Shape resonances of oriented molecules

We report ab initio calculations of the X-ray absorption cross section for the near edge X-ray absorption fine structure of C2H6, C2H4 and C2H2, at th e carbon K-edge, based on a full multiple scattering formalism. We find tha t shape resonances dominate the photoabsorption carbon K-edge spectra close to the edge, leading to a spectacular enhancement in cross section of orde r 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 t he amount of C-H character of this feature. A quantitative comparison betwe en theory and experiment, using spectra of oriented molecules on surfaces i s performed. Molecular orientation in the experiment is obtained by means o f physisorption which has the advantage of obtaining molecules very close t o their gas phase state. (C) 2000 Elsevier Science B.V. All rights reserved .