Multi-keV Electron Generation in the Interaction of Intense Laser Pulses with Xe Clusters

Abstract

We have observed the production of multi-keV electrons through the irradiation of Xe clusters by an intense, near infrared, femtosecond laser pulse. We find the electron kinetic energy distribution consists of two features, a "warm" peak of between 0.1 and 1 keV and a "hot" peak of energy between 2 and 3 keV. These measurements are consistent with a picture of rapid electron collisional heating in the cluster and exhibit good agreement with numerical modeling of the electron energy distribution.[S0031-9007(96)01420-2] PACS numbers: 36.40.Vz, 33.80.Rv, 36.40.Gk Though the nature of intense, short pulse laser interactions with single atoms and solid targets has been the subject of extensive experimental and theoretical investigation over the last 15 years [1], the dynamics of intense laser interactions with large molecules and atomic clusters has scarcely been studied during this time. The production of highly charged ions from individual atoms through multiphoton [2] and tunnel ionization [3] in a strong field has been thoroughly researched, as have the energy distributions of the electrons produced during these interactions [above threshold ionization (ATI)] [4][5][6]. Concurrently, the production of hot, high density plasmas by the intense irradiation of a solid by a short pulse laser has also been the subject of detailed studies [7,8]. Experiments on individual atoms have confirmed that the majority of electrons released by single atoms in a laser field of intensity ,10 16 W ͞cm 2 typically exhibits kinetic energies of ,100 eV [6]. Interactions with solids, on the other hand, have been shown to be much more efficient at coupling laser energy into electron energy. The electron temperature in these experiments is, however, usually clamped at a few hundred eV due to the conduction of the laser energy into surrounding cold, bulk material [9].Only recently has the nature of intense laser interactions with van der Waals bonded atomic clusters of 20-100 Å been addressed in experiments. These experiments have suggested that the laser-cluster interaction is much more energetic than that of isolated atoms, producing bright xray emission (100-5000 eV photons) when a low density gas containing clusters is illuminated [10][11][12]. The interactions also appear to be quite different than those of laser solid target interactions since a cluster, though like a solid, having high local density and therefore a high collision frequency, is unlike a solid because it is an isolated system, much smaller than a laser wavelength. Consequently, the laser interacts uniformly with all the atoms, much more like the interaction of a laser with a low density gas. Recent experiments by Ditmire et al. have indicated that the electrons in a cluster undergo rapid collisional heating for the short time ͑,1 ps͒ before the cluster disassembles in the laser [11]. These measurements indicated indirect evi-dence for keV electron production in the cluster through time resolved x-ray spectroscopic data. In fact, irradiation of Xe clusters at intensi...

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