Experimental study of debye clusters using brownian motion

Summary form only given. A two-dimensional Debye cluster is a system of n identical particles confined in a parabolic well and interacting through a screened Coulomb force (i.e., a Debye-Huckel or Yukawa potential) with a Debye length lambda. In the strong-coupling regime, the particles exhibit small-amplitude Brownian motions about their equilibrium positions. Experiments were performed for 27 clusters in a capacitively-coupled 9-W RF discharge with n=3 to 63 particles (9 mum-diam.) at a neutral argon pressure of 13.6 mtorr. Measured thermal motions were projected onto the center-of-mass and breathing modes and Fourier analyzed to give resonance curves from which the mode frequencies, amplitudes and damping rates were determined. The ratio of the breathing frequency to the center-of-mass frequency was compared with theory to self-consistently determine the Debye shielding parameter kappa, Debye length lambda, particle charge q and mode temperature. It is found that kappaap1, and that kappa decreases slightly with n. The particle charge averaged over all measurements is -14200plusmn200 e, and q also decreases weakly with n. The two center-of-mass modes and the breathing mode are found to have the same temperature, indicating that the clusters are in thermal equilibrium. The average cluster temperature is 399plusmn5 K