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arXiv:1311.7081 · Physical Review A

Production of ultracold hydrogen and deuterium via Doppler-cooled Feshbach molecules

Ian C. Lane

Published Aug 24, 2015• 56 citations
arXivHTMLPDF

Abstract

A counterintuitive scheme to produce ultracold hydrogen via fragmentation of laser cooled diatomic hydrides is presented where the final atomic H temperature is inversely proportional to the mass of the molecular parent. In addition, the critical density for formation of a Bose-Einstein condensate (BEC) at a fixed temperature is reduced by a factor $({m}_{H}/\phantom{\rule{0.0pt}{0ex}}{m}_{MH}{)}^{3/2}$ over directly cooled hydrogen atoms. The narrow Feshbach resonances between a $^{1}S_{0}$ atom and hydrogen are well suited to a tiny center of mass energy release necessary during fragmentation. With the support of ab initio quantum chemistry, it is demonstrated that BaH is an ideal diatomic precursor that can be laser cooled to a Doppler temperature of $\ensuremath{\sim}26\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$ with just two rovibronic transitions, the simplest molecular cooling scheme identified to date. Preparation of a hydrogen atom gas below the critical BEC temperature ${T}_{c}$ is feasible with present cooling technology, with optical pulse control of the condensation process.

DOI
10.1103/physreva.92.022511

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Ultracold molecules promise a revolutionary test bed for quantum science with applications ranging from experiments that probe the nature of our universe, to hosting new platforms for quantum computing. Cooling and trapping molecules in the ultracold regime is the first step to unlocking the wide array of proposed applications, and developing these techniques to control molecules is a key but challenging research field. In this thesis, we describe progress towards a new apparatus designed to cool and trap barium monohydride (BaH), a molecule that is amenable to laser cooling and has prospects as a precursor for ultracold atomic hydrogen. The same complexity that makes molecules interesting objects of study creates challenges for optical control. To mitigate some of these challenges, we first cool the molecules using cryogenic techniques and technologies. Our apparatus uses a cryogenic buffer gas to thermalize BaH within a contained cell. The molecules are extracted into a beam with millikelvin transverse temperature, and forward velocities <100 m/s. The BaH beam in this work is the brightest hydride beam to date, with molecule density and kinetic characteristics well suited for laser cooling and trapping.
Large parts of the periodic table cannot be cooled by current laser-based methods. We investigate whether zero energy fragmentation of laser cooled fluorides is a potential source of ultracold fluorine atoms. We report new ab initio calculations on the lowest electronic states of the BeF diatomic molecule including spin-orbit coupling, the calculated minima for the valence electronic states being within 1 pm of the spectroscopic values. A four colour cooling scheme based on the A(2)Π← X(2)Σ(+) transition is shown to be feasible for this molecule. Multi-Reference Configuration Interaction (MRCI) potentials of the lowest energy Rydberg states are reported for the first time and found to be in good agreement with experimental data. A series of multi-pulse excitation schemes from a single rovibrational level of the cooled molecule are proposed to produce cold fluorine atoms.
Laser cooling of polyatomic molecules to the ultracold regime may enable some new science and technology applications; however, the related study is still at its very early stage. Here, by means of accurate ab initio and dynamical calculations, we identify two new candidate tetratomic molecules that are suitable for laser cooling and demonstrate the feasibility and advantage of two laser cooling schemes that are able to produce ultracold CaCCH and SrCCH molecules. The internally contracted multiconfiguration reference configuration interaction method is applied, and excellent agreement is achieved between the computed and experimental spectroscopic data. We find that the X2Σ1/2+→A2Π1/2 transitions for both candidates feature diagonal Franck–Condon factors, short radiative lifetimes, and no interference from intermediate electronic states. In addition, the crossings with higher electronic states do not interfere. We further construct feasible laser cooling schemes for CaCCH and SrCCH, each of which allows scattering 104 photons for direct laser cooling. The estimated Doppler temperatures for both CaCCH and SrCCH are on the order of μK.
Abstract I present a novel scheme for producing cold (magnetically trappable) atomic hydrogen, based on threshold photodissociation of the BaH+ molecular ion. BaH+ can be sympathetically cooled using laser cooled Ba+ in an ion trap, before it is photodissociated on the single photon A1Σ+ ← X1Σ+ transition. The small mass ratio between Ba+ and BaH+ ensures a strong overlap within the ion trap for sympathetic cooling, while the large mass ratio between BaH+ and H means that the released hydrogen can be up to 139 times colder than the parent molecular ions. I examine the hydrogen production rate, and describe how the trap dynamics and photodissociation laser detuning influence the achievable energies. The low infrastructure costs and the ion trap nature of the scheme make it suitable for loading hydrogen into an antihydrogen experiment. This would support a direct matter–antimatter comparison, which could provide important clues as to why our Universe contains so little antimatter.
On the repulsive side of the Feshbach resonance fermionic atoms can bind into diatomic molecules which at low enough temperature condense into molecular Bose-Einstein condensate. Sufficiently close to the resonance the molecules are bound so weakly that a significant fraction of unbound fermionic atoms survives even below the temperature of condensation. In this paper we show that laser cooling of the unbound fermionic atoms can reduce temperature of mixture of atoms and molecules below the condensation temperature. Numerical simulations for reasonable experimental parameters show that it is possible to achieve molecular condensate after a few seconds of laser cooling.