On the occurrence of Balmer spectra in expanding microplasmas from laser irradiated liquid hydrogen

Abstract

Balmer spectra are investigated which are obtained from hydrogen droplets irradiated by ultra-short intense laser pulses. A unified quantum statistical description of bremsstrahlung, the Stark broadening and the van der Waals profile of hydrogen spectral lines is used, which allows to include many-particle effects. Analyzing the line profiles, a low ionization degree of a dense plasma is inferred, where the main contribution to the spectral line shape originates from the interaction with the neutral components. Effective temperatures and densities of the radiating microplasma are deduced. A dynamical description is given within plasma hydrodynamics, explaining the formation of excited atomic states in the expanding system and the occurrence of the observed Balmer lines only below a critical density.