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Physical Review A

Vibrational branching fractions for laser cooling of nonlinear strontium-containing molecules

Alexander Frenett, Zack Lasner, Lan Cheng, John M. Doyle

Published Aug 14, 2024• 8 citations
Publisher

Abstract

The authors explore candidates for a next-generation search for the electron electric dipole moment (eEDM) by experimentally measuring the vibrational loss channels in three Sr-containing nonlinear molecules. They conclude that SrNH${}_{2}$ is the optimal choice for a future laser-cooled molecule-based eEDM experiment.

DOI
10.1103/physreva.110.022811

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Frontiers in Chemistry•Sep 23, 2022•5 citations
The authors explore candidates for a next-generation search for the electron electric dipole moment (eEDM) by experimentally measuring the vibrational loss channels in three Sr-containing nonlinear molecules. They conclude that SrNH2{}_{2}2​ is the optimal choice for a future laser-cooled molecule-based eEDM experiment.
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 search for electric dipole moments of elementary particles with molecules has been instrumental in setting very strict limits on possible new physics, but a challenge for the next generation of such experiments is to use molecules which allow both laser cooling and very high suppression of systematic effects. While polyatomic molecules have been previously suggested for such purposes, the present work goes one step further by studying molecules containing multiple metals and showing that this approach may open the door for the control of metal species that cannot easily be laser-cooled otherwise.
Bright beams of cold molecules provide valuable quantum sensors for measuring permanent electric dipole moments that violate fundamental symmetries, which enables these tabletop experiments to probe energy scales exceeding those available with current or near-future particle colliders. The precision of such experiments generally scales with the number of molecules that are detected per unit time, typically a few meters downstream of the place where the molecular beam is created. So, improving the precision -and thereby probing new physics at higher and higher energy scales- requires increasing the downstream flux of molecules. This can be achieved by collimating the molecular beam to maintain a high number density. These molecular experiments can borrow techniques for control and manipulation from the extremely well-developed field of atomic physics. One such technique is laser cooling, which is a powerful method to achieve this boost in brightness, whose implementation however remains challenging due to the complex energy level structure of molecules. In this work, we demonstrate and analyze laser cooling of a focused cryogenic beam of BaF molecules. We conclude that with some follow-up steps this method will increase the beam brightness by a factor 100, a crucial milestone towards a high precision measurement of the electron electric dipole moment (eEDM).
Permanent electric dipole moments (EDMs) are signatures of time-reversal and parity violation and act as sensitive probes for physics beyond the Standard Model (BSM). The work in this thesis has been done as part of the NL-eEDM collaboration, which plans to measure the electron EDM using a beam of BaF molecules. The sensitivity of such a table-top precision experiment benefits from utilizing an intense, slow beam of molecules. We plan to achieve that through a combination of Stark deceleration and transverse laser cooling of a cryogenic beam of BaF. The work presented in this thesis has primarily been done to produce intense, slow molecular beams for an electron EDM measurement. We constructed a supersonic beam source of SrF and BaF molecules. The source produces high intensity beams with low translational and rotational temperatures. Time-resolved detection of laser-induced fluorescence from pulsed excitation of electronic states in BaF molecules was performed to determine the lifetimes of the A^{2}Pi_{1/2} and A^{2}Pi_{3/2} states. The lifetimes of these states were determined to be 57.1(3) ns and 47.9(7) ns, respectively. The transition between the A^{2}Pi_{1/2} and X^{2}Sigma^{+} states is the main laser cooling transition in BaF. We decelerated a beam of SrF molecules from a cryogenic source to a standstill inside a traveling-wave Stark decelerator, with the number of decelerated molecules on the order of 1000. BaF molecules will be decelerated with the same technique to perform an electron EDM measurement.
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 molecules to the ultracold regime is the prerequisite for many novel science and technologies. It is desirable to take advantage of theoretical approaches to explore polyatomic molecular candidates, which are capable of being cooled to the ultracold regime. In this work, we explore two polyatomic candidates, CaNC and SrNC, which are suitable for laser cooling. These molecules possess impressively large permanent dipole moments (∼6 Debye), which is preferred for applications using an external electric field. High-level ab initio calculations are carried out to reveal electronic structures of these molecules, and the calculated spectroscopic constants agree very well with the available experimental data. For each molecule, the Franck-Condon factor matrix is calculated and shows a diagonal distribution. The radiative lifetimes for CaNC and SrNC are estimated to be 15.5 and 15.8 ns, respectively. Based upon the features of various electronic states and by choosing suitable spin-orbit states, we construct two feasible laser cooling schemes for the two molecules, each of which allows scattering nearly 10000 photons for direct laser cooling. These indicate that CaNC and SrNC are excellent ultracold polyatomic candidates with strong polarity.