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Physical Review A

Autoionization of very-high- n strontium Rydberg states

G. Fields, X. Zhang, F. B. Dunning, S. Yoshida, Joachim Burgdörfer

Published Jan 31, 2018• 19 citations
Publisher

Abstract

We study, using a combination of experiment and theory, the excitation and decay of very-high-$n\phantom{\rule{4pt}{0ex}}(n\ensuremath{\sim}280\text{--}430)$ strontium autoionizing Rydberg states formed by near-resonant driving of the $5s\phantom{\rule{0.16em}{0ex}}^{2}S_{1/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}$ core-ion transition. The branching ratio between decay through radiative transitions and through autoionization is explored. Autoionization rates are measured as a function of both the $n$ and $\ensuremath{\ell}$ quantum numbers of the Rydberg electron. The nonstationary decay dynamics is studied by creating and manipulating Rydberg wave packets and by varying the laser pulse that drives the core excitation.

DOI
10.1103/physreva.97.013429

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We study, using a combination of experiment and theory, the excitation and decay of very-high-n(n\ensuremath∼280–430)n\phantom{\rule{4pt}{0ex}}(n\ensuremath{\sim}280\text{--}430)n(n\ensuremath∼280–430) strontium autoionizing Rydberg states formed by near-resonant driving of the 5s2S1/2\ensuremath→5p2P1/25s\phantom{\rule{0.16em}{0ex}}^{2}S_{1/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}5s2S1/2​\ensuremath→5p2P1/2​ core-ion transition. The branching ratio between decay through radiative transitions and through autoionization is explored. Autoionization rates are measured as a function of both the nnn and \ensuremathℓ\ensuremath{\ell}\ensuremathℓ quantum numbers of the Rydberg electron. The nonstationary decay dynamics is studied by creating and manipulating Rydberg wave packets and by varying the laser pulse that drives the core excitation.
The production of very-high-nnn ($n\ensuremath{\sim}300$--500) strontium Rydberg atoms is explored using a crossed-laser-atom-beam geometry. nnn1S0{}^{1}{S}_{0}1S0​ and nnn1D2{}^{1}{D}_{2}1D2​ states are created by two-photon excitation via the 5s5p5s5p5s5p 1P1{}^{1}{P}_{1}1P1​ intermediate state using radiation with wavelengths of $\ensuremath{\sim}$461 and $\ensuremath{\sim}$413 nm. Rydberg atom densities as high as $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ cm${}^{\ensuremath{-}3}$ have been achieved, sufficient that Rydberg-Rydberg interactions can become important. The isotope shifts in the Rydberg series limits are determined by tuning the 461-nm light to preferentially excite the different strontium isotopes. Photoexcitation in the presence of an applied electric field is examined. The initially quadratic Stark shift of the nnn1P1{}^{1}{P}_{1}1P1​ and nnn1D2{}^{1}{D}_{2}1D2​ states becomes near-linear at higher fields and the possible use of n1D2n{}^{1}{D}_{2}n1D2​ states to create strongly polarized, quasi-one-dimensional electronic states in strontium is discussed. The data are analyzed with the aid of a two-active-electron (TAE) approximation. The two-electron Hamiltonian, within which the Sr2+{}^{2+}2+ core is represented by a semi-empirical potential, is numerically diagonalized allowing the calculation of the energies of high-nnn Rydberg states and their photoexcitation probabilities.
Rydberg atoms are increasingly being used as a means of introducing long-range, tunable interactions into atomic systems through their strong dipole-dipole interactions. When creating such interactions, however, there are a variety of potential effects that can occur, such as state (n and/or L) changing or Penning ionization, which can complicate the dynamics of the system. In the following work, such interactions are investigated for Rydberg-Rydberg collisions using very high-n, n ~300, strontium Rydberg atoms in an atomic beam configuration. By exploiting Rydberg blockade and the inherent distribution of atomic velocities present in an atomic beam, it is possible to create a 1D array of moving Rydberg atoms which will naturally overtake each other over time. Measurements of both Penning ionization and state changing for such collisions are made and compared with classical trajectory Monte-Carlo (CTMC) simulations, with both indicating large collisional cross sections on the order of 1 x 10-6 cm2 for Penning ionization and nearly 8 x 10-5 cm2 for state changing interactions. In a second series of experiments, previous work involving doubly-excited Rydberg states is expanded upon by investigating autoionization with high-n, L > 3, Rydberg states. Past research involving autoionization has shown that the effect is strongly L-dependent, with high-L states having significantly smaller autoionizing rates. To create well-defined, L > 3 states, the use of microwave fields to drive various high-n, n ~ 150, n1F3 → (n+1)1G4, (n+2)1H5 transitions is investigated, with spectroscopic measurements of these transitions providing quantum defect values for n1G4 and n1H5 states. These measurements provide a basis for future studies into the L-dependence of high-n, n ~ 300, autoionizing Rydberg atoms, with the eventual goal of creating a long-lived, doubly-excited “planetary” Rydberg state.
Rydberg atoms have a long history of scientific research, but they have recently seen a resurgence in popularity due to their advantages when dealing with coupled internal Rydberg states, or externally-coupled Rydberg atoms. The use of convenient RF and/or microwave generation techniques allows for straightforward coupling of many nearby Rydberg states, enabling quantum simulation studies, such as the generation of a synthetic lattice. Strontium is particularly attractive for Rydberg atom studies due to its optically active core, which allows for two-electron excited states and its conveniently accessible transition frequencies that match those available using commercial diode lasers. Before these experiments and applications can be realized however, precise knowledge of Rydberg state energies and quantum defects is required. Therefore, this work undertakes an extensive study of the quantum defects of both singly and doubly-excited strontium Rydberg atoms for a wide range of n and l. Precision microwave spectroscopy between nearby Rydberg states produces an improved self-consistent set of quantum defects for singly-excited 1S0, 1P1, 1D2, and 1F3 states, described by their corresponding Rydberg-Ritz parameters. Quantum defects and lifetimes for autoionizing two-electron-states are also presented. These elucidate the effects of core penetration by the “outer” excited Rydberg electron on transitions within the core ion and on the Rydberg level itself.
Alkaline earth Rydberg atoms are very promising tools for quantum technologies. Their highly excited outer electron provides them with the remarkable properties of Rydberg atoms and, notably, with a huge coupling to external fields or to other Rydberg atoms while the ionic core retains an optically active electron. However, low angular-momentum Rydberg states suffer almost immediate autoionization when the core is excited. Here, we demonstrate that strontium circular Rydberg atoms with a core excited in a 4D metastable level are impervious to autoionization over more than a few millisecond time scale. This makes it possible to trap and laser-cool Rydberg atoms. Moreover, we observe singlet to triplet transitions due to the core optical manipulations, opening the way to a microwave to optical quantum interface.
This paper is devoted to the autoionization process of large-angular-momentum Rydberg states. After a brief account of autoionization formalism in nonrelativistic theory and single-configuration approximation, we apply it to atomic states with one electron of large angular momentum. The dominant contribution is the direct one and the exchange contribution is ignored. In this framework, the autoionization width varies with the principal quantum number as ${n}^{\mathrm{\ensuremath{-}}3}$ and exponentially with the angular momentum. When two excited electrons interact with a spherically symmetric core, it is demonstrated that the interaction between the Rydberg electron and the core polarized by the valence electron is proportional to the interaction between the two active electrons. Using newly derived radial matrix elements, we obtain a generally good agreement between our results and experiments on barium and strontium. Large-l Rydberg states are shown to be stable versus autoionization as soon as l is about 7. Possible refinements of our method are reviewed.