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Journal of Physics B Atomic Molecular and Optical Physics

Polarization dependence of n = 2 positronium transition rates to Stark-split n = 30 levels via crossed-beam spectroscopy

A. C. L. Jones, T. H. Hisakado, H. J. Goldman, Harry W. K. Tom, Allen Paine Mills

Published Feb 29, 2016• 13 citations
Publisher

Abstract

Abstract We produce Rydberg Ps by a two-step laser excitation from and from to states of principal quantum level that are Stark split by a motionally induced electric field. Our measurements are largely free of first-order Doppler shifts such that we are able to investigate the impact of laser polarization on the population of the closely spaced Stark levels. We find a variation in the distribution that is primarily dependent on the IR laser polarization with respect to the direction of the motionally induced electric field. With the IR light polarized parallel to the electric field F , the ratio of excitation probability to the levels of maximal Stark splitting compared to that of excitation to the states of minimal Stark splitting is found to be 3.37 ± 0.51, whereas with the IR light polarized perpendicular to F , the excitation ratio is 0.87 ± 0.64. Our results agree with those of Wall et al (2015 Phys. Rev. Lett. 114 173001 ) obtained with n = 11 and will be useful in the preparation of high- n states of Ps for a variety of experiments, including measuring the interaction of Ps with gravity, in precision time-of-flight (TOF) energy spectroscopy, and precision optical spectroscopy of Ps.

