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arXiv:1705.00113 · Physical Review A

High-resolution optical spectroscopy with a buffer-gas-cooled beam of BaH molecules

G. Z. Iwata, Rees L. McNally, Tanya Zelevinsky

Published Aug 7, 2017• 44 citations
arXivHTMLPDF

Abstract

Barium monohydride (BaH) is an attractive candidate for extending laser cooling and trapping techniques to diatomic hydrides. The apparatus and high-resolution optical spectroscopy presented here demonstrate progress toward this goal. A cryogenic buffer-gas-cooled molecular beam of BaH was constructed and characterized. Pulsed laser ablation into cryogenic helium buffer gas delivers $\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{10}$ molecules/sr/pulse in the $X^{2}\mathrm{\ensuremath{\Sigma}}^{+}$ (${v}^{\ensuremath{'}\ensuremath{'}}=0,{N}^{\ensuremath{'}\ensuremath{'}}=1$) state of primary interest. More than $1\ifmmode\times\else\texttimes\fi{}{10}^{7}$ of these molecules per pulse enter the downstream science region with forward velocities below 100 m/s and transverse temperature of 0.1 K. This molecular beam enabled high-resolution optical spectra of BaH in quantum states relevant to laser slowing and cooling. The reported measurements include hyperfine structure and magnetic $g$ factors in the $X^{2}\mathrm{\ensuremath{\Sigma}}^{+}$, $B^{2}\mathrm{\ensuremath{\Sigma}}^{+}$, and $A^{2}\mathrm{\ensuremath{\Pi}}_{1/2}$ states.

DOI
10.1103/physreva.96.022509

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Barium monohydride (BaH) is an attractive candidate for extending laser cooling and trapping techniques to diatomic hydrides. The apparatus and high-resolution optical spectroscopy presented here demonstrate progress toward this goal. A cryogenic buffer-gas-cooled molecular beam of BaH was constructed and characterized. Pulsed laser ablation into cryogenic helium buffer gas delivers \ensuremath∼1\ifmmode×\else\texttimes\fi1010\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{10}\ensuremath∼1\ifmmode×\else\texttimes\fi1010 molecules/sr/pulse in the X2\ensuremathΣ+X^{2}\mathrm{\ensuremath{\Sigma}}^{+}X2\ensuremathΣ+ (v\ensuremath′\ensuremath′=0,N\ensuremath′\ensuremath′=1{v}^{\ensuremath{'}\ensuremath{'}}=0,{N}^{\ensuremath{'}\ensuremath{'}}=1v\ensuremath′\ensuremath′=0,N\ensuremath′\ensuremath′=1) state of primary interest. More than 1\ifmmode×\else\texttimes\fi1071\ifmmode\times\else\texttimes\fi{}{10}^{7}1\ifmmode×\else\texttimes\fi107 of these molecules per pulse enter the downstream science region with forward velocities below 100 m/s and transverse temperature of 0.1 K. This molecular beam enabled high-resolution optical spectra of BaH in quantum states relevant to laser slowing and cooling. The reported measurements include hyperfine structure and magnetic ggg factors in the X2\ensuremathΣ+X^{2}\mathrm{\ensuremath{\Sigma}}^{+}X2\ensuremathΣ+, B2\ensuremathΣ+B^{2}\mathrm{\ensuremath{\Sigma}}^{+}B2\ensuremathΣ+, and A2\ensuremathΠ1/2A^{2}\mathrm{\ensuremath{\Pi}}_{1/2}A2\ensuremathΠ1/2​ states.
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.
We describe a simple experimental apparatus for laser ablation of barium monohydride (BaH) molecules and the study of their rovibrational spectra relevant to direct laser cooling. BaH is a promising candidate for laser cooling and ultracold fragmentation, both of which are precursors to novel experiments in many-body physics and precision measurement. We present a detailed analysis of the properties of ablation plumes that can improve the understanding of surface ablation and deposition technologies. A range of absorption spectroscopy and collisional thermalization regimes has been studied. We directly measured the Franck-Condon factor of the $B{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}({v}^{\ensuremath{'}}=0)\ensuremath{\leftarrow}X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}({v}^{\ensuremath{'}\ensuremath{'}}=1)$ cycling transition. Prospects for production of a high luminosity cryogenic BaH beam are outlined.
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We demonstrate buffer-gas cooling, high-resolution spectroscopy, and cycling fluorescence of cold barium monofluoride (BaF) molecules. Our source produces an intense and internally cold molecular beam containing the different BaF isotopologues with a mean forward velocity of 190m/s190\phantom{\rule{0.16em}{0ex}}\mathrm{m}/\mathrm{s}190m/s. For a well-collimated beam of 138Ba19F^{138}\mathrm{Ba}\phantom{\rule{0.16em}{0ex}}^{19}\mathrm{F}138Ba19F we observe a flux of >1010>{10}^{10}>1010 molecules ${\mathrm{sr}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{pulse}}^{\ensuremath{-}1}$ in the $X{\phantom{\rule{0.16em}{0ex}}}^{2}\mathrm{\ensuremath{\Sigma}}$, N=1N=1N=1 state in our downstream detection region. Studying the absorption line strength of the intermediate ${A}^{\ensuremath{'}}\mathrm{\ensuremath{\Delta}}$ state we infer a lifetime of ${\ensuremath{\tau}}_{\mathrm{\ensuremath{\Delta}}}=790\ifmmode\pm\else\textpm\fi{}346\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$, significantly longer than previously estimated. Finally, highly diagonal Franck-Condon factors and magnetic remixing of dark states allow us to realize a quasicycling transition in 138Ba19F^{138}\mathrm{Ba}\phantom{\rule{0.16em}{0ex}}^{19}\mathrm{F}138Ba19F that is suitable for future laser cooling of this heavy diatomic molecule.
We report an experimental investigation on the Doppler-free saturated absorption spectroscopy of buffer-gas-cooled Barium monofluoride (BaF) molecules in a 4 K cryogenic cell. The obtained spectra with a resolution of 19 MHz, much smaller than previously observed in absorption spectroscopy, clearly resolve the hyperfine transitions. Moreover, we use these high-resolution spectra to fit the hyperfine splittings of excited A ( v = 0) state and find the hyperfine splitting of the laser-cooling-relevant A 2 Π 1/2 ( v = 0, J = 1/2,+) state is about 18 MHz, much higher than the previous theoretically predicted value. This provides important missing information for laser cooling of BaF molecules.