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arXiv:2105.10760 · The Journal of Chemical Physics

Accurate prediction and measurement of vibronic branching ratios for laser cooling linear polyatomic molecules

Chaoqun Zhang, Benjamin L. Augenbraun, Zack Lasner, Nathaniel B. Vilas, John M. Doyle, Lan Cheng

Published Sep 3, 2021• 48 citations
arXivHTMLPDF

Abstract

We report a generally applicable computational and experimental approach to determine vibronic branching ratios in linear polyatomic molecules to the 10−5 level, including for nominally symmetry-forbidden transitions. These methods are demonstrated in CaOH and YbOH, showing approximately two orders of magnitude improved sensitivity compared with the previous state of the art. Knowledge of branching ratios at this level is needed for the successful deep laser cooling of a broad range of molecular species.

DOI
10.1063/5.0063611

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Molecules controlled at the single-quantum-state level offer wide-ranging applications spanning many of the frontiers of modern physics. Within the past decade, simple diatomic molecules have been brought under full quantum control both via assembly from pre-cooled atoms and by direct laser cooling. Polyatomic molecules\footnote{Molecules that contain three or more atoms.} have attracted focus as novel quantum resources providing distinct advantages (and challenges) compared to both atoms and diatomic molecules. For example, nearly all polyatomic molecules have long-lived states that can be fully polarized at low applied electric fields. These and other features generic to polyatomic molecules can be applied to quantum simulation, quantum chemistry, and tests of fundamental physics beyond the Standard Model. While these structural features of polyatomic molecules are highly promising for applications, they complicate the process of bringing molecules under full quantum control. In this thesis, we describe a series of experimental and theoretical advances toward the production of ultracold complex polyatomic molecules. We demonstrate the first laser cooling of the polyatomic radical ytterbium monohydroxide (YbOH), a promising candidate to search for particles and interactions beyond the Standard Model of particle physics. We develop a novel deceleration technique (``Zeeman-Sisyphus deceleration") capable of slowing molecular beams to trappable velocities while scattering fewer than 10 optical photons. This deceleration method is demonstrated using CaOH molecules, a lighter analog of YbOH. In order to determine a viable pathway to full three-dimensional cooling and trapping of YbOH, we develop a spectroscopic technique to measure vibrational branching ratios with relative intensity sensitivity around 1 part in $10^5$, approximately two orders of magnitude beyond the previous state of the art. We also perform the first spectroscopic observations and characterizations of the related molecule ytterbium monomethoxide, YbOCH$_3$, a promising species to probe beyond-the-Standard-Model physics. In addition, we show theoretically that an entire class of asymmetric top molecules can be laser cooled using essentially the same techniques used for much simpler species. Measurements of the vibrational branching ratios for a few prototypical asymmetric to molecules (CaSH and CaNH$_2$) are presented to confirm these predictions.

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Abstract Doppler and Sisyphus cooling of 174YbOH are achieved and studied. This polyatomic molecule has high sensitivity to physics beyond the Standard Model and represents a new class of species for future high-precision probes of new T-violating physics. The transverse temperature of the YbOH beam is reduced by nearly two orders of magnitude to < 600 μK and the phase-space density is increased by a factor of > 6 via Sisyphus cooling. We develop a full numerical model of the laser cooling of YbOH and find excellent agreement with the data. We project that laser cooling and magneto-optical trapping of long-lived samples of YbOH molecules are within reach and these will allow a high sensitivity probe of the electric dipole moment of the electron. The approach demonstrated here is easily generalized to other isotopologues of YbOH that have enhanced sensitivity to other symmetry-violating electromagnetic moments.

