Surface Review and Letters, Vol. 9, No. 1 (2002) 111–116 c  World Scientific Publishing Company ELECTRONIC ENERGY DISSIPATION PROCESSES IN DOPED RARE GAS CLUSTERS WITH A SHELL-LIKE STRUCTURE T. LAARMANN, K. VON HAEFTEN, H. WABNITZ and T. M ¨ OLLER HASYLAB at DESY, Notkestr. 85, 22603 Hamburg, Germany Energy relaxation processes of photoexcited ArM clusters (M< 50) covered with a shell of Kr atoms (up to 30 atoms) which are embedded in large NeN clusters (N = 3500) are investigated with energy- resolved fluorescence spectroscopy. In the energy range of the Ne cluster absorption (16.5–18 eV) a strong energy transfer to the embedded Ar subcluster is observed, which results in desorption of electronically excited Ar ∗ atoms. Ar ∗ move through the Ne cluster, desorb and emit visible and near- infrared light in the vacuum (4p → 4s). By coating the Ar clusters with Kr atoms, the Ar lines disappear and 5p → 5s transitions of Kr become dominant. Additionally, new bands occur, which are assigned to transitions of perturbed atomic Ar 4p states inside Ne clusters. A simple kinematic model is suggested to describe the suppression of desorption of excited Ar atoms from Ne clusters. 1. Introduction The electronic structure and dynamics of clusters has attracted considerably interest for many years since it gives new insight into the properties of condensed matter. Investigations on doped species offer the op- portunity to get information about the host cluster and the impurity atoms or molecules, because the ex- citation is usually localized on the impurity. 1,2 Clus- ters can also serve as matrix for atoms, molecules and radicals. Because of their low chemical reactivity and the transparency within the vacuum-ultraviolet (VUV) spectral range, rare gases are especially well suited. 3 Supersonic free jets of rare gas clusters of- fer the possibility of isolating and characterizing molecules and complexes at very low temperatures, which are not accessible by other techniques. 4 As an example, recent studies on small (H 2 O) 6 clus- ters embedded in large helium droplets demonstrated the preparation of the metastable cyclic water hex- amer, which is only a local minimum on the asso- ciated potential energy surface. 5 The molecular re- arrangement into the lowest energy configuration, a cage structure, is suppressed, because of the low He cluster temperature (0.38 K) and the rapid cooling mechanism. 6 The recent progress allows the investigation of chemical reactions inside large clusters by adding a third material. 7 The large cluster, which serves as a reaction medium, has many degrees of freedom, and can therefore be considered as a good thermo- stat as long as the reactants stay in contact with the cluster. The use of different host cluster mate- rials allows the study of condensation and quench- ing reactions at different temperatures, ranging from ∼ 0.4 K in He, ∼ 10 K in Ne and ∼ 32 K in Ar clusters, respectively. 8,9 Further advantages are of course, first, the possibility of controlled surface or bulk localization of a finite number of reactants and, second, the finite size of the reaction medium. Desorption processes are the condensed matter analogs of unimolecular reactions and predissocia- tion in gas phase molecules. It is well known that desorption of excited atoms and molecules play an important role in the relaxation cascade of light rare gas clusters (He, Ne) photoexcited in the VUV spec- tral range and which was also observed for small Ar clusters. 10,11 The aim of the present contribution is to provide experimental data on hindered desorp- tion processes in Ar M Kr K Ne N clusters by modify- ing surface properties, which shed light on electronic energy dissipation processes in doped clusters and 111