We have observed resonant phenomena in the intense-field ionization of argon that do not easily fit into the standard models of above-threshold ionization. High-resolution angleresolved measurements of the energy region around 6-10 times the ponderomotive potential U P in the ATI electron spectrum show at least three separate series of peaks, each with a distinct intensity threshold for their onset. These series do not shift in energy and have characteristic values of intensity for their

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... nce, pointing to an intensity-selective resonant enhancement effect. In order to further investigate this phenomenon, we have observed VUV fluorescence photons from the focal region following ionization. Current theories for the creation of high-energy electrons do not seem to explain these phenomena without significant modifications. We discuss possible physical mechanisms and implications for extending the standard model.