FINE VIBRATIONAL STRUCTURE IN CORE-TO-BOUND SPECTRA OF POLYATOMIC-MOLECULES

Near-edge x-ray-absorption fine-structure (NEXAFS) spectra of 1-butene (C4H8), acrylonitrile (C2H3CN) and 1,3-butadiene (C4H6) multilayers w ere recorded with high resolution (better than 100 meV at the carbon e dge) and a theoretical method to interpret the vibrational structure o f the observed C-1s-->pi bands is presented. The method is based on l ocal (quadratic) approximations for the potential energy surfaces of b oth 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 equivalen t core model. The method proved to be useful in qualitative terms, pro viding information on the nature of the excited modes and on the geome trical changes following the electronic excitation. We have also calcu lated the C-1s-->pi spectrum of ethylene (C2H4) and compared it to th ose obtained for C4H8, C2H3CN, and C4H6 Since the latter can be consid ered as substituted ethylenes, the validity of the building-block sche me for the vibrational splittings in inner-shell absorption spectra co uld be assessed by this comparison.