Many experiments on pure and doped helium clusters result in the production of a broad distribution of charged fragment ions after electron impact or laser ionisation. These ion yield distributions occasionally exhibit distinct stability patterns. We use the diffusion quantum Monte Carlo technique to study these stability patterns and structural features of helium clusters with open shell atomic and molecular dopants. These simulations require reliable many-body potentials which we construct from high level ab initio CCSD(T) calculations for several electronic states mixed by spin-orbit coupling and including non-additive interactions arising from induction. For small clusters numerically exact calculations of rovibrational properties are used to establish the quality of our potential surfaces. The approach is illustrated with applications to I
q@He
n, q=-1,0,+1,+2 clusters which appear in ionisation and photodissociation experiments. Spin-orbit coupling due to the iodine atom is found to have a profound effect on the structure of the mixed clusters. Radial and angular helium density profiles are computed and show that even in the presence of pronounced radial structuring exchange between 'shells' remains possible and angular disorder remains significant. The identification of a well defined ionic snowball core appears incorrect. We present first results for CO
+ ions in helium clusters. He-CO
+ is an interesting astrophysical collision system but its interest for helium cluster studies is the similarity of the CO
+ rotational constant with the one of neutral CO. Our present understanding of rotation inside helium clusters relies on studies of molecules where changing the molecule implies changing dynamical parameters and the interaction potential. The CO/CO
+ case allows to study the specific effect of changing only the interaction energy. CO
+@He
n should be relatively easy to make by cluster ionisation or aggregation in ion drift tubes and its strong dipole and vibrational transition moment make it an easy target for high resolution spectroscopy. We have computed accurate ab initio surfaces for the two lowest electronic states of He-CO
+ to predict rovibrational spectroscopic and collisional properties. A many-body model using these surfaces is used to study larger CO
+He
n clusters.
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 {sup 39}K 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 He{sub N}X 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 |{Phi}{sub 0}|{sup 2} at the core, the second one is a reduction of |{Phi}{sub 0}|{sup 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)