Detailed study of pyridine at the C1s and N1s ionization thresholds: The influence of the vibrational fine structure

High resolution, vibrationally resolved, near-edge x-ray absorption fine st ructure (NEXAFS) spectra at the C 1s and N 1s ionization thresholds of pyri dine and deuterated d(5)-pyridine in the gas phase have been recorded. The high resolution of 65 meV (150 meV) at the C s (N 1s) ionization thresholds reveals vibrational structures in the spectra. Detailed ab initio and dens ity functional theory (DFT) calculations were performed to interpret the ex perimental spectra and to assign the observed peaks. In particular we focus ed on the previously unexplained intensity ratio for the two components of the C1s -->1 pi* transition. For this transition the vibrational structure is included through a linear coupling model in the DFT calculations and lea ds to the experimentally observed similar to2:3 intensity ratio between the two pi* components in the C1s spectrum rather than the similar to3:2 ratio obtained without vibrational effects. After inclusion of relaxation effect s in the excited states, in addition to the vibrational effects, both theor etical methods yield almost perfect agreement with experiment. (C) 2001 Ame rican Institute of Physics.