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

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 1 Author to whom correspondence should be addressed. Electronic mail: ziaja@mail.desy.de 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.

Energetics, Ionization, and Expansion Dynamics of Atomic Clusters Irradiated with Short Intense Vacuum-Ultraviolet Pulses

“…As suggested in ref. [22], this small discrepancy between experiment and our predictions may be due to the fact that at low fluences the many-body recombination effects within the cold electron plasma can be important [20]. They could then lead to higher ionization states that we do not observe within this model.…”

“…Each of these various approaches lead to the significantly enhanced energy absorption in agreement with the experimental data [11]. However, one would expect that if all proposed enhancement factors are included within one model, it would probably lead to absorption rates much higher than those experimentally observed.In order to evaluate the contribution of various processes to the ionization dynamics, we have constructed a unified model [22], [23] based on kinetic equations including the following predominant interactions: photoionization, collisional ionization, elastic scattering of electrons on ions, inverse bremsstrahlung heating, electrostatic interactions between charges and with laser field, shifts of energy levels within atomic potentials due to …”

“…In order to evaluate the contribution of various processes to the ionization dynamics, we have constructed a unified model [22], [23] based on kinetic equations including the following predominant interactions: photoionization, collisional ionization, elastic scattering of electrons on ions, inverse bremsstrahlung heating, electrostatic interactions between charges and with laser field, shifts of energy levels within atomic potentials due to…”

“…In order to evaluate the contribution of various processes to the ionization dynamics, we have constructed a unified model [22], [23] based on kinetic equations including the following predominant interactions: photoionization, collisional ionization, elastic scattering of electrons on ions, inverse bremsstrahlung heating, electrostatic interactions between charges and with laser field, shifts of energy levels within atomic potentials due to the plasma environment, and shielded electron-electron interactions. Using a non-equilibrium Boltzmann solver, we followed the evolution of the Xe clusters of various size (N atoms = 20 − 90000) irradiated with rectangular VUV pulses of intensity 10 12 −10 14 W/cm 2 and duration, ≤ 50 fs.…”

Charge recombination in soft x-ray laser produced nanoplasmas

“…Theoretically, cluster-core recombination has been suggested to explain differences in observed and calculated charge states [10]. Recent theoretical work predicts that such charge redistribution and neutralization processes will turn the cluster Coulomb explosion into a much slower (>ps) hydrodynamic expansion [26,27].…”