Ab Initio Spin Densities of Small Alkali-Metal Doped Helium Droplets

Full text: In recent experiments of our group on alkali-metal-doped helium droplets, it could be shown that the hyperfine structure of the dopants is strongly affected by the helium surrounding. A change of the hyperfine splitting of 39K is observed as a function of the nozzle temperature, indicating a dependence on the helium droplet size. Hence we started a theoretical investigation based on an open shell coupled cluster approach to study this size effect within molecular orbital theory. In our theoretical model we are restricted to very small HeNX clusters (N=1- 5,X=Li,Na,K) due to the high computational demands of the chosen method. However, it is possible to identify two antagonistic effects of the helium surrounding on the electronic wavefunction of the alkali dopant: The first effect is a compression of the wavefunction which leads to higher spin densities |Φ0|2 at the core, the second one is a reduction of |Φ0|2 due to the Pauli repulsion, i.e. the deformation of the s-orbitals. Despite of the computational limitations we also try to give estimations for larger systems. Here we apply a combination of the quantum chemistry approach with a stochastic geometry optimization. Assuming only two-body-interactions we use the simulated annealing technique to obtain geometry predictions. (author)