Correlated electronic decay in expanding clusters triggered by intense XUV pulses from a Free-Electron-Laser

Tim Oelze,Bernd Schütte,Maria Krikunova,Torsten Golz,Maria Müller,Markus Drescher,Nikola Stojanovic,Alaa Al-Shemmary,Ulrike Frühling,Jan P Müller,Marek Wieland

doi:10.1038/srep40736

Abstract

Irradiation of nanoscale clusters and large molecules with intense laser pulses transforms them into highly-excited non- equilibrium states. The dynamics of intense laser-cluster interaction is encoded in electron kinetic energy spectra, which contain signatures of direct photoelectron emission as well as emission of thermalized nanoplasma electrons. In this work we report on a so far not observed spectrally narrow bound state signature in the electron kinetic energy spectra from mixed Xe core - Ar shell clusters ionized by intense extreme-ultraviolet (XUV) pulses from a free-electron-laser. This signature is attributed to the correlated electronic decay (CED) process, in which an excited atom relaxes and the excess energy is used to ionize the same or another excited atom or a nanoplasma electron. By applying the terahertz field streaking principle we demonstrate that CED-electrons are emitted at least a few picoseconds after the ionizing XUV pulse has ended. Following the recent finding of CED in clusters ionized by intense near-infrared laser pulses, our observation of CED in the XUV range suggests that this process is of general relevance for the relaxation dynamics in laser produced nanoplasmas.

Highlights

Irradiation of nanoscale clusters and large molecules with intense laser pulses transforms them into highly-excited non- equilibrium states
Nanoscale objects exposed to intense ultrashort light pulses are transformed into highly-excited non-equilibrium states[1,2,3]
A peak structure at ≈13.7 eV is well pronounced above a thermal electron distribution in Xe core - Ar shell clusters with 〈Ν〉= 4000 atoms (a)

Summary

Introduction

Irradiation of nanoscale clusters and large molecules with intense laser pulses transforms them into highly-excited non- equilibrium states. In this work we report on a so far not observed spectrally narrow bound state signature in the electron kinetic energy spectra from mixed Xe core Ar shell clusters ionized by intense extreme-ultraviolet (XUV) pulses from a free-electron-laser. This signature is attributed to the correlated electronic decay (CED) process, in which an excited atom relaxes and the excess energy is used to ionize the same or another excited atom or a nanoplasma electron. Quasi-free energetic electrons excited within nanoscale samples and bulk solids thermalize through multielectron as well as lattice collisions[4,7] Another deexcitation mechanism is related to an autoionization of excited atomic states. High density of atomic excited states populated within a single cluster results in collective autoionization processes in which several excited atoms are involved as predicted theoretically[17] and observed experimentally at a free electron laser (FEL)[18,19,20]

Methods

Results

Conclusion