Pump laser wavelength-dependent control of the efficiency of kilovolt x-ray emission from atomic clusters

An explanation is presented for the recently reported striking differences in the kilovolt Xe L-shell (3d --> 2p) x-ray emission from Xe cluster targe ts excited by comparable terawatt ultraviolet (248 nm) and infrared (800 nm ) femtosecond laser pulses under nearly identical experimental conditions ( Kondo K et al. 1997 J. Phys. B.: At. Mel. Opt. Phys. 30 2707-16). A classic al analysis of these results, within the framework of the first Born approx imation for electron-atom collisions producing inner-shell ionization, stro ngly suggests that both the similar to 3000 times stronger Xe(L) emission u nder ultraviolet laser excitation and the observed differences in the x-ray spectra are caused primarily by the different ultraviolet and infrared pum p laser wavelengths. The kinematics of photoionized electrons in the intens e laser fields (10(18) 10(19) W cm(-2)) and the Coulomb-driven expansion of the electron distribution photoionized from the atomic cluster both indica te that the strong pump-laser wavelength scaling in the production of kilov olt x-rays from Xe clusters results from the more localized and controlled electron-cluster interactions afforded by a shorter optical period.