Limiting cross sections for multiphoton coupling

Abstract

The availability of extraordinarily bright femtosecond ultraviolet sources is rapidly extending the study of nonlinear atomic responses into an unexplored regime involving intensities in the range of ∼10 20 W/cm 2 . An estimate is made, covering approximately ten orders of magnitude in intensity, of the effective cross section for nonlinear energy transfer to atoms undergoing subpicosecond irradiation. This treatment, which includes: (a) threshold measurements for low stages of ionization in the low intensity regime ( 10 19 W/cm 2 ), indicates that the cross section for energy transfer in the high intensity (>10 19 W/cm 2 ) high Z limit falls in a relatively narrow range between simply established upper and lower bounds. The values of these limits are σ m =8 πλ c 2 (upper) and the magnitude of the total photoabsorption cross section of Cf at the K edge (lower). Based on this analysis, the maximum cross section for heavy atoms in the high intensity limit is expected to be approximately max ∼10 −20 cm 2 , a value which represents an energy transfer rate of ∼1 W/atom for an assumed intensity of 10 20 W/cm 2 . Coupling of this strength would enable the creation of highly energetic and strongly nonequilibrium states of matter and motivates the conclusion that stimulated emission in the X-ray range can be Estimation de la section efficace effective du transfert d'energie non lineaire vers des atomes soumis a une irradiation subpicoseconde. (Mesures de seuil d'ionisation, determination experimentale pour le transfert total d'energie a intensite intermediaire de la section efficace moyenne, et estimations theoriques pour le regime en champ fort). Application a l'emission stimulee dans le domaine des rayons X