HARD X-RAY EMISSION BY CLUSTERS IN AN INTENSE FEMTOSECOND LASER FIELD AT THE COLLECTIVE RECOMBINATION
Krainov V.P., Sofronov A.V.

Received: March 26, 2008

PACS: 36.40.-c, 32.80.Rm

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We consider the new mechanism of X-ray generation by clusters under irradiation by femtosecond laser pulses, the so called collective photo-recombination. We develop the theory of the photo-recombination of electrons that pass from atomic clusters at the outer ionization to the ground level of a homogeneously charged cluster. Such a cluster is considered as a quantum potential well. The dipole approximation is inapplicable for this process. We conclude that X-ray photons in the collective photo-recombination on a charged cluster as a whole have the energy that is much larger than that for the photo-recombination on separate atomic ions inside the cluster. For a typical cluster of electrons, with the radius R=300 and the number density of plasma electrons cm^-3, we obtain that at a 5 % outer ionization of this cluster, the energy of hard X-ray photons is 7.2 keV.