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Spectroscopy and manipulation of positronium in excited states

A. Alonso

Published Apr 28, 2018• 0 citations
OpenAlex

Abstract

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.

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Methods and techniques in the production, manipulation, and accumulation of positrons in a `Surko-type' buffer-gas trap are presented. The pulsed output of the trap is optimised for laser excitation of positronium (Ps). The techniques of positron time compression and single-shot positron annihilation lifetime spectroscopy (SSPALS) are discussed in detail and used throughout the study. A small array or Ps converter materials have been characterised at various temperatures. Ps cooling as a function of positron implantation is presented and discussed. Direct laser irradiation of the samples, and the effect on the Ps yield has also been studied. In some cases, particularly at cryogenic temperatures, laser induced paramagnetic centres are formed and this has a detrimental effect on the Ps yield as they cause triplet Ps to convert to the shorter-lived singlet state. Whereas in single crystal semiconductors, the laser has an enhancing effect due to an exciton-like positron-electron surface state. Many of the Ps converter materials studied here are formed from porous structures. The long term confinement of Ps within isolated cavities has also been observed, where a lifetime measurement technique using an excitation laser is presented. General methods of Ps-laser spectroscopy are also presented and the applications of which are discussed including laser-enhanced time-of-flight spectroscopy (LEPTOF), and the production, and electrostatic guiding of highly excited Rydberg Ps. Using these techniques a crossed beam scattering experiment involving Rydberg Ps, electrons, and Argon ions was carried out. This work presents important considerations for experiments to create a laser-cooled Bose-Einstein condensate (BEC) of Ps in an engineered porous material at cryogenic temperatures.

•Mar 28, 2018•0 citations

Electrostatically Guided Rydberg Positronium

Adam Deller, A. Alonso, B. S. Cooper +2 more

We report experiments in which positronium (Ps) atoms were guided using inhomogeneous electric fields. Ps atoms in Rydberg-Stark states with principal quantum number n=10 and electric dipole moments up to 610 D were prepared via two-color two-photon optical excitation in the presence of a 670 V cm^{-1} electric field. The Ps atoms were created at the entrance of a 0.4 m long electrostatic quadrupole guide, and were detected at the end of the guide via annihilation gamma radiation. When the lasers were tuned to excite low-field-seeking Stark states, a fivefold increase in the number of atoms reaching the end of the guide was observed, whereas no signal was detected when high-field-seeking states were produced. The data are consistent with the calculated geometrical guide acceptance.

Physical Review Letters•Aug 12, 2016•37 citations
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.
The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.
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.
We numerically study laser deceleration, cooling and trapping of host fast ortho-positronium (Ps) bunches as produced in silica microchannel targets using positron bunches from a Surko trap. While Doppler laser cooling routinely allows to cool neutral atom species below milli-Kelvin kinetic temperatures, the fast ground state annihilation and the high recoil shift to line-width ratio in Ps pose new challenging limitations on timing and the required laser power. Earlier theoretical work and first experiments already exhibit viable prospects for radiation pressure cooling to create unprecedented cold and dense Ps ensembles. Our extended numerical studies confirm very good prospects to simultaneously decelerate, cool and optically trap Ps atoms in a standard Doppler cooling geometry on the 13S1^3\mathrm{S}13S--23P2^3\mathrm{P}23P transition. Including the full Zeeman state manifolds and adding additional transverse lasers on the 23P2^3\mathrm{P}23P--33D3^3\mathrm{D}33D transition improves timing and final temperature to allow for optical trapping at the end. The required laser powers and geometry parameters to implement effective slowing and trapping within a time frame, where the majority of atoms is not annihilated, is in reach of current technology of mJ\mathrm{mJ}mJ energy pulses of approximately 100 ns100\ \mathrm{ns}100 ns duration. Future extensions to collectively enhance cooling and trapping in optical cavities or hollow-core fibres should finally allow fast preparation of ultra-cold Ps systems as future basis of superradiant lasing and Bose--Einstein condensates.
Methods and techniques in the production, manipulation, and accumulation of positrons in a `Surko-type' buffer-gas trap are presented. The pulsed output of the trap is optimised for laser excitation of positronium (Ps). The techniques of positron time compression and single-shot positron annihilation lifetime spectroscopy (SSPALS) are discussed in detail and used throughout the study. A small array or Ps converter materials have been characterised at various temperatures. Ps cooling as a function of positron implantation is presented and discussed. Direct laser irradiation of the samples, and the effect on the Ps yield has also been studied. In some cases, particularly at cryogenic temperatures, laser induced paramagnetic centres are formed and this has a detrimental effect on the Ps yield as they cause triplet Ps to convert to the shorter-lived singlet state. Whereas in single crystal semiconductors, the laser has an enhancing effect due to an exciton-like positron-electron surface state. Many of the Ps converter materials studied here are formed from porous structures. The long term confinement of Ps within isolated cavities has also been observed, where a lifetime measurement technique using an excitation laser is presented. General methods of Ps-laser spectroscopy are also presented and the applications of which are discussed including laser-enhanced time-of-flight spectroscopy (LEPTOF), and the production, and electrostatic guiding of highly excited Rydberg Ps. Using these techniques a crossed beam scattering experiment involving Rydberg Ps, electrons, and Argon ions was carried out. This work presents important considerations for experiments to create a laser-cooled Bose-Einstein condensate (BEC) of Ps in an engineered porous material at cryogenic temperatures.
We report experiments in which positronium (Ps) atoms were guided using inhomogeneous electric fields. Ps atoms in Rydberg-Stark states with principal quantum number n=10 and electric dipole moments up to 610 D were prepared via two-color two-photon optical excitation in the presence of a 670 V cm^{-1} electric field. The Ps atoms were created at the entrance of a 0.4 m long electrostatic quadrupole guide, and were detected at the end of the guide via annihilation gamma radiation. When the lasers were tuned to excite low-field-seeking Stark states, a fivefold increase in the number of atoms reaching the end of the guide was observed, whereas no signal was detected when high-field-seeking states were produced. The data are consistent with the calculated geometrical guide acceptance.