Skinning of argon clusters by Coulomb explosion induced with an intense femtosecond laser pulse

Abstract

The energy distributions of ions emitted from argon clusters Coulomb exploded at an intensity of $<{10}^{17}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ with an intense femtosecond laser have been experimentally studied. The power $m$ of energy $E$ of the ion energy distribution $(dN∕dE\ensuremath{\sim}{E}^{m})$ is expected to be $1∕2$ for spherical ion clusters, but it is in fact reduced smaller than $1∕2$ as the laser intensity is decreased. This reduction can be well interpreted as resulting from the instantaneous ionization of the surface of the cluster. The validity of this interpretation was confirmed by experiments with double pulse irradiation. A cluster irradiated by the first pulse survives as a skinned cluster, and the remaining core part is Coulomb exploded by the second pulse. It is shown that a cluster can be skinned by an intense short laser pulse, and the laser-intensity dependence of the skinned layer thickness can be reasonably explained by the laser-induced space charge field created in the cluster.