Far-infrared Rydberg-Rydberg transitions in a magnetic field:: Deexcitation of antihydrogen atoms

The dynamics of (de)excitation between highly excited Rydberg states (15 < n < 60 of Rb) in a magnetic field of 0.85 +/- 0.05 T is studied with far-infrared pulses (90-110 and 50 mu m) originating from a free electron laser. We measured the excitation spectrum to states around n=40 from a deeper bound state near n=25. Moreover, starting from a highly excited state (30 < n < 60) below and in the n-mixing regime we investigated the efficiency of the deexcitation channel vs the ionization channel. We measured deexcitation efficiencies well above 50% for some of the states. However, starting deep in the n-mixing regime the deexcitation efficiency is less than 10%. The measurements were in good agreement with fully quantum mechanical calculations. Calculations for deexcitation of n=35 states in H found the largest amount of deexcitation for m=0 and almost none for m=20. In recent experiments at CERN, antihydrogen is produced in high n states in a strong magnetic field with a wide distribution of m. Our measurements and calculations suggest that deexcitation stimulated by infrared photons is not an efficient method for accelerating cascade to the ground state.