Microwave ionization of Rydberg atoms
Microwave ionization of Rydberg atoms
Haruka Maeda
Dept. of Phys.& Math., Aoyama Gakuin Univ., Fuchinobe, Sagamihara, Kanagawa 252-5258, Japan
When exposed to intense microwave fields, Rydberg atoms are ionized. It is well known that microwave ionization of Rydberg atoms are described either by a photon point of view or treating the microwave as classical oscillating fields [1]. In extremely intense microwave fields, tunneling ionization or over-the-barrier ionization takes place, in which Rydberg atoms “feel” electric fields of the microwave fields. In low frequency regime, i.e., Ω < 1 where Ω = ωn^3 is a scaled frequency, ω is microwave (angular) frequency, and n is principal quantum number of the initial Rydberg state, alkali atoms such as Na or Li are ionized at the electric field amplitudes blow 1/n^4, i.e., classical field ionization limit, to as low as 1/n^5 [2]. We use atomic units unless otherwise specified. It is pointed out that the 1/n^5 field correlates with Ingliss-Teller limit, at which n and n+1 Stark manifolds intersect [1]. At this field the atom can evolve from the initial n state to n+1 state via single Landau-Zener (LZ) transition. Once the atom undergoes the n→n+1 LZ transition, the atom is sooner or later driven into very high-lying states or even into continuum after many cycles of the microwave field, because the n→n+1 LZ transition is the rate limiting step. H.B. van Linden van den Heuvell et al. show that the ionization process of alkali Rydberg atoms via the LZ transition can be well understood based on the photon point of view [3].
Recently we have experimentally studied microwave ionization of Li Rydberg atoms in low-frequency regime. In our experiments we have measured final-state distribution of Li Rydberg atoms exposed to linearly polarized 17.5-GHz microwave pulses by varying microwave-field amplitudes in the energy region of 23 ≤ n ≤ 72, which is equivalent to 0.03 ≤ Ω ≤ 1. Our measurements show that microwave field amplitudes at which 50-% ionization of initial Rydberg atoms occurs tend to deviate from 1/n5 scaling law and increase as n increases, whereas the microwave amplitudes at which transitions of the Rydberg atoms into neighboring state occur nearly match the Ingliss-Teller limit, 1/3n^5. Our results indicate that the n→n+1 LZ transition does not necessarily play a role of the rate limiting step in the ionization process.
References
[1] see, for example, T.F. Gallagher, “Rydberg Atoms”, (Cambridge Univ. Press, New York, 1994).
[2] P. Pillet et al, Phys. Rev. A 30, 280 (1984).
[3] H.B. van Linden van den Heuvell et al., Phys. Rev. Lett. 53, 1901 (1984).
