Size distribution and mass fraction of microclusters in laser-irradiated plasmas

Laser interactions with a mixture of a gaseous plasma and microclusters depend strongly on the cluster-size distribution, which is usually difficult to measure directly. We present a new method for recovering the cluster-size distribution and cluster mass fraction from measurements of a time-dependent refractive index of the medium in a two-pulse pump-probe experiment. The refractive index is determined from power absorption and phase shift of the probe measured for various delays between the two pulses. The primary cause of absorption is plasma resonances in clusters with peak density above the critical density, which makes the approach especially suitable for determining the tail of the cluster-size distribution. We demonstrate the feasibility of the method by analyzing the data from recent pump-probe experiments at the University of Texas. We have determined that the distribution of clusters over the initial cluster radii for these experiments is well approximated by a lognormal distribution shifted with respect to zero radius.