title: Site-dependent spectral shifts in core-to-pi* excitations of pyridine clusters.
abstract: 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 1s 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 --> 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*( v = 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 variable-size pyridine clusters.
authors.affiliation: Institute for Molecular Science, Okazaki 444-8585, Japan.
authors.forename: I L
authors.initials: IL
authors.lastname: Bradeanu
authors.forename: N
authors.initials: N
authors.lastname: Kosugi
authors.forename: R
authors.initials: R
authors.lastname: Flesch
authors.forename: E
authors.initials: E
authors.lastname: Rühl
mesh_terms: D002244:Carbon / Q000737:chemistry; D004735:Energy Transfer; D013058:Mass Spectrometry; D008962:Models, Theoretical; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry; D011725:Pyridines / Q000737:chemistry*
chemical_list: D011725:Pyridines; D002244:Carbon; D009584:Nitrogen