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arXiv:1810.06736 · Physical Review A

Laser-stimulated deexcitation of Rydberg antihydrogen atoms

D. Comparat, C. Malbrunot

Published Jan 16, 2019• 6 citations
arXivHTMLPDF

Abstract

Antihydrogen atoms are routinely formed at CERN in a broad range of Rydberg states. Ground-state antiatoms, those useful for precision measurements, are eventually produced through spontaneous decay. However given the long lifetime of Rydberg states the number of ground-state antihydrogen atoms usable is small, in particular for experiments relying on the production of a beam of antihydrogen atoms. Therefore, it is of high interest to efficiently stimulate the decay in order to retain a higher fraction of ground-state atoms for measurements. We propose a method that optimally mixes the high angular momentum states with low ones enabling us to stimulate, using a broadband frequency laser, the deexcitation toward low-lying states, which then spontaneously decay to the ground state. We evaluate the method in realistic antihydrogen experimental conditions. For instance, starting with an initial distribution of atoms within the $n=20--30$ manifolds, as formed through the charge exchange mechanism, we show that more than 80% of antihydrogen atoms will be deexcited to the ground state within 100 ns using a laser producing 2 J at 828 nm.

DOI
10.1103/physreva.99.013418

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An efficient method for production of cold antihydrogen $(\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{})$ is proposed. Alkali-metal atoms laser excited to a Rydberg state are charge exchanged with cold trapped positrons, producing Rydberg states of positronium. In a second Rydberg-state charge exchange, the positronium atoms give up their Rydberg positrons to cold trapped antiprotons, producing Rydberg states of $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$. These $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$ atoms soon decay down to the ground state, and, because they are cold, could be trapped in a magnetic trap. The efficiency of the process results from the extremely large cross sections for Rydberg charge exchange. Classical trajectory Monte Carlo calculations indicate an $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$ instantaneous production rate of 10${}^{6}/$s.

