Ion imaging of spatially inhomogeneous nanoplasmas in NaCl particles.

Studying photoemission from free, unsupported aerosol particles is a powerful method for gaining insight into light-matter interactions at the nanoscale. We used single-shot velocity map imaging to experimentally measure kinetic energy and angular distributions of ions emitted following interaction of sub-micrometer NaCl particles with femtosecond pulses of near infrared (NIR, 800 nm) and ultraviolet (UV, 266 nm) light. We combined this with time-dependent simulations of light propagation through the particles and a rate equation approach to computationally address the origin of the observed ion emission. For both NIR and UV pulses, ion emission is caused by the formation of an under-dense nanoplasma with similar densities, although using an order of magnitude weaker UV intensities. Such conditions result in remarkably similar ion fragments with similar kinetic energies, and no obvious influence of the plasma formation mechanism (photoionization or collisional ionization). Our data suggests that Coulomb explosion does not play a significant role for ion emission, and we discuss alternative mechanisms that can lead to material ablation from under-dense nanoplasma. Finally, we show how finite size effects play an important role in photoemission through generation of spatially inhomogeneous nanoplasmas, which result in asymmetric ion emission that depends on particle size and laser wavelength. By utilizing the single-particle information available from our experiments, we show how finite size effects and inhomogeneous nanoplasma formation can be exploited to retrieve the size and orientation of individual submicrometer aerosol particles.