Fine vibrational structure in core-to-bound spectra of polyatomic molecules

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Abstract
Near‐edge x‐ray‐absorption fine‐structure (NEXAFS) spectra of 1‐butene (C4H8), acrylonitrile (C2H3CN) and 1,3‐butadiene (C4H6) multilayers were recorded with high resolution (better than 100 meV at the carbon edge) and a theoretical method to interpret the vibrational structure of the observed C1s→π* bands is presented. The method is based on local (quadratic) approximations for the potential energy surfaces of both ground and excited electronic states, with input data obtained by ab initio calculations of normal modes for the ground state and energy gradients for the excited state within the framework of the equivalent core model. The method proved to be useful in qualitative terms, providing information on the nature of the excited modes and on the geometrical changes following the electronic excitation. We have also calculated the C1s→π* spectrum of ethylene (C2H4) and compared it to those obtained for C4H8, C2H3CN, and C4H6. Since the latter can be considered as substituted ethylenes, the validity of the building‐block scheme for the vibrational splittings in inner‐shell absorption spectra could be assessed by this comparison.