Dynamic stabilization in 1σu → 1πg excited nitrogen clusters

High-resolution 1s near-edge spectra of molecular nitrogen and variable size nitrogen clusters ob rained using monochromatic synchrotron radiation from the high brilliance BESSY-IS storage ring facility are reported. The vibrationally 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 isolated molecule, but the vibrational structure and linewidths an essentially unchanged. 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 field. This leads to changes in intermolecular interactions compared to the ground-state cluster Such spectral shifts are expected to occur generally in molecular clusters and in the corresponding condensed phase.