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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 ( $\tilde{C}$) state. Here, we report the first detailed spectroscopic characterization of the $\tilde{C}$state in CaOPh and SrOPh. We find that nonadiabatic couplings between the $\tilde{A}, \tilde{B}$, and $\tilde{C}$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 $\sim 0.1{\mathrm{cm}}^{-1}$. 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 $\tilde{A}$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. 

To date, computational methods for modeling defects (vacancies, adsorbates, etc.) have relied on periodic supercells in which the defect is far enough from its repeated image that they can be assumed non-interacting. Yet, the relative proximity and periodic repetition of the defect’s images may lead to spurious, unphysical artifacts, especially if the defect is charged and/or open-shell, causing a very slow convergence to the thermodynamic limit (TDL). In this article, we introduce a “defectless” embedding formalism such that the embedding field is computed in a pristine, primitive-unit-cell calculation. Subsequently, a single (i.e., “aperiodic”) defect, which can also be charged, is introduced inside the embedded fragment. By eliminating the need for compensating background charges and periodicity of the defect, we circumvent all associated unphysicalities and numerical issues, achieving a very fast convergence to the TDL. Furthermore, using the toolbox of post-Hartree–Fock methods, this scheme can be straightforwardly applied to study strongly correlated defects, localized excited states, and other problems for which existing periodic protocols do not provide a satisfactory description. 

This study reports simulations of the lowest band in the electronic absorption spectrum of pyrazine carried out using a multi-state-multimode vibronic Hamiltonian parameterized using equation-of-motion coupled-cluster methods. The simulations explain the main spectral features and show how peaks of vibronic nature appear. The most complete vibronic model includes four electronic states and six vibrational modes. The simulations reveal that non-adiabatic coupling with bright states located as high as 3 eV above the studied state can lead to discernible features in the absorption spectrum. This study demonstrates the power of fully ab initio treatments of electronic and vibrational structure and their utility in understanding the mechanisms leading to complex molecular spectra. 

The energetic disorder induced by fluctuating liquid environments acts in opposition to the precise control required for coherence-based sensing. Overcoming fluctuations requires a protected quantum subspace that only weakly interacts with the local environment. We report a ytterbium complex that exhibited an ultranarrow absorption linewidth in solution at room temperature with a full width at half maximum of 0.625 milli–electron volts. Using spectral hole burning, we measured an even narrower linewidth of 410 pico–electron volts at 77 kelvin. Narrow linewidths allowed low-field magnetic circular dichroism at room temperature, used to sense Earth-scale magnetic fields. These results demonstrated that ligand protection in lanthanide complexes could substantially diminish electronic state fluctuations. We have termed this system an “atomlike molecular sensor” (ALMS) and proposed approaches to improve its performance. 

Achieving ultranarrow absorption linewidths in the condensed phase enables optical state preparation of specific non-thermal states, a prerequisite for quantum-enabled technologies.