The role of atomic physics (collisional ionization, Coulomb binary collisions, recombination) in laser-produced plasma.

We present a comprehensive investigation of atomic physics in laser-produced plasma using high-intensity laser pulses irradiating sub‑micron planar targets. Building on recent findings, we explore three key processes in laser-produced plasma: (1) collisional ionization, (2) Coulomb binary collisions, and (3) recombination.

Through combined PIC simulations and experimental validation, we demonstrate that collisional ionization is responsible for generating very high charge states, e.g., up to Z = 75 for gold ions.

Notably, for the first time, we have demonstrated the influence of Coulomb binary collisions firstly on the transverse emittance growth of ions and secondly on the final charge state distribution of ions, and specifically the generation of lower ionization levels. In these simulations, we took into account the measured spatio‑temporal laser profiles that had major influences on ion acceleration. Using experimentally measured laser pulse profiles in simulations, rather than idealized profiles, leads to more realistic results and better agreement with experimental data.

We then identified previously overlooked recombination effects, particularly dielectronic, radiative, and three-body recombination, that can alter the charge-state distribution of ions during laser-driven ion acceleration. These effects become more pronounced in thicker targets where ions spend more time in environments with higher recombination rates. Simulations indicate that recombination can lead to a decrease in the average charge state of ions, particularly in thicker targets, aligning with experimental observations.