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

Abstract

Intense laser-produced plasma is a complex amalgam of ions, electrons and atoms both in ground and excited states. Little is known about the spatial composition of the excited states that are an integral part of most gaseous or cluster plasma. In cluster-plasma, Rydberg excitations change the charge composition of the ions through charge transfer reactions and shape the angular distributions. Here, we demonstrate a non-invasive technique that reveals the anisotropic Rydberg excited cluster sheath by measuring anisotropy in fast neutral atoms. The sheath is stronger in the direction of light polarization and the enhanced charge transfer by the excited clusters results in larger neutralization.

Anisotropic negative-ion emission from cluster nanoplasmas

“…In our earlier experiments, we demonstrated that neutralatom emission is more anisotropic than the positive-ion emission from single-cluster explosions [15]. A priori, one would expect that the anisotropy in the negative-ion emission should be more than that of neutrals, since anions are formed from a further electron-transfer reaction that reduces the neutral atoms.…”

“…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].…”

“…much more efficient. Subsequently, neutral-atom generation is much more anisotropic than the anisotropy of the positive ions that are Coulomb exploded from the isolated single clusters [15]. In fact the charge-transfer rate can be used to measure the anisotropy in the Rydberg excited sheath formed around the laser focus [15].…”

Summary of laser plasma physics sessions at the first AAPPS-DPP conference

“…Krishnmaurthy [L-I27] reported on the acceleration of neutral atoms in laserproduced plasmas. It was shown that laser plasma ion accelerators can be converted to compact-efficient MeV neutral atom accelerators (Rajeev et al 2013(Rajeev et al , 2015Malay et al 2017). In particular, with nanocluster exposed to intense laser pulses, it is possible to convert more than 90% of the accelerated ions to neutral atoms.…”