Femtosecond non-equilibrium dynamics of clusters irradiated with short intense VUV pulses

TL;DR

This study models the ultrafast non-equilibrium ionization dynamics of atomic clusters exposed to intense femtosecond VUV pulses using a kinetic Boltzmann approach, comparing results with experimental data and analyzing key effects.

Contribution

It introduces a computationally efficient kinetic Boltzmann model that incorporates various ionization processes and plasma effects, providing new insights into high charge state formation under VUV irradiation.

Findings

High charge states require enhanced electron heating rates.

Plasma environment has a lesser effect on ionization dynamics.

Model results agree with experimental data from FLASH facility.

Abstract

The kinetic Boltzmann equation is used to model the non-equilibrium ionization phase that initiates the evolution of atomic clusters irradiated with single pulses of intense vacuum ultraviolet radiation. The duration of the pulses is < 50 fs and their intensity in the focus is < 10^{14} W/cm^2. This statistical model includes various processes contributing to the sample dynamics at this particular radiation wavelength, and is computationally efficient also for large samples. Two effects are investigated in detail: the impact of the electron heating rate and the effect of the plasma environment on the overall ionization dynamics. Results on the maximal ion charge, the average ion charge and the average energy absorbed per atom estimated with this model are compared to the experimental data obtained at the free-electron-laser facility FLASH at DESY. Our analysis confirms that the dynamics within the irradiated samples is complex, and the total ionization rate is the resultant of various processes. In particular, within the theoretical framework defined in this model the high charge states as observed in experiment cannot be obtained with the standard heating rates derived with Coulomb atomic potentials. Such high charge states can be created with the enhanced heating rates derived with the effective atomic potentials. The modification of ionization potentials by plasma environment is found to have less effect on the ionization dynamics than the electron heating rate. We believe that our results are a step towards better understanding the dynamics within the samples irradiated with intense VUV radiation.