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Physical Review A

Far-infrared Rydberg-Rydberg transitions in a magnetic field: Deexcitation of antihydrogen atoms

A. Wetzels, Andreas Gürtler, Lambertus D. Noordam, F. Robicheaux

Published Jun 16, 2006• 12 citations
Publisher

Abstract

The dynamics of (de)excitation between highly excited Rydberg states ($15<n<60$ 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 $n=40$ from a deeper bound state near $n=25$. Moreover, starting from a highly excited state $(30<n<60)$ below and in the $n$-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 $n$-mixing regime the deexcitation efficiency is less than 10%. The measurements were in good agreement with fully quantum mechanical calculations. Calculations for deexcitation of $n=35$ states in H found the largest amount of deexcitation for $m=0$ and almost none for $m=20$. In recent experiments at CERN, antihydrogen is produced in high $n$ states in a strong magnetic field with a wide distribution of $m$. Our measurements and calculations suggest that deexcitation stimulated by infrared photons is not an efficient method for accelerating cascade to the ground state.

DOI
10.1103/physreva.73.062507

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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±\else\textpm\fi0.05T\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{T}\ifmmode±\else\textpm\fi0.05T is studied with far-infrared pulses (90--110 and 50\ensuremathμm50\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}50\ensuremathμm) 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.
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.
The short-pulse dynamics of excitation and stimulated emission between highly excited states (15<n<60)(15<n<60)(15<n<60) is studied with picosecond far-infrared pulses (40--60 $\ensuremath{\mu}\mathrm{m}$) originating from a free electron laser. We measured the excitation of a Rydberg wave packet in rubidium around n=40n=40n=40 excited from a deeper bound $(n\ensuremath{\approx}20)$ Rydberg state by a picosecond far-infrared laser pulse. Moreover, starting from a stationary n=40n=40n=40 state we found that upon irradiating with a short far-infrared laser pulse, adjacent Rydberg states are populated by resonant Raman transfer via a deeper (n=20)(n=20)(n=20) bound state. Finally, we discuss the dynamics of wave-packet excitation at a Cooper minimum in lithium, where sign change of the excitation matrix elements causes excitation at large distances from the atomic core. Possible measurement methods for this effect are discussed and found to yield different information.
We have studied the radiative decay of atomic hydrogen in strong magnetic fields of up to 4T4\phantom{\rule{0.3em}{0ex}}\mathrm{T}4T. We have followed the radiative cascade from completely l,ml,ml,m mixed distributions of highly excited states as well as from distributions that involve highly excited states with $\ensuremath{\mid}m\ensuremath{\mid}\ensuremath{\sim}n$. We have found that the time it takes to populate the ground state is not affected by the magnetic field for the initial states with $n\ensuremath{\lesssim}20$. For higher nnn manifolds, the electrons in the most negative mmm states are substantially slowed down by the magnetic field resulting in a much longer lifetime. We show that less than 10% of the antihydrogen atoms with $n\ensuremath{\sim}35$ generated in antihydrogen experiments at 4K4\phantom{\rule{0.3em}{0ex}}\mathrm{K}4K will decay to their ground states before they hit the wall of the vacuum container unless they are trapped. We have also found that the decay time is mainly determined by the fraction of atoms that were initially in highest negative mmm states due to the fact that only $\ensuremath{\mid}\ensuremath{\Delta}m\ensuremath{\mid}+\ensuremath{\mid}\ensuremath{\Delta}\ensuremath{\pi}\ensuremath{\mid}=1$ transitions are allowed in the magnetic field. We give a semiclassical method for calculating the decay rates for circular states and show that when the initial states have high-$\ensuremath{\mid}m\ensuremath{\mid}$, semiclassical rates agree with the full quantum mechanical rates within a couple of percent for states with effective $n\ensuremath{\gtrsim}20$.
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.
We have performed calculations of two successive charge transfers from Rydberg states in a strong magnetic field. In the first charge transfer, a positron interacts with a highly excited atom to form positronium. In the second stage, the positronium interacts with an antiproton to give antihydrogen. For many parameters, our results are in qualitative agreement with previous calculations with no magnetic field. However, we do find that there are important changes which may affect the usefulness of the method for efficient formation of antihydrogen that can be trapped.
We have performed calculations of two successive charge transfers from Rydberg states in a strong magnetic field. In the first charge transfer, a positron interacts with a highly excited atom to form positronium. In the second stage, the positronium interacts with an antiproton to give antihydrogen. For many parameters, our results are in qualitative agreement with previous calculations with no magnetic field. However, we do find that there are important changes which may affect the usefulness of the method for efficient formation of antihydrogen that can be trapped.