Dynamic stabilization in 1 sigma(u) -> 1 pi(g) excited nitrogen clusters

High-resolution 1s near-edge spectra of molecular nitrogen and variable siz e nitrogen clusters ob rained using monochromatic synchrotron radiation fro m the high brilliance BESSY-IS storage ring facility are reported. The vibr ationally resolved 1 sigma (u) --> 1 pi (g) core-to-valence excitation band of clusters shows a distinct redshift of 6 +/- 1 meV relative to the isola ted molecule, but the vibrational structure and linewidths an essentially u nchanged. This shift is assigned to dynamic stabilization of 1 sigma (u) -- > 1 pi (g) exicited molecules in clusters, arising from the dynamic dipole moment generated by core-hole localization in the low-symmetry cluster fiel d. This leads to changes in intermolecular interactions compared to the gro und-state cluster Such spectral shifts are expected to occur generally in m olecular clusters and in the corresponding condensed phase.