are now achievable allow one to gather relevant information also on subtle quantum-mechanical effects, such as those due to the occurrence of near- degenerate core–hole configurations, and make this type of spectroscopy very interesting and challenging for the theory. Recently, Kempgens et al. [5] have reported the first experimental identification of the core-level splitting in the Cls photoelectron spectrum of C 2 H 2 , a feature ascribed to the presence of two near-degenerate core–hole states, classified as j 2 R g : 1r 1 g i and j 2 R u : 1r 1 u i in the independent- particle scheme. In this paper, we present an ab initio calculation and theoretical analysis of the C1s photoelectron spectrum of C 2 H 2 measured by Kempgens et al. [5]. The results of this study have been obtained using a quantum-mechanical ap- proach [6–8] already successfully applied to the interpretation of XPS, Auger and autoionization spectra of atoms and molecules. In this paper, we * Corresponding author. Address: Scuola Normale Superi- ore, I-56100 Pisa, Italy. Tel.: +39-050-509-248; fax: +39-050- 563-513. E-mail address: colle@sns.it (R. Colle). 0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01535-0 Nuclear Instruments and Methods in Physics Research B 213 (2004) 65–70 www.elsevier.com/locate/nimb