Abstract
Rydberg states of atomic helium with principal quantum numbers ranging from n=20 to n=100 have been prepared by non-resonance-enhanced single-color two-photon excitation from the metastable 2 {^3}S{_1} state. Photoexcitation was carried out using linearly and circularly polarized pulsed laser radiation. In the case of excitation with circularly polarized radiation, Rydberg states with azimuthal quantum number |m_{\ell}|=2 were prepared in zero electric field, and in homogeneous electric fields oriented parallel to the propagation axis of the laser radiation. In sufficiently strong electric fields, individual Rydberg-Stark states were resolved spectroscopically, highlighting the suitability of non-resonance-enhanced multiphoton excitation schemes for the preparation of long-lived high-|m_{\ell}| hydrogenic Rydberg states for deceleration and trapping experiments. Applications of similar schemes for Doppler-free excitation of positronium atoms to Rydberg states are also discussed.
Highlights
Among their many often extreme properties, Rydberg states of atoms and molecules can possess long radiative lifetimes
In the absence of external perturbations, low-angular-momentum hydrogenic Rydberg states with principal quantum number n > 30 and small quantum defects typically exhibit lifetimes exceeding 1 μs which scale with n3
Because of these long lifetimes, optical and microwave transitions involving these states exhibit narrow natural linewidths. They are very well suited for precision spectroscopic measurements, including, for example, the accurate determination of molecular ionization and dissociation limits [2] and studies of the role of nuclear spins in photoionization [3,4]. These long lifetimes are of importance for applications of Rydberg states in quantum information processing [5,6], and in experiments with antihydrogen and positronium atoms directed toward (i) spectroscopic investigations of matter-antimatter asymmetries [7], and (ii) measurements of the acceleration of particles composed of antimatter in the gravitational field of the Earth [8,9,10]
Summary
Among their many often extreme properties, Rydberg states of atoms and molecules can possess long radiative lifetimes. These cases include, for example, precision spectroscopic studies of the ionization and dissociation limits of H2 and its isotopomers [2], and the preparation of long-lived excited states of positronium [11] In these cases, it is foreseen to minimize effects of Doppler broadening in the photoexcitation process by using counterpropagating laser beams to drive non-resonance-enhanced single-color two-photon transitions to the Rydberg states [12,13]. In addition to the importance of the results presented here for Doppler-free Rydberg-state photoexcitation, when driven using circularly polarized laser radiation, such multiphoton excitation schemes represent a general approach to the preparation of long-lived non-corepenetrating Rydberg states of small molecules With their small quantum defects and long fluorescence lifetimes, these states are ideally suited for deceleration and electric trapping experiments [18,19], directed toward studies of excited-state decay processes and low-energy scattering with other gasphase targets or surfaces [20]. Comparisons are drawn between the single-color twophoton spectra reported here and those expected in the case of Doppler-free two-photon excitation of positronium using laser radiation with similar temporal and spectral properties
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