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We have developed a novel method of time-of-f light (TOF) photoelectron spectroscopy that permits observation of multiphoton ionizations with extremely high precision, especially for low-probability events. By scanning the laser-produced ionization region across a pinhole we can select specific laser peak intensities. The volumes occupied by low intensities rise rapidly compared with traditional straight TOF spectroscopy, resulting in high signal gains. This technique presents a new way of observing fundamental laser-matter interactions.

We have measured and analyzed the clear emergence of ac-Stark-shifted multiphoton resonances with successive photon orders in xenon. The remarkable quality of our data illustrates the unambiguous evolution through parity-allowed resonances at the eight-and subsequent nine-photon levels. This marks a significant advance in showing that the transient resonance model is valid and strong optical-field ionization remains multiphoton in character at higher intensities. Furthermore, a simple Landau-Zener picture is sufficient to understand the basic principles.