Revisit of recombination processes of low-charge-state ion generation during picosecond intense laser–gas interaction

The origin of [Formula: see text] ions in picosecond intense laser–gas interaction was experimentally and numerically investigated. A nanosecond laser pulse was used to modulate the plasma condition, which helps to distinguish the origin of the helium ions with different charge states. By comparing the energy spectra of [Formula: see text] and [Formula: see text] ions in the experiments with/without plasma shaping, we found neither ion–atom nor electron–ion recombination is responsible for the [Formula: see text] generation, which is supported by two-dimensional hydrodynamic simulations and theoretical calculations.