Journal of Electron Spectroscopy and Related Phenomena 113 (2000) 57–65 www.elsevier.nl / locate / elspec Shape resonances of oriented molecules a ,1 a a a a, b * D. Arvanitis , N. Haack , G. Ceballos , H. Wende , K. Baberschke , A.L. Ankudinov , b J.J. Rehr a ¨ ¨ Institut f ur Experimentalphysik, Freie Universitat Berlin, Arnimallee 14, D-14195 Berlin-Dahlem, Germany b Department of Physics, University of Washington, Seattle, WA 98185-1560, USA Received 12 April 2000; received in revised form 11 July 2000; accepted 12 July 2000 Abstract We report ab initio calculations of the X-ray absorption cross section for the near edge X-ray absorption fine structure of CH,CH and C H , at the carbon K-edge, based on a full multiple scattering formalism. We find that shape resonances 2 6 2 4 2 2 dominate the photoabsorption carbon K-edge spectra close to the edge, leading to a spectacular enhancement in cross section of order 3–4 Mbarn. Our analysis highlights the strong influence of the molecular vibrations on the intensity, asymmetric profile and energy of this spectral feature. Calculations with and without hydrogen atoms, allow to estimate the amount of C–H character of this feature. A quantitative comparison between theory and experiment, using spectra of oriented molecules on surfaces is performed. Molecular orientation in the experiment is obtained by means of physisorption which has the advantage of obtaining molecules very close to their gas phase state.  2000 Elsevier Science B.V. All rights reserved. 1. Introduction obtaining quantitative information on bonding strength and structural parameters [2,3]. Despite a The broad structures in the continuum region of considerable accumulation of experimental data in the X-ray absorption spectra of molecules have been the gas phase and on surfaces no systematic quantita- the focus of many studies in the past. Since the tive analysis of these resonances was performed in contribution of Dehmer et al. [1] an early interpreta- the literature for several years. Experimental studies tion was, that these resonances originate from the mostly focused on the empirical correlation between temporary trapping of the excited core electron by a resonance energy and internuclear separation [2–4] centrifugal barrier of the molecular potential in a and not on a full analysis including oscillator quasibound state. In particular for K-edge excitation, strength and spectral shape. In several instances a in the spectral range of 0.2–1 keV, these features are pronounced asymmetry in the spectral shape for important in the spectra of light molecules for these resonances is observed. This asymmetry was first discussed in the literature in terms of a strong coupling between vibrational and electronic motion *Corresponding author. Tel.: 149-30-838-56148; fax: 149-30- leading to a breakdown of the Franck Condon 838-53646. principle in the particular case of the nitrogen E-mail address: bab@physik.fu-berlin.de (K. Baberschke). 1 molecule [1]. A cross section of several Mbarn was Permanent address: Department of Physics, Uppsala University, Box 530, S-75121 Uppsala, Sweden. calculated at the resonance maximum but no quan- 0368-2048 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0368-2048(00)00198-5