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 the carbon K-edge, based on a full multiple scattering formalism. We find that shape resonances 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.