Coherent 0.5-keV X-Ray Emission from Helium Driven by a Sub-10-fs Laser

Abstract

Helium atoms ionized by intense few-cycle light pulses in the barrier suppression regime emit spatially coherent extreme ultraviolet continuum extending to photon energies greater than E<inf>ph</inf> = 0.5 keV (λ<2.5 nm). The high-energy end of the continuum in the range of E<inf>ph</inf>>=0.2 keV (λ<=6 nm) was characterized spectrally over a considerable dynamic range using energy-dispersive detection. The sub-10-fs laser pulse duration was found to be crucial for generating radiation with the highest photon energies at the low (<0.5 mJ) pump energy levels used in the experiments. The single-atom quantum theory of high-order harmonic generation combined with Maxwell's wave equation provides a satisfactory account for the experimental observations. © 1998 The American Physical Society.