EVIDENCE FOR RESONANT EFFECTS IN HIGH-ORDER ATI SPECTRA

A We have observed resonant phenomena in the intense-field ionization of argon that do not easily fit into the standard models of above-thre shold ionization. High-resolution angle-resolved measurements of the e nergy region around 6-10 times the ponderomotive potential U-P in the ATI electron spectrum show at least three separate series of peaks, ea ch with a distinct intensity threshold for their onset. These series d o not shift in energy and have characteristic values of intensity for their appearance. pointing to an intensity-selective resonant enhancem ent effect. In order to further investigate this phenomenon, we have o bserved VUV fluorescence photons from the focal region following ioniz ation. Current theories for the creation of high-energy electrons do n ot seem to explain these phenomena without significant modifications. We discuss possible physical mechanisms and implications for extending the standard model.