Lithium Rydberg states interacting with high-intensity, far infrared radiation

We have studied the interaction of lithium Rydberg states with high intensity, far infrared radiation. Two sources have been used, a free-electron laser, tunable from 10 to 100 μm, and a biased GaAs-wafer, emitting broadband THz radiation when illuminated with a sub-picosecond optical pulse. Studies on multiphoton ionization of the Li 23s (with 50 μ m <λ< 70 μ m) have yielded the remarkable result that it is less likely to absorb two photons than to first emit a stimulated photon and subsequently absorb three(J. H. Hoogenraad et al.), Phys. Rev. Lett, 75,4579 (1995). Direct two-photon ionization is impeded by the presence of a Cooper minimum in the bound-bound ns-n'p matrix elements. The alternative route (23s arrow 17p arrow 22/23d arrow 50p/f arrow ɛ l) evades this Cooper minimum. Further studies on the Cooper minimum using the GaAs-wafer are presented. Starting in a high Rydberg state, the low energy of the THz photons ensures that multiple Rydberg states are resonant within the bandwidth of the pulse, both for absorbing and for emitting a photon. However, absorption from the ns to the n'p series is again hampered by the Cooper minimum so that the Rydberg state effectively acts as an amplifier for the THz pulse.