Competition between multiphoton xenon cluster excitation and plasma wave Raman scattering at 248 nm

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Abstract
The role of Raman heating of plasma electrons and the subsequent thermal generation of Xe(M) emission from Xe cluster targets has been examined through studies of the spatial and spectral properties of the observed x-rays. Several experiments conducted on gaseous targets composed of Ne, Ar, Xe and Xe:He, Xe:Ne and Xe:Ar mixtures excited by intense subpicosecond ( fs) ultraviolet (248 nm) pulses have been performed. The results show (i) that forward Raman scattering from a Xe or Xe:He plasma does not occur at significant levels, (ii) that the Xe(M) emission ( keV) observed is not generated by thermal excitation from a hot plasma, and (iii) that targets composed of either Ne, Ar or a mixture of Xe:Ne failed to produce observable x-ray (1 - 3 keV) emission from either Ne or Ar. These findings, viewed in the light of prior experimental results, indicate that both electron cavitation caused by the high ponderomotive force and absorption of the Raman wave by the clusters can strongly suppress the growth of the Raman mode. Consequently, the main mechanism of energy deposition generating the observed Xe(M) emission is direct absorption of the incident 248 nm wave by the clusters.