Laser-excited intershell rotation of finite Coulomb clusters in a dusty plasma

The dynamical stability of finite two-dimensional clusters of charged micrometer sized particles is studied experimentally in a dusty plasma, when the particles experience a screened interaction. Far clusters containing 19 and 20 particles the intershell rotation barrier height is measured quantitatively by applying a well defined torque on the clusters' outer shell by the radiation pressure of two opposing laser beams. Moreover, a complex behavior is found for the 19-particle cluster where structural transitions occur together with the intershell rotation showing that the presence of shielding can basically change the least stable modes in finite clusters. The measured crystal properties have been compared with Monte Carlo simulations. Quantitative agreement is found when nonideal effects are taken into account.