Autoionization dynamics of He nanodroplets resonantly excited by intense XUV laser pulses

TL;DR

This study investigates how helium nanodroplets of various sizes ionize under intense XUV laser pulses, revealing complex electron spectra evolution from atomic-like peaks to nanoplasma emissions, supported by detailed numerical simulations.

Contribution

It provides a comprehensive analysis of helium droplet ionization dynamics under intense XUV irradiation, combining experimental photoelectron spectroscopy with advanced rate equation simulations.

Findings

Observation of size-dependent electron spectra evolution.

Identification of multiple ionization and relaxation processes.

Demonstration of complex transition from atomic-like to nanoplasma emission.

Abstract

The ionization dynamics of helium droplets in a wide size range from 220 to 10^6 He atoms irradiated with intense femtosecond extreme ultraviolet (XUV) pulses of 10^9 {\div} 10^{12} W/cm2 power density is investigated in detail by photoelectron spectroscopy. Helium droplets are resonantly excited in the photon energy range from ~ 21 eV (corresponding to the atomic 1s2s state) up to the atomic ionization potential (IP) at ~ 25 eV. A complex evolution of the electron spectra as a function of droplet size and XUV intensity is observed, ranging from atomic-like narrow peaks due to binary autoionization, to an unstructured feature characteristic of electron emission from a nanoplasma. The experimental results are analyzed and interpreted with the help of numerical simulations based on rate equations taking into account various processes such as multi-step ionization, interatomic Coulombic decay (ICD), secondary inelastic collisions, desorption of electronically excited atoms, collective autoionization (CAI) and further relaxation processes.