Dynamics of small clusters irradiated by intense laser fields

Summary form only given. We have developed a new model describing the interaction of an intense (10/sup 18/-10/sup 19/ W/cm/sup 2/) ultrashort laser pulse (10-100 fs) with small clusters (5-20 A) to characterize the radiative characteristics as the cluster evolves in time. We refer to the new model as an "ignition and oscillation" model. When clusters of rare gas atoms are excited by TW UV radiation, X-ray emission with wavelengths of a few angstroms is produced by highly charged ions. In recent experiments with Xenon, X-ray emission spectra from the L-shell (3d-2p) were recorded. The target consisted of small Xe clusters excited by UV (248 nm) and IR (800 nm) laser systems under comparable conditions. It was found that the Xe(L) emission spectra depended strongly on the laser wavelength; the X-ray intensity produced by the UV laser was about thousand times stronger than its IR counterpart. These striking differences in Xe(L) X-ray emission suggested that there is a fundamental difference in the mechanisms responsible for production of KeV X-rays. To address this issue, we modeled the cluster dynamics with our version of the plasma model accounting for the cluster ionization and the impact of the outer (ejected) electrons. We found that under these conditions nearly all the electrons leave the cluster. These electrons are readily accelerated by the ponderomotive potential of the laser field to energies of 10-100 KeV, sufficient to produce collisional inner-shell ionization. Since these electrons play a key role in understanding the differences in the Xe(L) X-ray emission spectra, we studied them in more detail. In particular, the equation of motion of electrons has been studied both inside and outside the cluster, and the expansion of both the cluster ion core and electron cloud has been investigated. We found that the electron cloud expansion is very fast, and the laser frequency therefore plays a critical role in the probability that the high-energy oscillating e- ectrons have time to interact with the cluster and produce X-ray radiation. The resulting laser wavelength dependence of the X-ray intensity (I(/spl lambda/)/spl sim//spl lambda//sup -n/) is very strong, with n=5.