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International Symposium on Molecular Spectroscopy (ISMS)

FUNCTIONALIZED AROMATIC MOLECULES FOR LASER COOLING AND TRAPPING

Benjamin L. Augenbraun, John M. Doyle, Eric Hudson, Justin Ryan Caram, Wesley C. Campbell, Anastassia N. Alexandrova, Zack Lasner, Guanming Lao, Claire E. Dickerson, Debayan Mitra, Guo‐Zhu Zhu, Sean Burchesky

Published Jun 20, 2022• 0 citations
Publisher

Abstract

Rapid and repeated scattering of laser photons (``optical cycling'') underlies many uses of atoms and small molecules for quantum science and measurement. Larger polyatomic molecules are also appealing targets, partly because these species may be decorated with functional groups offering unique scientific opportunities. In this talk, we discuss a large class of aromatic molecules that can be functionalized with an alkaline-earth metal atom to enable optical cycling. We describe the gas-phase production of Ca- and Sr-bearing derivatives of phenyl (Ph) and naphthyl radicals and, using dispersed fluorescence spectroscopy, we show that these molecules contain multiple electronic transitions suitable for optical cycling and laser cooling. We present high-resolution laser excitation spectra for molecules such as fluorinated-CaOPh and SrOPh and compare these to the well-known alkaline-earth monoalkoxides and monoamides. These data inform ongoing work to laser cool and magneto-optically trap a functionalized aromatic molecule.

DOI
10.15278/isms.2022.mm04

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Ultracold organic chemistry enables studies of reaction dynamics and mechanisms in the quantum regime. Access to ultracold molecules hinges on the ability to efficiently scatter multiple photons via quasi-closed cycling transitions. Optical cycling in polyatomic molecules is challenging due to their complex electronic structure. Using equation-of-motion coupled-cluster calculations, we demonstrate that an alkaline earth metal attached to various aromatic ligands (such as benzene, phenol, cyclopentadienyl, and pyrrolide) offers nearly closed cycling transitions with only a few additional repump lasers. We also show that aromatic ligands such as benzene can accommodate multiple cycling centers in various geometrical arrangements, opening new avenues in quantum information science, precision measurements, and ultracold chemistry.
We present a high-resolution laser spectroscopic study of the à 2 B 2 –X̃ 2 A 1 and B̃ 2 B 1 –X̃ 2 A 1 transitions of calcium(I) phenoxide, CaOPh (CaOC 6 H 5 ). The rotationally resolved band systems are analyzed using an effective Hamiltonian model and are accurately modeled as independent perpendicular ( b - or c -type) transitions. The structure of calcium monophenoxide is compared to previously observed Ca-containing radicals, and implications for direct laser cooling are discussed. This work demonstrates that functionalization of aromatic molecules with optical cycling centers can preserve many of the properties needed for laser-based control.
Laser induced electronic excitations that spontaneously emit photons and decay directly to the initial ground state ("optical cycling transitions") are used in quantum information and precision measurement for state initialization and readout. To extend this primarily atomic technique to large, organic compounds, we theoretically investigate optical cycling of alkaline earth phenoxides and their functionalized derivatives. We find that optical cycle leakage due to wave function mismatch is low in these species, and can be further suppressed by using chemical substitution to boost the electron-withdrawing strength of the aromatic molecular ligand through resonance and induction effects. This provides a straightforward way to use chemical functional groups to construct optical cycling moieties for laser cooling, state preparation, and quantum measurement.
or SrOPh) and variants featuring electron-withdrawing groups designed to suppress vibrational excitation during spontaneous emission from the electronically excited state. Optical cycling closure of these species, which is the decoupling of the vibrational state changes from spontaneous optical decay, is found by dispersed laser-induced fluorescence spectroscopy to be high, in accordance with theoretical predictions. A high-resolution, rotationally resolved laser excitation spectrum is recorded for SrOPh, allowing the estimation of spectroscopic constants and identification of candidate optical cycling transitions for future work. The results confirm the promise of strontium phenoxides for laser cooling and quantum state detection at the single-molecule level.
An experimentally feasible strategy for direct laser cooling of polyatomic molecules with six or more atoms is presented. Our approach relies on the attachment of a metal atom to a complex molecule, where it acts as an active photon cycling site. We describe a laser cooling scheme for alkaline earth monoalkoxide free radicals taking advantage of the phase space compression of a cryogenic buffer-gas beam. Possible applications are presented including laser cooling of chiral molecules and slowing of molecular beams using coherent photon processes.
The rich internal structure of polyatomic molecules has inspired a number of theoretical proposals exploring a myriad of unique scientific and engineering frontiers, including quantum simulation and next-generation searches for physics beyond the standard model. Establishing full control over these complex quantum objects is likely necessary to harness their full potential; however, the same rich internal structure that gives rise to the immense scientific potential of polyatomic molecules also makes manipulating them a daunting task. In the past decade there has been considerable progress in the production, cooling, and control of diatomic species using direct laser cooling and magneto-optical trapping. This dissertation focuses on extending direct laser cooling and trapping to polyatomic molecules. Direct laser cooling of triatomic radical calcium monohydroxide (CaOH) is demonstrated. This molecules is a prototypical example of a broad class of polyatomic molecules that includes symmetric and asymmetric tops. Transverse magneto-optical trapping is demonstrated, establishing the ability to scatter an unprecedented number of photons in a polyatomic species. An understanding of previously unappreciated intricacies of molecular structure is developed, that explains observations of novel vibrational decay pathways. High resolution spectroscopy of hybrid vibrational modes is performed. A laser cooling scheme is constructed that will support future efforts to use the full range of laser cooling and high-fidelity detection techniques required to implement an optical tweezer array of polyatomic molecules. Our experimental efforts demonstrate the feasibility of direct laser cooling applied to CaOH, up ending long-standing assumptions of the inextricable complexity of polyatomic molecules.