We present a detailed investigation of the propagation properties of beams of ultrashort terahertz (THz) pulses emitted from large-aperture (LA) antennas. The large area of the emitter is demonstrated to have substantial influence on the temporal pulse profile in both the near field and the far field. We perform a numerical analysis based on scalar and vectorial broadband diffraction theory and are able to distinguish between near-field and far-field contributions to the total THz signal. We find that the THz beam from a LA antenna propagates like a Gaussian beam and that the temporal profile of the THz pulse, measured in the near field, contains information about the temporal and spatial field distribution on the emitter surface, which is intrinsically connected to the carrier dynamics of the antenna substrate. As a result of pulse reshaping, focusing of the THz beam leads to a reduced relative pulse momentum, with implications in THz field-ionization experiments.

We present measurements of the electron ejection direction in the ionization of high (n 90) Rydberg states of rubidium subjected to few-cycle radio-frequency (RF) pulses. For weak pulses we find a strong asymmetry for even (cosine) pulses and no asymmetry for odd (sine) pulses. This asymmetry disappears for pulses longer than four RF cycles. For strong pulses, very large asymmetry is found for both sine and cosine pulses that persists up to eight RF cycles and is attributed to initial state depletion effects within a cycle.

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The asymptotic velocity distribution of electrons ionized in half-cycle-pulse excitation of high Rydberg states (n=34), placed in a static electric field, is studied using electron velocity-map imaging. At weak half-cycle pulse strengths, the electrons escape over the saddle point in the potential. For strong half-cycle pulses, the electrons are emitted in the direction of the field kick. The much slower and less intense half cycle of opposite polarity, which necessarily follows the main half-cycle pulse, strongly affects the momentum distribution and reduces the excess energy of the electrons significantly.

The dynamics of ͑de͒excitation between highly excited Rydberg states ͑15Ͻ n Ͻ 60 of Rb͒ in a magnetic field of 0.85 ±0.05 T is studied with far-infrared pulses ͑90-110 and 50 m͒ 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 ͑30Ͻ n Ͻ 60͒ 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.