Asymmetric emission of high-energy electrons in the two-dimensional hydrodynamic expansion of large xenon clusters irradiated by intense laser fields

Abstract

Energy spectra and angular distributions have been measured of electrons that are emitted upon disassembly of Xe_n clusters (n=20000-150000) following irradiation by intense (10¹⁵-10¹⁶W cm^-2) laser pulses whose durations are varied over the 100-2200 fs range. The cluster explosion dynamics occur in the hydrodynamic regime. For the smaller clusters in the range that we have studied, a single-electron temperature adequately describes the measured electron energy distribution; in the case of larger clusters, a two-temperature fit becomes necessary. The total electron emission is found to be unexpectedly asymmetric and exhibits a resonance when the laser-pulse duration is ~1ps. These results are rationalized by extending the hydrodynamic model to also take into account the force that the light field exerts on the polarization charge that is induced on the surface of the cluster. We show that the magnitude of this electrostrictive force is comparable to those of the Coulombic and hydrodynamic forces, and it exhibits resonance behavior. Contrary to the findings of the only other earlier report, we find that the low-energy component in the electron energy distribution is connected to the resonance in energy absorption by the cluster. The high-energy component seems to be produced by a mechanism that is not so strongly influenced by the resonance.