Probing collective multi-electron dynamics in xenon with high-harmonic spectroscopy

High-harmonic spectroscopy provides a unique insight into the electronic structure of atoms and molecules(1-5). Although attosecond science holds the promise of accessing the timescale of electron-electron interactions, until now, their signature has not been seen in high-harmonic spectroscopy. We have recorded high-harmonic spectra of atoms to beyond 160 eV, using a new, almost ideal laser source with a wavelength of 1 : 8 mu m and a pulse duration of less than two optical cycles. We show that we can relate these spectra to differential photoionization cross-sections measured with synchrotron sources. In addition, we show that the high-harmonic spectra contain features due to collective multi-electron effects involving inner-shell electrons, in particular the giant resonance in xenon. We develop a new theoretical model based on the strong-field approximation and show that it is in agreement with the experimental observations.