Exchange interaction in Kr 3d excitations of small krypton clusters

X-ray absorption spectra of small krypton clusters with average size of ∼15 atoms were measured with Kr2+ ion yields at the Kr 3d5/2 and 3d3/2 region. The second lowest Rydberg transition 3d−16p is redshifted from the corresponding atomic transition, and shows an asymmetric broad shoulder feature. This characteristic feature is attributable to some surface sites with different coordination numbers such as corner, edge and face as previously observed in the Kr 3d photoexcitation and photoelectron spectra. Diffuse Rydberg states are influenced predominantly with induced polarization of the surrounding atoms. On the other hand, the lowest transition 3d−15p is blueshifted, and shows a rather narrow peak. The blueshift arises from short-range repulsive interaction, that is, exchange interaction of the Rydberg electron with the surrounding atoms. The larger coordination number of the nearest neighbor atoms increases both the exchange (blueshift) and induced polarization (redshift) effects. In the higher Rydberg state, the blueshift effect is drastically reduced but the redshift effect is most dominant. Cancelation of these two effects can be expected in the lowest Rydberg transition. The present experimental analysis enables us to discuss the site-dependent and state-dependent exchange interaction.