Pressure Ionization and Density Diagnostics in Subpicosecond Laser-Produced Plasmas

Abstract

The atomic physics of high-density plasmas is studied extensively for its relevance to astrophysics¹, inertial confinement fusion,^2,3 x-ray lasers,⁴ and to the interaction of ultrashort lasers with solids. ^5-7 Of utmost importance is the knowledge of the plasma parameters of electron density, N_e, and temperature, T_e, as they govern the atomic physics in the plasma, from its ionization balance to its emission and absorption. The structure and behavior of atoms and ions, for example, can be radically affected by the presence of strong fields in high-density plasmas¹, leading to such effects as extreme line broadening and pressure ionization.^1,2,9 Pressure ionization and line-merging have been used in laboratory plasmas as a density diagnostic of spatially- and/or temporally-integrated spectra. ^2,10–13 But in laser-produced plasmas, conditions often vary rapidly over time and space, so it is important to resolve both these dimensions for accurate diagnostics. Furthermore, several models are available to quickly extract densities from spectroscopic data but are very different and need to be carefully benchmarked in order to identify which apply for any given set of plasma parameters. Precise data for model validation is rare and usually comes from plasmas limited in density and temperature range.¹³ Here, we compare four models under a wide range of densities and temperatures in plasmas created with ultrafast laser pulses. These 100-fs laser pulses have the advantage over nanosecond pulses of depositing the energy of the laser impulsively, in a small target layer. Thus, the spectroscopic measurements are conducted after the laser pulse, in a freely expanding plasma, without the added complication of further energy deposition during the plasma evolution.