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Physical Review A

Optical pumping of metastable NH radicals into the paramagnetic ground state

Sebastiaan Y. T. van de Meerakker, Boris G. Sartakov, Allard P. Mosk, Rienk T. Jongma, Gerard Meijer

Published Sep 15, 2003• 21 citations
Publisher

Abstract

We here report on the optical pumping of both ${}^{14}\mathrm{NH}$ and ${}^{15}\mathrm{NH}$ radicals from the metastable $a{}^{1}\ensuremath{\Delta}$ state into the $X{}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}}$ ground state in a molecular beam experiment. By inducing the hitherto unobserved spin-forbidden $A{}^{3}\ensuremath{\Pi}\ensuremath{\leftarrow}a{}^{1}\ensuremath{\Delta}$ transition, followed by spontaneous emission to the $X{}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}}$ state, a unidirectional pathway for population transfer from the metastable state into the electronic ground state is obtained. The optical pumping scheme demonstrated here opens up the possibility to accumulate NH radicals in a magnetic or optical trap.

DOI
10.1103/physreva.68.032508

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We here report on the optical pumping of both 14NH{}^{14}\mathrm{NH}14NH and 15NH{}^{15}\mathrm{NH}15NH radicals from the metastable a1\ensuremathΔa{}^{1}\ensuremath{\Delta}a1\ensuremathΔ state into the X3\ensuremathΣ\ensuremath−X{}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}}X3\ensuremathΣ\ensuremath− ground state in a molecular beam experiment. By inducing the hitherto unobserved spin-forbidden A3\ensuremathΠ\ensuremath←a1\ensuremathΔA{}^{3}\ensuremath{\Pi}\ensuremath{\leftarrow}a{}^{1}\ensuremath{\Delta}A3\ensuremathΠ\ensuremath←a1\ensuremathΔ transition, followed by spontaneous emission to the X3\ensuremathΣ\ensuremath−X{}^{3}{\ensuremath{\Sigma}}^{\ensuremath{-}}X3\ensuremathΣ\ensuremath− state, a unidirectional pathway for population transfer from the metastable state into the electronic ground state is obtained. The optical pumping scheme demonstrated here opens up the possibility to accumulate NH radicals in a magnetic or optical trap.
We report on the Stark deceleration and electrostatic trapping of 14NH^{14}\text{N}\text{H}14NH $(a^{1}\ensuremath{\Delta})$ radicals. In the trap, the molecules are excited on the spin-forbidden $A^{3}\ensuremath{\Pi}\ensuremath{\leftarrow}a^{1}\ensuremath{\Delta}$ transition and detected via their subsequent fluorescence to the $X{^{3}\ensuremath{\Sigma}}^{\ensuremath{-}}$ ground state. The 1∕e1∕e1∕e trapping time is $1.4\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{s}$, from which a lower limit of 2.7 s for the radiative lifetime of the $a^{1}\ensuremath{\Delta}$, v=0v=0v=0, J=2J=2J=2 state is deduced. The spectral profile of the molecules in the trapping field is measured to probe their spatial distribution. Electrostatic trapping of metastable NH followed by optical pumping of the trapped molecules to the electronic ground state is an important step toward accumulation of these radicals in a magnetic trap.
We report on the production of a pulsed molecular beam of metastable NH (a1Δa ^1Δa1Δ) radicals and present first results on the Stark deceleration of the NH (a1Δ,J=2,MΩ=−4a ^1Δ, J=2, MΩ=-4a1Δ,J=2,MΩ=−4) radicals from 550 m/s to 330 m/s. The decelerated molecules are excited on the spin-forbidden A3Π←a1ΔA ^3Π\leftarrow a ^1ΔA3Π←a1Δ transition, and detected via their subsequent spontaneous fluorescence to the X3Σ−,v"=0X ^3Σ^{-}, v"=0X3Σ−,v"=0 ground-state. These experiments demonstrate the feasibility of our recently proposed scheme [Phys. Rev. A 64 (2001) 041401] to accumulate ground-state NH radicals in a magnetic trap.
We propose a scheme to accumulate fermionic or bosonic NH radicals (14NH{(}^{14}\mathrm{NH}(14NH or 15NH,{}^{15}\mathrm{NH},15NH, respectively) in a magnetic trap in order to increase phase-space density. NH molecules prepared in a long-lived metastable state that exhibits a large Stark effect can be brought to a standstill using Stark deceleration. By applying an intense laser field, the NH radicals can then be forced to spontaneously decay to the electronic ground state. In the ground state the NH molecules are no longer sensitive to electric fields but show a significant Zeeman effect instead, enabling confinement and reloading of ground-state molecules in a magnetic trap.
We demonstrate laser cooling and magneto-optical trapping of europium. The atoms are optically pumped to a metastable state and then loaded from an atomic-beam source via conventional Zeeman slowing and magneto-optical trapping techniques using a $J=13/2\ensuremath{\leftrightarrow}J=15/2$ quasicyclic transition. The trapped populations contained up to $1\ifmmode\times\else\texttimes\fi{}{10}^{7}$ atoms, and a two-body loss rate is estimated as $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{3}/\mathrm{s}$ from the nonexponential loss of atoms at high densities. We also observed leakage out of the quasicyclic transition to the two metastable states with J=9/2J=9/2J=9/2 and 11/211/211/2, which is adequate to pump the laser-cooled atoms back to the J=7/2J=7/2J=7/2 ground state.
In the optical pumping process, spontaneous emission plays a unique role in redistributing the population of the ground level among the magnetic sub-levels. It is clear that the branching ratio of the relevant transition in the optical pumping must be nearly equal to unity in order to avoid the loss of population involved. Unfortunately, most complicated atoms do not have a simple energy level structure and cannot be polarized using optical pumping. We propose here a new polarizing method of a purely laser-driven process that avoids spontaneous emission and can be used in many heavier atoms. For J = 0 ground-state atoms, a complete population transfer to the meta-stable state can be used as efficient laser isotope separation, whereas a coherently prepared superposition state of the non-magnetic ground state and one of the magnetic states of the meta-stable level can be used as a tool to study the decoherent process. For J of the ground level no less than unity, partly removing the population on one of its sub-magnetic states will enhance the magnetic resonance signal significantly.