Abstract

High-order harmonic generation by means of high-energy, femtosecond laser pulse has proved to be a convenient way to generating coherent radiation in the extreme-ultraviolet (XUV) spectral regions. Recent experiments [1,2] show that harmonic duration is much shorter than the pump one while divergence measurements show that the harmonic beam should be focussable to a fairly small spotsize and reach intensities high enough for nonlinear interactions. In this contribution we report the first electron energy spectrum(^*) resulting from non-resonant multiphoton transitions induced in the XUV domain using harmonics produced in xenon by the 120 fs, 10 mJ Ti:sapphire laser at the Palaiseau facility. An annular beam geometry[3] is used to separate the fundamental from the harmonics which are focussed in a magnetic bottle electron spectrometer filled with 10^-2 torr of helium by a (W-Re)-coated spherical mirror (R=70mm). The intensity of the pump laser is adjusted to .9 mJ for a cutoff at harmonics 15. Consequently, no one-photon ionization of helium is observed. Photoelectrons are detected only when the XUV radiation reflected by the mirror is focussed in the sensitivity zone of the magnetic bottle (Fig.1). This behavior is the signature of a strong intensity dependence of the ionization process and rules out one-photon processes.

© 1997 Optical Society of America