DOI
10.1088/0953-4075/49/6/064006

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Physical Review Letters•Jan 26, 2012•93 citations
Abstract We produce Rydberg Ps by a two-step laser excitation from and from to states of principal quantum level that are Stark split by a motionally induced electric field. Our measurements are largely free of first-order Doppler shifts such that we are able to investigate the impact of laser polarization on the population of the closely spaced Stark levels. We find a variation in the distribution that is primarily dependent on the IR laser polarization with respect to the direction of the motionally induced electric field. With the IR light polarized parallel to the electric field F , the ratio of excitation probability to the levels of maximal Stark splitting compared to that of excitation to the states of minimal Stark splitting is found to be 3.37 ± 0.51, whereas with the IR light polarized perpendicular to F , the excitation ratio is 0.87 ± 0.64. Our results agree with those of Wall et al (2015 Phys. Rev. Lett. 114 173001 ) obtained with n = 11 and will be useful in the preparation of high- n states of Ps for a variety of experiments, including measuring the interaction of Ps with gravity, in precision time-of-flight (TOF) energy spectroscopy, and precision optical spectroscopy of Ps.
Rydberg positronium (Ps) atoms have been prepared in selected Stark states via two-step (1s→2p→nd/ns) optical excitation. Two methods have been used to achieve Stark-state selection: a field ionization filter that transmits the outermost states with positive Stark shifts, and state-selected photoexcitation in a strong electric field. The former is demonstrated for n=17 and 18 while the latter is performed for n=11 in a homogeneous electric field of 1.9 kV/cm. The observed spectral intensities and their dependence on the polarization of the laser radiation are in agreement with calculations that include the perturbations of the intermediate n=2 manifold. Our results pave the way for the generation of Rydberg Ps atoms with large electric dipole moments that are required for the realization of schemes to control their motion using inhomogeneous electric fields, an essential feature of some proposed Ps free-fall measurements requiring focused beams of long-lived atoms.
The Stark splitting of the second excited level of atomic hydrogen (with principal quantum number n=3n=3n=3) has been measured by pulsed laser spectroscopy in order to determine small electric fields by optical means. Doppler-free excitation with two counterpropagating laser beams at 205nm205\phantom{\rule{0.3em}{0ex}}\mathrm{nm}205nm allows us to measure the n=3n=3n=3 Stark spectra either by an optogalvanic signal after ionization by a third laser photon or by detection of the Balmer-$\ensuremath{\alpha}$ fluorescence photons emitted by the excited atoms. Three different cases of laser polarization have been investigated in order to evaluate the possibilities and limitations of this one-step laser excitation method for electric field determination, especially at field strengths below 200V∕cm200\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}200V∕cm. The method provides the best field sensitivity achieved with lower excited levels that can be reached directly by two-photon absorption from the ground state (in contrast to the more sensitive Rydberg levels). For this purpose, an advanced tunable pulsed uv laser with narrow bandwidth almost at the Fourier limit has been used. The high spectral resolution allowed us to deduce electric field strengths as small as 50V∕cm50\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}50V∕cm from the frequency shift between two selected isolated Stark components. As a main advantage, this simple analysis of the measured spectra does not rely on knowledge of the line intensities which may be strongly affected by the measuring conditions.
The production of long-lived Rydberg positronium (Ps) and correction for Doppler shifts in the excitation laser frequencies are crucial elements of proposed measurements of the gravitational freefall of antimatter and for precision measurements of the optical spectrum of Ps. Using a two-step optical transition via 2P2P2P levels, we have prepared Ps atoms in Rydberg states up to the term limit. The spectra are corrected for the first-order Doppler shift using measured velocities, and the Balmer transitions are resolved for $15\ensuremath{\le}n\ensuremath{\le}31$. The excitation signal amplitude begins to decrease for n>50n>50n>50, consistent with the onset of motional electric field ionization in the 3.5-mT magnetic field at the Ps formation target.
A Surko-type positron trap has been used to produce intense positron pulses, and thus a dilute positronium (Ps) gas in vacuum with an initial density of 10^7 cm-3. This gas was excited with lasers for a variety of experiments. n = 2 atoms were prepared using a single-photon excitation scheme (1S to 2P) in controlled electric and magnetic fields for studies of Stark and Zeeman effects, measurements of Ps cooling via Doppler spectroscopy, and for the produciton of ensembles of atoms in pure 2S states. The first measurements of Ps excitation to specific Rydberg-Stark states were performed via a two-color two-photon excitation scheme ( 1S to 2P to nS/nD). Ps atoms in low-field–seeking Rydberg Stark states were prepared at the entrance of an electrostatic quadrupole guide which was used to control the trajectories and velocity distributions of the Ps atoms. These advances, together with the results from numerical simulations, suggest that these methods could be used in to perform Stark deceleration and trapping of Rydberg Ps, high-precision Ps spectroscopy, and to create positron-atom bound states.
We have measured certain ratios of transition intensities between excited hydrogen Stark sublevels over a range of electric-field values. CO2{\mathrm{CO}}_{2}CO2​ lasers were used to excite a fast beam of hydrogen atoms. The \ensuremath{\pi}-polarized transitions studied were from extremal Stark sublevels of principal quantum number n=10 to extremal and next extremal sublevels of n=30 and 44, at fields from 2 to 700 V/cm. The observed ratios are compared with those obtained using a non-relativistic zero-field formula attributed to Gordon [Ann. Phys. (Leipzig 2, 1031 (1929)] and with calculations employing matrix diagonalization including fine structure. Even for those relatively high n values, we find at low field significant deviations from results from Gordon's formula due to fine structure in the lower state as well as deviations at high field due to n mixing in the upper state induced by the static electric field. The experimental results and the matrix diagonalization calculations agree to within a few percent, which is the order of magnitude of possible saturation effects.
We demonstrate experimentally the production of Rydberg positronium (Ps) atoms in a two-step process, comprising incoherent laser excitation, first to the 2(3)P state and then to states with principal quantum numbers ranging from 10 to 25. We find that excitation of 2(3)P atoms to Rydberg levels occurs very efficiently (~90%) and that the ~25% overall efficiency of the production of Rydberg atoms is determined almost entirely by the spectral overlap of the primary excitation laser and the Doppler broadened width of the 1 (3)S-2(3)P transition. The observed efficiency of Rydberg Ps production can be explained if stimulated emission back to the 2P states is suppressed, for example, by intermixing of the Rydberg state Stark sublevels. The efficient production of long-lived Rydberg Ps in a high magnetic field may make it possible to perform direct measurements of the gravitational free fall of Ps.