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

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.

THz pulse doubler at FLASH: double pulses for pump–probe experiments at X-ray FELs

“…The X-ray pulse arrival time for the same electron bunch is locked to the respective THz pulse down to a few femtoseconds due to natural synchronization (Frü hling et al, 2009;Tavella et al, 2011;Oelze et al, 2017).…”

“…The X-ray free-electron laser FLASH1 at DESY in Hamburg, Germany, has a worldwide unique feature -a THz undulator behind the main X-ray undulator section, that allows users to perform pump-probe experiments with ultrashort highintensity THz and soft X-ray pulses (Gensch et al, 2008;Frü hling et al, 2009;Tavella et al, 2011;Stojanovic & Drescher, 2013;Oelze et al, 2017). The wavelength range can be tuned from 1 to 300 mm (1 to 300 THz), depending on the electron beam energy.…”

“…Folding of the THz beam adds additional optical path, adding up to a delay of 21.5 ns ($ 6.5 m of optical path) with respect to X-ray pulses at the experimental station. One solution to ensure temporal overlap between THz and X-ray pulses at the experiment is to introduce additional optical path for the X-rays in back-reflection geometry (Feldhaus et al, 2005;Frü hling et al, 2009;Oelze et al, 2017) using normal-incidence multilayer mirrors specially designed for a particular wavelength (Bajt et al, 2007). Although this method has been successfully demonstrated (Frü hling et al, 2009; Oelze et al, 2017), there are two major drawbacks when utilizing this technique (within the specificities of the discussed setup): low reflectivity and narrow spectral range of the X-ray mirrors.…”

Tracing transient charges in expanding clusters

“…3(b). Generally, the electron emission can be attributed to correlated electronic decay processes, where one electron relaxes from an excited to the ground state and transfers the excess energy to a second electron that leaves the cluster [31,32]. Since in the discussed processes an inner-valence vacancy is filled, they can more specifically be described as Auger decay [37], which, to our knowledge, has not been previously observed following ionization of clusters by intense NIR pulses.…”

“…Previous studies have shown that only those nonradiative decay processes, which occur at least a few picoseconds after the laser pulse has ended, can be identified in electron kinetic energy spectra [17,18,31,32]. At earlier times, the cluster potential strongly influences the kinetic energies of emitted electrons.…”