[PAST EVENT] Physics Colloquium - Tom Gallagher
Dr. Tom Gallagher, University of Virginia, Title of Talk: Exploring Strong fields with Rydberg Atoms and Microwaves
Abstract: Multiphoton ionization of atoms by intense laser pulses has led to surprising results, notably above threshold ionization and the production of high lying states accompanying ionization. Using Rydberg atoms and microwaves, the same phenomena can be observed and studied in detail. Although the production of high lying Rydberg states was not observed in early microwave ionization experiments, in better controlled recent experiments it has been. How the high lying states are produced is clarified by spectroscopy in the presence of a microwave field, which reveals a long periodic structure consisting of peaks removed from the ionization limit by integral multiples of the microwave frequency. The Floquet states represented by these peaks exhibit the same ponderomotive AC Stark shift as the limit, and production of high lying states occurs when they are shifted into resonance with an initial Rydberg state. The series of Floquet states extends well above the ionization limit, i. e., laser excitation above the limit in the presence of the microwave field leads to bound states. These observations can be described by a simple model, which is verified by time domain measurements.