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arXiv:1512.06316 · Physical Review A

BaH molecular spectroscopy with relevance to laser cooling

Marco G. Tarallo, G. Z. Iwata, Tanya Zelevinsky

Published Mar 17, 2016• 46 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physreva.93.032509

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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 B2\ensuremathΣ+(v\ensuremath′=0)\ensuremath←X2\ensuremathΣ+(v\ensuremath′\ensuremath′=1)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)B2\ensuremathΣ+(v\ensuremath′=0)\ensuremath←X2\ensuremathΣ+(v\ensuremath′\ensuremath′=1) cycling transition. Prospects for production of a high luminosity cryogenic BaH beam are outlined.
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 ggg factors in the $X^{2}\mathrm{\ensuremath{\Sigma}}^{+}$, $B^{2}\mathrm{\ensuremath{\Sigma}}^{+}$, and $A^{2}\mathrm{\ensuremath{\Pi}}_{1/2}$ 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 reported a detailed experimental study of the cold collision of barium monofluoride (BaF) with buffer gas and the high-resolution spectroscopy relevant with direct laser cooling. BaF molecules are efficiently produced with laser ablation and buffer-gas cooled in a cryogenic apparatus. The laser cooling relevant transition $|X\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}},v=0,N=1\ensuremath{\rangle}$ to $|A\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Pi}},{v}^{\ensuremath{'}}=0,{J}^{\ensuremath{'}}=1/2\ensuremath{\rangle}$ is identified. The collision cross section with buffer gas is measured to be $1.4(7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{4.pt}{0ex}}{\text{cm}}^{\ensuremath{-}2}$, which is very suitable for buffer-gas cooling. Both rotational and vibrational temperatures are effectively cooled, and a large amount of molecules are quenched into the desired states. Our study provides an important benchmark for further laser cooling of the BaF molecule.
We present the first experimental demonstration of radiation pressure force deflection and direct laser cooling for barium monohydride (BaH) molecules resulting from multiple photon scattering. Despite a small recoil velocity (2.7 mm s −1 ) and a long excited state lifetime (137 ns), we use 1060 nm laser light exciting the X → A electronic transition of BaH to deflect a cryogenic buffer-gas beam and reduce its transverse velocity spread. Multiple experimental methods are employed to characterize the optical cycling dynamics and benchmark theoretical estimates based on rate equation models as well as solutions of the Lindblad master equation for the complete multilevel system. Broader implications for laser cooling and magneto-optical trapping of heavy-metal-containing molecules with narrow transition linewidths are presented. Our results pave the way for producing a new class of ultracold molecules—alkaline earth monohydrides—via direct laser cooling and trapping, opening the door to realizing a new method for delivering ultracold hydrogen atoms (Lane 2015 Phys . Rev . A 92 , 022511).
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.
Currently, there is great interest in laser cooling of barium monofluoride (BaF) molecules for precision tests of fundamental symmetries. We use high-resolution absorption spectroscopy to characterize several as yet imprecisely known transition frequencies required to realize such cooling. We extract an improved set of molecular constants for the bosonic 138Ba19F^{138}\mathrm{Ba}^{19}\mathrm{F}138Ba19F and 136Ba19F^{136}\mathrm{Ba}^{19}\mathrm{F}136Ba19F isotopologues, confirm the existence of a significant hyperfine splitting in the excited state of the laser cooling cycle, and investigate the effects of this splitting on the achievable cooling forces. As a direct application of our spectroscopic insights, we experimentally demonstrate nearly background-free fluorescence imaging of a BaF molecular beam in a glass cell vacuum chamber. We expect such high-fidelity detection to be useful for various types of precision measurement scenarios.