Observation of ions with energies above 100 keV produced by the interaction of a 60-fs laser pulse with clusters

The x-ray spectra of a plasma generated by heating CO2 and Ar clusters with high-intensity femtosecond laser pulses with q(las)similar or equal to 10( 18) W/cm(2) are investigated. Spatially resolved x-ray spectra of a cluster plasma are obtained for the first time. Photoionization absorption is obse rved to influence the spectral line profiles. The recorded features of the x-ray emission spectra definitely indicate the existence of a large relativ e number of excited ions (similar or equal to 10(-2)-10(-3)) with energies of 0.1-1 MeV in such a plasma. Possible mechanisms underlying the accelerat ion of ions to high energies are discussed. It is shown that the experiment al results can be attributed to the influence of ponderomotive forces in st anding waves generated by the reflection of laser radiation from the cluste rs. (C) 1999 American Institute of Physics. [S1063-7761(99)01106-3].