arXiv:1910.11318•New Journal of Physics•Jan 7, 2020•163 citations
We report a generally applicable computational and experimental approach to determine vibronic branching ratios in linear polyatomic molecules to the 10−5 level, including for nominally symmetry-forbidden transitions. These methods are demonstrated in CaOH and YbOH, showing approximately two orders of magnitude improved sensitivity compared with the previous state of the art. Knowledge of branching ratios at this level is needed for the successful deep laser cooling of a broad range of molecular species.
Molecules controlled at the single-quantum-state level offer wide-ranging applications spanning many of the frontiers of modern physics. Within the past decade, simple diatomic molecules have been brought under full quantum control both via assembly from pre-cooled atoms and by direct laser cooling. Polyatomic molecules\footnote{Molecules that contain three or more atoms.} have attracted focus as novel quantum resources providing distinct advantages (and challenges) compared to both atoms and diatomic molecules. For example, nearly all polyatomic molecules have long-lived states that can be fully polarized at low applied electric fields. These and other features generic to polyatomic molecules can be applied to quantum simulation, quantum chemistry, and tests of fundamental physics beyond the Standard Model. While these structural features of polyatomic molecules are highly promising for applications, they complicate the process of bringing molecules under full quantum control. In this thesis, we describe a series of experimental and theoretical advances toward the production of ultracold complex polyatomic molecules. We demonstrate the first laser cooling of the polyatomic radical ytterbium monohydroxide (YbOH), a promising candidate to search for particles and interactions beyond the Standard Model of particle physics. We develop a novel deceleration technique (``Zeeman-Sisyphus deceleration") capable of slowing molecular beams to trappable velocities while scattering fewer than 10 optical photons. This deceleration method is demonstrated using CaOH molecules, a lighter analog of YbOH. In order to determine a viable pathway to full three-dimensional cooling and trapping of YbOH, we develop a spectroscopic technique to measure vibrational branching ratios with relative intensity sensitivity around 1 part in 10510^5105, approximately two orders of magnitude beyond the previous state of the art. We also perform the first spectroscopic observations and characterizations of the related molecule ytterbium monomethoxide, YbOCH3_33​, a promising species to probe beyond-the-Standard-Model physics. In addition, we show theoretically that an entire class of asymmetric top molecules can be laser cooled using essentially the same techniques used for much simpler species. Measurements of the vibrational branching ratios for a few prototypical asymmetric to molecules (CaSH and CaNH2_22​) are presented to confirm these predictions.
Abstract Alkaline earth monoalkoxide free radicals (MORs) have molecular properties conducive to direct laser cooling to sub-millikelvin temperatures. Using dispersed laser induced fluorescence measurements from a pulsed supersonic molecular beam source we determine vibrational branching ratios and Franck–Condon factors for the MORs CaOH and CaOCH 3 . With narrow linewidth continuous-wave dye laser excitation, we precisely measure fluorescence branching for both X ˜ – A ˜ and X ˜ – B ˜ electronic systems in each molecule. Weak symmetry-forbidden decays to excited bending states with non-zero vibrational angular momentum are observed. Normal mode theoretical analysis combined with ab initio structural calculations are performed and compared to experimental results. Our measurements and analysis pave the way for direct laser cooling of these (and other) complex nonlinear polyatomic molecules. We also describe a possible approach to laser cooling and trapping of molecules with fewer symmetries like chiral species.
The vibrational branching ratios from the lowest excited electronic state for SrOCH3\textrm{SrOCH}_3SrOCH3​, SrNH2\textrm{SrNH}_2SrNH2​, and SrSH\textrm{SrSH}SrSH are measured at the <0.1%< 0.1\%<0.1% level. Spectra are obtained by driving the X~−A~\tilde{X} - \tilde{A}X~−A~ transitions and dispersing the fluorescence on a grating spectrometer. We also perform ab initio\textit{ab initio}ab initio calculations for the energies of vibrational levels relevant for laser cooling, as well as branching ratios to support the interpretations of all molecular spectra. Symmetry group analysis is applied in conjunction with our data to study rotational closure in these molecules. These analyses indicate favorable prospects for laser cooling SrNH2\textrm{SrNH}_2SrNH2​ and other similar alkaline-earth(-like) amides for future beyond the Standard Model physics searches using polyatomic molecules with long-lived parity doublets.
The vibrational branching ratios of SrOH for radiative decay to the ground electronic state, $X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$, from the first two electronically excited states, $A{\phantom{\rule{0.16em}{0ex}}}^{2}\mathrm{\ensuremath{\Pi}}$ and $B{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$, are determined experimentally at the $\ensuremath{\sim}{10}^{\ensuremath{-}5}$ level. The observed small branching ratios enable the design of a full, practical laser-cooling scheme, including magnetooptical trapping and sub-Doppler laser cooling, with >104>{10}^{4}>104 photon scatters per molecule. Ab initio calculations sensitive to weak vibronic transitions are performed to facilitate the experimental measurement and analysis, and show good agreement with the experiment.
Laser cooling of large, complex molecules is a long-standing goal, instrumental for enabling new quantum technology and precision measurements. A primary consideration for the feasibility of laser cooling, which determines the efficiency and technical requirements of the process, is the number of excited-state decay pathways leading to vibrational excitations. Therefore, the assessment of the laser-cooling potential of a molecule begins with estimate of the vibrational branching ratios of the first few electronic excited states theoretically to find the optimum cooling scheme. Such calculations, typically done within the Born-Oppenheimer and harmonic approximations, have suggested that one leading candidate for large, polyatomic molecule laser cooling, alkaline earth phenoxides, can most efficiently be laser cooled via the third electronically excited ( C ̃ ) state. Here, we report the first detailed spectroscopic characterization of the C ̃ state in CaOPh and SrOPh. We find that nonadiabatic couplings between the A ̃ , B ̃ , and C ̃ states lead to substantial mixing, giving rise to vibronic states that enable additional decay pathways. Based on the intensity ratio of these extra decay channels, we estimate a nonadiabatic coupling strength of ∼ 0.1 cm − 1 . While this coupling strength is small, the large density of vibrational states available at photonic energy scales in a polyatomic molecule leads to significant mixing. Only the lowest excited state A ̃ is exempt from this coupling because it is highly separated from the ground state. Thus, this result is expected to be general for large molecules and implies that only the lowest electronic excited state should be considered when judging the suitability of a molecule for laser cooling.
Abstract Doppler and Sisyphus cooling of 174YbOH are achieved and studied. This polyatomic molecule has high sensitivity to physics beyond the Standard Model and represents a new class of species for future high-precision probes of new T-violating physics. The transverse temperature of the YbOH beam is reduced by nearly two orders of magnitude to < 600 μK and the phase-space density is increased by a factor of > 6 via Sisyphus cooling. We develop a full numerical model of the laser cooling of YbOH and find excellent agreement with the data. We project that laser cooling and magneto-optical trapping of long-lived samples of YbOH molecules are within reach and these will allow a high sensitivity probe of the electric dipole moment of the electron. The approach demonstrated here is easily generalized to other isotopologues of YbOH that have enhanced sensitivity to other symmetry-violating electromagnetic moments.