High resolution near-edge spectroscopy of molecules and molecular van der Waals clusters

High resolution near-edge excitation of isolated molecules and homogeneous molecular van der Waals clusters is reported. Resonant excitation near the element K-edges of isolated and clustered carbon monoxide and nitrogen gives rise to small spectral shifts that can only be detected by high spectral resolution. The experimental results indicate for the vibrationally resolved 1s --> pi* transitions of isolated molecules that symmetric line shapes cannot be used rather than the lifetime-broadened rotational fine structure. Clustered molecules exhibit characteristic redshifts of the order of a few meV relative to the corresponding isolated molecules. Changes in Franck-Condon factors and changes in spectral line shape are observed, as well. The experimental results are discussed in the framework of the quasiatomic approach with respect to intermolecular interactions and dynamic localization of resonant core-to-valence excitations.