Physical Review A•Mar 1, 1998•87 citations
Antihydrogen atoms are routinely formed at CERN in a broad range of Rydberg states. Ground-state antiatoms, those useful for precision measurements, are eventually produced through spontaneous decay. However given the long lifetime of Rydberg states the number of ground-state antihydrogen atoms usable is small, in particular for experiments relying on the production of a beam of antihydrogen atoms. Therefore, it is of high interest to efficiently stimulate the decay in order to retain a higher fraction of ground-state atoms for measurements. We propose a method that optimally mixes the high angular momentum states with low ones enabling us to stimulate, using a broadband frequency laser, the deexcitation toward low-lying states, which then spontaneously decay to the ground state. We evaluate the method in realistic antihydrogen experimental conditions. For instance, starting with an initial distribution of atoms within the n=20−−30n=20--30n=20−−30 manifolds, as formed through the charge exchange mechanism, we show that more than 80% of antihydrogen atoms will be deexcited to the ground state within 100 ns using a laser producing 2 J at 828 nm.
Antihydrogen atoms are routinely formed at the Antiproton Decelerator at CERN in a wide range of Rydberg states. To perform precision measurements, experiments rely on ground-state antimatter atoms which are currently obtained only after spontaneous decay. In order to enhance the number of atoms in ground state, we propose and assess the efficiency of different methods to stimulate their decay. First, we investigate the use of THz radiation to simultaneously couple all nnn-manifolds down to a low-lying one with sufficiently fast spontaneous emission toward ground state. We further study a deexcitation scheme relying on state mixing via microwave and/or THz light and a coupled (visible) deexcitation laser. We obtain close to unity ground-state fractions within a few tens of $\ensuremath{\mu}\mathrm{s}$ for a population initiated in the n=30n=30n=30 manifold. Finally, we study how the production of antihydrogen atoms via stimulated radiative recombination can favorably change the initial distribution of states and improve the overall number of ground-state atoms when combined with stimulated deexcitation.
The dynamics of (de)excitation between highly excited Rydberg states (15<n<6015<n<6015<n<60 of Rb) in a magnetic field of 0.85 $\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ is studied with far-infrared pulses (90--110 and $50\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$) originating from a free electron laser. We measured the excitation spectrum to states around n=40n=40n=40 from a deeper bound state near n=25n=25n=25. Moreover, starting from a highly excited state (30<n<60)(30<n<60)(30<n<60) below and in the nnn-mixing regime we investigated the efficiency of the deexcitation channel vs the ionization channel. We measured deexcitation efficiencies well above 50% for some of the states. However, starting deep in the nnn-mixing regime the deexcitation efficiency is less than 10%. The measurements were in good agreement with fully quantum mechanical calculations. Calculations for deexcitation of n=35n=35n=35 states in H found the largest amount of deexcitation for m=0m=0m=0 and almost none for m=20m=20m=20. In recent experiments at CERN, antihydrogen is produced in high nnn states in a strong magnetic field with a wide distribution of mmm. Our measurements and calculations suggest that deexcitation stimulated by infrared photons is not an efficient method for accelerating cascade to the ground state.
This work reports on experiments in which antihydrogen atoms have been produced in cryogenic Penning traps from antiproton and positron plasmas by two different methods and on experiments that have been carried out subsequently in order to investigate the antihydrogen atoms. By the first method antihydrogen atoms have been formed during the process of positron cooling of antiprotons in so called nested Penning traps and detected via a field ionization method. A linear dependence of the number of detected antihydrogen atoms on the number of positrons has been found. A measurement of the state distribution has revealed that the antihydrogen atoms are formed in highly excited states. This suggests along with the high production rate that the antihydrogen atoms are formed by three-body recombination processes and subsequent collisional deexcitations. However current theory cannot yet account for the measured state distribution. Typical radii of the detected antihydrogen atoms lie in the range between 0.4 µm and 0.15 µm. The deepest bound antihydrogen atoms have radii below 0.1 µm. Antihydrogen atoms with that size have chaotic positron orbits so that for the first time antihydrogen atoms have been detected that cannot be described by the GCA-model. The kinetic energy of the weakest bound antihydrogen atoms has been measured to about 200 meV, which corresponds to an antihydrogen velocity of approximately 6200 m/s. A simple model suggests that these atoms are formed from only one deexcitation collision and methods that might lead to a decrease of the antihydrogen velocity are presented. By the second method antihydrogen atoms have been synthesized in charge-exchange processes. Lasers are used to produce a Rydberg cesium beam within the cryogenic Penning trap that collides with trapped positrons so that Rydberg positronium atoms are formed via charge-exchange reactions. Due to their charge neutrality the Rydberg positronium atoms are free to leave the positron trapping region. The Rydberg positronium atoms that collide with nearby stored antiprotons form antihydrogen atoms in charge-exchange reactions. So far, 14 +/- 4 antihydrogen atoms have been detected background-free via a field-ionization method. The antihydrogen atoms produced via the two-step charge-exchange mechanism are expected to have a temperature of 4.2 K, the temperature of the antiprotons from which they are formed. A method is proposed by which the antihydrogen temperature can be determined with an accuracy of better than 1 K from a measurement of the time delay between antihydrogen annihilation events and the laser pulse that initiates the antihydrogen production via the production of Rydberg cesium atoms. First experiments have been carried out during the last days of the 2004 beam time, but the number of detected antihydrogen annihilations has been too low for a determination of the antihydrogen temperature. Trapped antiprotons have been directly exposed to laser light delivered by a Titanium:Sapphire laser in order to investigate if the laser light causes any loss on the trapped antiprotons. Experiments have shown that no extra loss occurs for laser powers of less than 590 mW. This is an important result against the background of the future plan to confine antihydrogen atoms in a combined Penning-Ioffe trap and then to carry out laser spectroscopy on these atoms, since it reveals that laser light does not cause an increase of the pressure in the trapping region to the extend that annihilations with the background gas become noticeable. The ATRAP Collaboration plans to precisely investigate antihydrogen atoms. The ultimate goal is to test the CPT-theorem by a high precision measurement of the 1S-2S transition of antihydrogen and a comparison with the precisely known value of the corresponding transition in hydrogen. This thesis presents the achievement of the first step towards this challenging goal: the production of cold antihydrogen itself.
An efficient method for production of cold antihydrogen $(\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{})$ is proposed. Alkali-metal atoms laser excited to a Rydberg state are charge exchanged with cold trapped positrons, producing Rydberg states of positronium. In a second Rydberg-state charge exchange, the positronium atoms give up their Rydberg positrons to cold trapped antiprotons, producing Rydberg states of $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$. These $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$ atoms soon decay down to the ground state, and, because they are cold, could be trapped in a magnetic trap. The efficiency of the process results from the extremely large cross sections for Rydberg charge exchange. Classical trajectory Monte Carlo calculations indicate an $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}$ instantaneous production rate of 106/{}^{6}/6/s.