Ionization of Clusters in Strong X-Ray Laser Pulses

Abstract

The effect of intense X-ray laser interaction on argon clusters is studied theoretically with a mixed quantum/classical approach. In comparison to a single atom we find that ionization of the cluster is suppressed, which is in striking contrast to the observed behavior of rare-gas clusters in intense optical laser pulses. We have identified two effects responsible for this phenomenon: A high space charge of the cluster in combination with a small quiver amplitude and delocalization of electrons in the cluster. We elucidate their impact for different field strengths and cluster sizes. PACS numbers: 33.90.+h, 32.80.Hd, 36.40.Wa, 41.60.Cr The advent of femtosecond laser pulses has triggered numerous activities in the field of laser-matter interaction. In particular novel, non-linear processes in atoms induced by the available high intense fields from these ultrashort pulses have been a challenge for experiment and theory [1]. To understand more generally the energy transfer from intense laser light to matter, complex targets like atomic clusters have been studied [2,3,4,5]. These experiments received considerable attention due to the observed dramatic effects like emission of very fast ions [2] and electrons [3] or the production of coherent X-ray radiation [4]. So far such studies are almost exclusively restricted to visible or infrared wavelengths [6]. The new X-ray free electron laser (xfel) sources, under construction at DESY in Hamburg [7] and at the LCLS in Stanford [8], will change this situation. They can deliver intense laser pulses at high frequencies (from VUV to hard X-ray) and thus open a new regime of strong-field atomic physics.Here we present theoretical investigations of X-ray (frequencyhω = 350 eV, i. e. wavelength λ ≈ 3.5 nm) laser interaction with argon clusters at intensities of I ≈ 3.5·10 14 . . . 3.5·10 18 W/cm 2 . In this laser regime the interaction is notably different from long-wavelength pulses which is evident from the ponderomotive energy E pond ∼ I/ω 2 . It represents the average kinetic energy of a free electron in a laser field, while ∆x ∼ √ I/ω 2 is the quiver amplitude of the electron, i. e., the spatial excursion in the laser field. For the given laser parameters one finds E pond ≈ 0.4 meV . . . 4 eV, vastly different from E pond ≈ 20 eV . . . 200 keV for a 780-nm-laser at the same intensities. The small ponderomotive energy has two important consequences: (i) Despite the high intensities the laser-atom interaction is of non-relativistic and perturbative nature. The latter is also clear from the so-called Keldysh parameter γ which gives the ratio between the tunneling time and the laser period [9] and can be rewritten in terms of the binding energy E bind and the ponderomotive energy E pond as γ = (E bind /2E pond ) 1/2 . For the L and M -shells of argon with E bind = 326, 249, 29.3, 15.8 eV we always find γ > 1 and for the inner shells even γ ≫ 1. Obviously the laser period is far too small for field-or tunnel-ionization. Rather, ionization is due to single-photon absorption....

Charge recombination in soft x-ray laser produced nanoplasmas

“…Even though it is crucial for the success of the imaging experiments, understanding the interaction of intense x-ray pulses with atomic systems and the underlying dynamics is still in its infancy. To date, virtually all studies about the ionization as well as nuclear dynamics of nanometer-sized structures in intense (soft) x-ray pulses are of theoretical nature [3][4][5][6] and no experimental data are available.…”

“…Even though it is crucial for the success of the imaging experiments, understanding the interaction of intense x-ray pulses with atomic systems and the underlying dynamics is still in its infancy. To date, virtually all studies about the ionization as well as nuclear dynamics of nanometer-sized structures in intense (soft) x-ray pulses are of theoretical nature [3][4][5][6] and no experimental data are available.For the experimental investigations of matter in intense light pulses atomic clusters are ideal because their size can be tuned from the molecular to the bulk-like regime and there is no energy dissipation into surrounding media. The ionization dynamics of clusters in intense laser pulses depend considerably on the radiation wavelength.…”

Experimental Evidence for Interatomic Coulombic Decay in Ne Clusters

“…The ICD mechanism may also have important implications for proposed experiments on condensed matter with ultrashort x-ray pulses, e.g., from a free electron laser. Since the energy dissipation by ICD occurs on a very short time scale, it may foreclose the currently often assumed tunneling mechanisms for interatomic charge equilibration [21].…”

Atomistic three-dimensional coherent x-ray imaging of nonbiological 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]. 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.…”

“…The time scales of electronic rearrangement, Auger decay, nuclear motion, nanoplasma formation, Coulomb explosion, and ion-electron recombination are inconveniently similar and comparable to the femtosecond XFEL pulse duration, placing inherent limitations on the structural precision attainable from coherent diffractive imaging. The dynamics initiated by the imaging pulse have been considered using continuum models [36,37], and with molecular dynamics approaches [21,38,39] which, unlike the continuum approach, hold the promise of atomistic tracking. In an early XFEL experiment, 2D imaging of a mimi virus was demonstrated to 30 nm [40].…”