Generation of energetic negative ions from clusters using intense laser fields

Intense laser fields are known to induce strong ionization in atoms. In nanoclusters, ionization is only stronger, resulting in very high charge densities that lead to Coulomb explosion and emission of accelerated highly charged ions. In such a strongly ionized system, it is neither conceivable nor intuitive that energetic negative ions can originate. Here we demonstrate that in a dense cluster ensemble, where atomic species of positive electron affinity are used, it is indeed possible to generate negative ions with energy and ion yield approaching that of positive ions. It is shown that the process behind such a strong charge reduction is extraneous to the ionization dynamics of single clusters within the focal volume. Normal and well-known charge transfer reactions are insufficient to explain the observations. Our analysis reveals the formation of a manifold of Rydberg excited clusters around the focal volume that facilitate orders of magnitudes more efficient electron transfer. This phenomenon, which involves an active role of laser-heated electrons, comprehensively explains the formation of copious accelerated negative ions from the nano-cluster plasma.