Site-dependent spectral shifts in core-to-π* excitations of pyridine clusters

Site- and element-selective core-to-pi* excitation in free pyridine clusters is investigated. The experimental results indicate the occurrence of site- and size-dependent spectral shifts in the C 1s and N 1s -> pi* excitation regime. Specifically, we observe in the C Is regime a substantial and site-dependent redshift of the low energy slopes of the C 1s -> pi* band by 90 meV in clusters relative to the bare molecule, whereas the high energy slopes of this band remain almost unchanged. In contrast, a size-dependent blueshift of the same order of magnitude is found for the entire N 1s -> 7 pi* band. This is distinctly different from previous results on van der Waals clusters, where exclusively redshifts in 1s -> pi* transitions are observed. The experimental results are compared to ab initio calculations, which serve to simulate the 1s -> pi*(nu = 0) transitions. These results clearly indicate that the spectral shifts are primarily a result of electrostatic interactions between the molecular moieties and that an antiparallel orientation of molecular units preferably dominates in variablesize pyridine clusters.