Simulation of the dynamics of laser-cluster interaction

Abstract

Abstract. We study the interaction of an intense (I 10 15 Wcm −2 ) femtosecond laser pulse with a large (N > 10000 atoms) rare-gas cluster. The simulations are based on a mean-field classical transport approach. The electronic dynamics during the interaction with the laser are discussed in more detail. In particular we point out the difference in behavior between the fast electrons and the collectively moving slow electrons and try to shed light on the acceleration mechanisms behind the high energy tail of the electron energy distribution. The benchmark for our simulations is experimental X-ray spectroscopy data. We show a comparison with the experimentally found total X-ray yields and charge-state distributions. IntroductionRanging in size from a few atoms to several million atoms agglomerated at solid densities, clusters form a link between the gas and the solid phase. One manifestation is the up-conversion of the incident infra-red laser into keV X-ray photons in rare-gas clusters, which takes place with a relatively high efficiency providing large X-ray yields just like in solids, yet is relatively debris-free, a property shared with gas targets [1]. Similar behavior has been observed for the emission of energetic electrons [2] or highly charged ions [3], thus making the interaction of intense short and ultra-short laser pulses with clusters a topic of considerable interest [4; 5].In a simple picture, the dynamics during the interaction of a strong laser pulse with a cluster can be summarized as follows [6]: the atoms of the cluster are first ionized by the incident laser pulse (inner ionization) and a cold "nano-plasma" of solid density is formed. The quasi-free electrons take part in a collective oscillation driven by the laser field and, moreover, interact with the field of the surrounding particles. Electron-impact ionization of cluster ions produces additional quasi-free electrons and inner-shell vacancies which are at the origin of characteristic X-ray radiation. As a fraction of the electrons leaves the cluster (outer ionization), a net positive charge is left behind and the cluster begins to expand before disintegrating completely in a Coulomb explosion.The size of the system and the abundance of mechanisms at play provide a considerable challenge for the theoretical description of the interaction. One approach to the problem is treating the cluster as a "nanoplasma" submitted to a laser field [7; 8]. The inclusion of the cluster boundaries and the inhomogeneous ionic background during cluster expansion is however problematic. Molecular dynamics simulations on the other hand are limited to clusters of about 1000 atoms [9][10][11], and a scaling of the results to larger cluster sizes is difficult. Clusters of ∼ 10 4 atoms have been simulated using a microscopic particle in cell (MPIC) code [12], but typical

Xe K-shell X-ray generation using conical nozzle and 25 TW laser

“…We found that the Xe K-shell X-ray photon number increases with increasing cluster radius from 8 to 12 nm, and then saturates at cluster radii between 12 and 17 nm. This result is similar to the result obtained by Deiss (2009) that an Ar cluster radius on the order of 10 nm is optimum for Ar K-shell X-ray generation.…”

“…A similar cluster radius dependence was found for Ar K-shell X-ray generation simulated by Deiss (2009). Laser-cluster interaction was simulated with a mean-field classical transport approach.…”

“…The threshold laser intensity for Ar K-shell X-ray generation was simulated by a mean-field classical transport approach by Deiss and coworkers (Deiss, 2009;Prigent et al, 2008), who found that an optimum cluster radius was of 10 nm order for Ar K-shell X-ray generation. The threshold intensity of 2 × 10 15 W/cm 2 was obtained by the simulation, and it is close to the 2.9 × 10 15 W/cm 2 observed experimentally, as shown in Table 3.…”