Interaction of a multi-electron atom with intense radiation in the VUV range: beyond the conventional model for high harmonic generation

We investigate the response of a multi-electron atom to an intense radiatio n at photon energies up to 40 eV. Such sources of radiation are just being tested, based on VUV free-electron laser systems. We observe the generation of high harmonics well beyond the cut-off predicted by the conventional th ree-step model (known also as a simple man's model), successfully applied a t optical wavelengths. The high harmonics (photon energies in keV range) ar e produced in the vicinity of the nucleus, i.e., by the core electrons whic h start to vibrate at higher laser frequencies. We predict the appearance o f even multiples of the driving frequency in the harmonic spectrum just bef ore the electrons in the consecutive, more tightly bound shell become activ e.