Anomalous Ion Charge Distribution from Cluster Nanoplasmas

Abstract

In nearly all the intense laser experiments with nanoclusters, the key observation has been that immense ionization drives highly charged ions to highest energies while low charge ions, if any, have lower kinetic energies. We show experimental measurements that are contrary to this established notion. The active role of outer-ionized electrons in a multicluster interaction is shown to effectively reduce high charge ions to low charge states with no loss of momentum. The role of Rydberg excited clusters, intrinsic in dense cluster ensembles, is identified and a quantitative analysis is shown to comprehensively explain the anomalous charge distribution and ion energies observed in experiments.

Anisotropic emission of neutral atoms: evidence of an anisotropic Rydberg sheath in nanoplasma

“…The mean charge is shown to reduce from 7.6 to 2.8 where Ar 2+ becomes the most dominant with the same energy as that of the highly charged ions. Our ECTREC formalism has been shown to reproduce well the charge histograms that are experimentally measured by accounting for the charge state dependent charge transfer rates [33].…”

“…Ions are reduced to neutrals by the enhanced charge transfer from Rydberg excited clusters (ECTREC). The charge transfer physics has also been shown to comprehend well the charge reduction for the different charge states produced in these clusters [33]. In these experiments the ionization of isolated Ar clusters is shown to produce a wide charge distribution with Ar 8+ as the most dominant ion and a mean charge of 7.6.…”

Anisotropic negative-ion emission from cluster nanoplasmas

“…It was found that while the ion energies change very little, the ion charge states, as observed in Thompson parabola spectrometry [17], are reduced to an anomalously low value [12]. The unusually large charge-transfer rate that occurs in the transit of the supersonic jet could not be explained by the normal charge-transfer reactions or the electron recombination processes.…”

“…Recently, experiments carried out nearly at 100 times larger cluster densities, where the spatial extent of the cluster jet is also much larger than the laser focus, have revealed hitherto unanticipated intracluster dynamics [12][13][14][15][16]. It was found that while the ion energies change very little, the ion charge states, as observed in Thompson parabola spectrometry [17], are reduced to an anomalously low value [12].…”

“…An ion passing through the Rydberg excited cluster sheath undergoes a very efficient charge transfer because the charge-transfer cross section with the electronically excited systems can be orders of magnitude larger [18]. If the enhanced charge transfer by the Rydberg excited clusters (ECTREC) is taken into account, then the anomalous charge distribution from the Coulomb explosion studies of the clusters could very well be explained [12]. In a system where the average charge state of the ions is reduced from 8+ to 2+, it is obvious that a substantial number of ions would be reduced to neutrals.…”