Size-Dependent Ultrafast Ionization Dynamics of Nanoscale Samples in Intense Femtosecond X-Ray Free-Electron-Laser Pulses

Abstract

All matter exposed to intense femtosecond x-ray pulses from the Linac Coherent Light Source (LCLS) free-electron laser is strongly ionized on femtosecond time scales. On these time scales, the ionization is competing with the lifetimes of the created inner-shell vacancies. In the present work, it is shown that for nanoscale objects the environment, i.e., nanoparticle size, is an important parameter for the time-dependent ionization dynamics in intense x-ray pulses because it has an in uence on the inner-shell vacancy liftimes. As a sample system, argon atoms and clusters with sizes between ⟨N⟩ = 55 and ⟨N⟩ = 1600 were chosen. The clusters were irradiated with 480 eV x-ray pulses reaching power-densities of up to a few 10 17 W/cm 2 . At this photon energy dominantly the argon L-shell is ionized and the remaining vacancies are most likely lled via Auger decay. To investigate the electron dynamics, the x-ray pulse length was tuned between 30 fs and 85 fs, using the novel LCLS slotted spoiler technique. The ionization products were measured with an ion time-of-ight spectrometer with a special slit aperture. This aperture e ciently suppresses atomic background, which yields cluster spectra of unprecedented quality. Spectra for di erent pulse lengths and a range of pulse energies were collected for atoms and all cluster sizes. In atoms, as in clusters, longer x-ray pulses are absorbed more e ciently than shorter x-ray pulses with the same number of photons. To shed light on size-dependent e ects in clusters, an independent measure for the time-dependent component of the absorption for every cluster size was found by means of x-ray induced transparency increase (XITI). The XITI increases from atoms to clusters and shows a clear cluster size dependence. A rate equation model for the ionization of atomic systems has been developed to support the interpretation of the experimentally determined XITI as a function of the cluster size. As a result, the Auger lifetimes of large argon clusters are found to be longer than for small clusters and isolated atoms. This is due to delocalization of the valence electrons in the x-ray induced nanoplasma, resulting in a smaller overlap between valence electrons and core holes. As a consequence, large nanometer sized samples absorb intense femtosecond x-ray pulses less e ciently than small ones.

Femtosecond X-ray-induced explosion of C60 at extreme intensity

“…Specifically, we demonstrate the effects of high per-atom X-ray absorption on the fragmentation dynamics of C 60 molecules in the gas phase. C 60 offers a number of advantages as a benchmark molecule when compared with isolated atoms [29][30][31][32][33] , small molecules [34][35][36][37][38][39][40] or van der Waals (vdW) clusters 41,42 , from the perspective of radiation damage. C 60 consists of chemically bonded carbon atoms with representative bond lengths and damage processes.…”

Nanoplasma Dynamics of Single Large Xenon Clusters Irradiated with Superintense X-Ray Pulses from the Linac Coherent Light Source Free-Electron Laser

“…All matter irradiated by the intense x-ray pulses will be highly ionized within femtoseconds. So far, investigations about the ionization dynamics under such conditions have focused on ensembles of atoms and small systems on the one hand [3][4][5][6] and macroscopic solids on the other hand [7]. A detailed understanding of the x-ray pulse-matter interaction for intermediate, i.e., mesoscopic, systems is of utmost relevance for x-ray imaging applications.…”

Atomistic three-dimensional coherent x-ray imaging of nonbiological systems

“…Existing XFEL measurements on Ar atoms through Ar 1000 clusters of ion and electron yields as a function of photon energy, pulse duration, and fluence [10,54] allow us to validate our model and approach to understanding 3D imaging for nanoscale systems. Furthermore, there is intrinsic interest in the study of atomic clusters where the composition and structure can be controlled as a testing ground for new regimes of intense laser-matter interaction [55,56].…”

“…Unprecedented focused intensities at ultrashort wavelengths have led to the discovery of xray phemonena such as nonlinear multiphoton absorption in atoms, molecules, and clusters [6][7][8][9][10], atomic x-ray lasing [11,12], induced transparency or saturable absorption [6,9,10,[13][14][15], stimulated emission [16][17][18], and second harmonic generation [19]. Understanding these fundamental processes underpins the use of ultraintense XFEL pulses and fuels the dream of 3D imaging of single biomolecules using the "diffract-before-destroy" method initially envisioned at the dawn of the XFEL-enabled era of x-ray science [20,21].…”

“…Furthermore, there is intrinsic interest in the study of atomic clusters where the composition and structure can be controlled as a testing ground for new regimes of intense laser-matter interaction [55,56]. Rare gas clusters, bound by easy-to-model van der Waals forces, have traditionally served as testbeds as intense lasers have evolved from optical to x-ray wavelengths [10,38,[57][58][59][60][61][62][63][64]. FEL-induced transient dynamics in rare gas clusters have been observed in imaging experiments in the XUV [65] and, more recently, in the x-ray regime [66] providing evidence for femtosecond time scale electronic damage.…”