Dynamic Stabilization in ${1\sigma }_{\mathit{u}}\to {1\pi }_{\mathit{g}}$ Excited Nitrogen Clusters

High-resolution 1s near-edge spectra of molecular nitrogen and variable size nitrogen clusters obtained using monochromatic synchrotron radiation from the high brilliance BESSY-II storage ring facility are reported. The vibrationally resolved $1{\sigma }_u\to 1{\pi }_g$ core-to-valence excitation band of clusters shows a distinct redshift of $6\pm 1\mathrm{meV}$ relative to the isolated molecule, but the vibrational structure and linewidths are essentially unchanged. This shift is assigned to dynamic stabilization of $1{\sigma }_u\to 1{\pi }_g$ excited 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.