Ionization of Na Rydberg Atoms by Subpicosecond Quarter-Cycle Circularly Polarized Pulses

Abstract

We report the first observation of ionization of Rydberg atoms by subpicosecond, circularly polarized THz radiation. The field amplitude in these pulses is non-negligible for only one-quarter of an optical cycle. The experiment is performed in the short-pulse regime, where the duration of the ionizing pulse is shorter than the classical Kepler period of the Rydberg electron. We find that the ionization probability for these atoms is remarkably insensitive to the time-varying polarization of the THz field.Over the years, multiphoton ionization of atoms has been recognized as a process of fundamental importance for understanding the dynamic interaction between atoms and intense radiation fields. Very recently, a number of in vestigations have led to the discovery of novel ionization dynamics during the interaction of highly excited Rydberg atoms with strong, subpicosecond pulses of THz radiation [1][2][3][4][5][6][7]. The unipolar electric field in these pulses resembles one-half of an optical cycle of an oscillating electric field so they are commonly referred to as "half-cycle" pulses (HCPs). To date, experimental and theoretical HCP stud ies have considered only linearly polarized fields. In this configuration, HCPs provide an impulsive unidirectional "kick" that can literally push a Rydberg electron off an atom [1][2][3]8]. Although the ionization dynamics are now relatively well understood for unidirectional fields, previ ous work with multi-cycle laser [9] and microwave pulses [10], as well as classical intuition, suggests that ionization by subpicosecond far-infrared pulses might proceed very differently with circularly polarized radiation.In this Letter, we describe the results of the first experi ment on the ionization of atoms by subpicosecond circu larly polarized THz pulses. The duration of these pulses is so brief that the electric field vector rotates by only 90 ± during the pulse. In the language of classical physics, an electron exposed to this "quarter-cycle" circularly po larized pulse is subjected to a rapidly rotating force in contrast to a unidirectional kick. Therefore, one should expect to observe interesting new ionization dynamics us ing these novel pulses.The interaction of atoms with these pulses is also in teresting from the point of view of collision physics. A quarter-cycle field pulse bears a strong resemblance to the time-dependent field seen by an atom undergoing a colli sion with a charged particle, as shown explicitly in Fig. 1. In each case, the field can be described as a combination of a half-cycle cosine pulse along one axis (the transverse collision field) and a "single-cycle" sine component along an orthogonal axis (the longitudinal collision field). In contrast to true collision experiments, the relative veloc ity, impact parameter, and orientation of the colliding par ticles can be precisely controlled in a "mock collision" by adjusting the duration, field amplitude, and polarization of the quarter-cycle field pulse.It is well known that circularly polarized radiation can...

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