Connecting field ionization to photoionization via 17- and 36-GHz microwave fields

Abstract

Here we present experimental results connecting field ionization to photoionization in Li Rydberg atoms obtained with 17-and 36-GHz microwave fields. At a low principal quantum number n, where the microwave frequency ω is much lower than the classical, or Kepler frequency, ω K = 1/n 3 , microwave ionization occurs by field ionization, at E = 1/9n 4. When the microwave frequency exceeds the Kepler frequency, ω > 1/n 3 , the field required for ionization is independent of n and given by E = 2.4ω 5/3 , in agreement with dynamic localization models, which cross over to a Fermi's Golden Rule approach at the photoionization limit. A surprising aspect of our results is that when ω ≈ 1/2n 2 , the one-and multiphoton ionization rates are similar, and even at the lowest microwave powers, all are 10 times lower than the perturbation theory rate calculated for single-photon ionization. Further, we show that when the Rydberg atoms are excited in the presence of the microwave field, the probability of an atom's being bound at the end of the microwave pulse passes smoothly across the limit. This microwave stimulated recombination to bound Rydberg states can be well described by a simple classical model. More generally, these results suggest that the problem of a Rydberg atom coupled to a high-frequency microwave field is similar to the problem of interchannel internal coupling in multilimit atoms, a problem well described by quantum defect theory.

Phase-Dependent Electron-Ion Recombination in a Microwave Field

“…which is in excellent agreement with the experimental observations of Shuman et al [10] and Gurian et al [12]. In Fig.…”

“…Collectively, these observations suggest that quasi stable Floquet states are formed in both cases, as suggested by Gurian et al and Tong et al [12,14]. In the He case, there are bound levels of He separated by frequencies close to the IR frequency, so it is easy to imagine a quasi stable wave packet based on these coupled states.…”

“…In the frequency domain recombination experiments, while the energy extent above the limit increases with field according to Eq. (2), the amplitude of the signal for a specific laser tuning close to the limit often decreases [10,12]. Since our ps laser excitation only addresses a 250 GHz wide band just above the limit, as the extent of the above threshold recombination grows beyond 250 GHz the recombination signal decreases.…”

Metastable states in microwave ionization

“…Although one might expect the classical model to work better for higher n states, where Ω > 1, it predicts that at a fixed frequency the microwave field required for ionization decreases with n [6]. However, experiments have shown that for Ω > 1 the ionization field only depends on the microwave frequency until the initial binding energy is within a few microwave photons of the ionization limit [7,8]. When the initial Rydberg state is within one or two microwave photons of the ionization limit, we would expect perturbation theory to give an accurate prediction of the ionization rate.…”

Dichotomy between tunneling and multiphoton ionization in atomic photoionization: Keldysh parameter γ versus scaled frequency Ω

“…Experiments such as the ones in Refs. [9,10] have suggested that the ratio of the laser frequency ω to the classical orbital frequency ω K of the bound electron, i.e., ω/ω K , could be just as relevant in deducing the dominant physical mechanisms as the laser frequency ω itself. This ratio is termed the scaled frequency and is given by = ωn 3 for atoms.…”