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arXiv:2209.13025 · The Journal of Physical Chemistry Letters

Laser Spectroscopy of Aromatic Molecules with Optical Cycling Centers: Strontium(I) Phenoxides

Guanming Lao, Guo‐Zhu Zhu, Claire E. Dickerson, Benjamin L. Augenbraun, Anastassia N. Alexandrova, Justin Ryan Caram, Eric R. Hudson, Wesley C. Campbell

Published Nov 22, 2022• 14 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1021/acs.jpclett.2c03040

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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.
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.
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.
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 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.
Rapid and repeated photon cycling has enabled precision metrology and the development of quantum information systems using atoms and simple molecules. Extending optical cycling to structurally complex molecules would provide new capabilities in these areas, as well as in ultracold chemistry. Increased molecular complexity, however, makes realizing closed optical transitions more difficult. Building on already established strong optical cycling of diatomic, linear triatomic, and symmetric top molecules, recent work has pointed the way to cycling of larger molecules, including phenoxides. The paradigm for these systems is an optical cycling center bonded to a molecular ligand. Theory has suggested that cycling may be extended to even larger ligands, like naphthalene, pyrene, and coronene. Herein, we study optical excitation and fluorescent vibrational branching of CaO-[Formula: see text], SrO-[Formula: see text], and CaO-[Formula: see text] and find only weak decay to excited vibrational states, indicating a promising path to full quantum control and laser cooling of large arene-based molecules.
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.