X-ray generation by laser-cluster interaction

Abstract

Abstract. We investigate the efficient heating of quasi-free electrons during the interaction of short infrared laser pulses with large rare-gas clusters. In the framework of our mean-field classical transport simulation we are able to explain the emission of characteristic x-rays at moderate laser intensities (I ∼ 10 15 Wcm −2 ) where the ponderomotive energy of the electrons is by far to low to allow for the creation of inner-shell vacancies. We identify large-angle elastic electron-ion scattering as an important heating mechanism at moderate laser intensities.

Simulation of the dynamics of laser-cluster interaction

“…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.…”

“…In order to take into account that the ions are screened by the surrounding quasi-free electrons, we estimate the screened ionic charge stateq j (t) from the local number of quasi-free electrons per ion [6]. As the cluster is much smaller than the wavelength of the laser pulse, the laser can be described as a uniform time-dependent external electric field linearly polarized in the z-direction:…”

“…This formula is also used to estimate the contribution of two-step ionization (electron-impact excitation from L-shell to M-shell followed by an impact ionization). Further ionization mechanisms included in the simulation are Auger decay of L-shell vacancies [23], over-barrier field ionization as well as tunnel ionization [6].…”

“…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.…”

Mechanisms of cluster ionization in strong laser pulses

“…a phase difference between the oscillating electron cloud and the driving field which does not originate from resonant absorption 9 . More recently (Deiss et al 2005), it was argued that the polarization acts against the dephasing and cannot be responsible for enhanced absorption. In any case, as long as the cloud of quasi-free electrons oscillates over a few laser cycles with the laser frequency, any energy gain from the laser field must be due to a phase difference or non-harmonic motion of the electron cloud with respect to the driving field.…”