No abstract

Conventional techniques of probing ionization dynamics at relativistic intensities for extended target systems such as clusters are difficult both due to problems of achieving good charge resolution and signal integration over the focal volume. Simultaneous measurement of arrival time, necessary for these systems, has normally involved complicated methods. We designed and developed a Thomson parabola imaging spectrometer that overcomes these problems. Intensity sampling method evolved in this report is proved to be mandatory for probing ionization dynamics of clusters at relativistic intensities. We use this method to measure charge resolved kinetic energy spectra of argon nanoclusters at intensities of 4 × 10(18) W cm(-2).

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.

Ion emissions from clusters in intense ultrashort laser fields have been studied predominantly using time-of-flight (TOF) spectroscopy so far. Assuming atomic ion emission, arrival time signal is converted to a charge-integrated kinetic-energy spectrum. We present here a Thomson parabola spectrum that decrypts the charge-integrated energy distribution to the charge-resolved kinetic-energy spectra (CRKES). TOF measurements compare well with the spectrum generated by encrypting back the CRKES. A quantitative measure of ionization probabilities of Ar 36 000 clusters to varied charge states at 7 × 10 15 W cm −2 is compared with three-dimensional microscopic particle-in-cell simulations. A good agreement between these detailed measurements and the simulations shows the possibility for the retrieval of charge distribution within a nanocluster.

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.