Diatomic molecules consisting of an alkaline earth atom (group IIa) bound to an alkali atom (group Ia) have attracted recent attention as molecules that may be suitable for direct laser cooling. At ultra-cold temperatures these molecules may be manipulated via their spins and molecular dipole moments. Diatoms composed of an alkaline earth atom bound to a coinage metal (group Ib) offer similar desirable properties, but there are very few experimental data for these species. We have now recorded electronic spectra for BeCu using one-color resonantly enhanced photoionization spectroscopy (REMPI). Progressions of vibronic bands were observed within the range from 32,894 to 35,273 cm⁻¹ (energy of the first photon). Two-color REMPI measurements indicated that these excited states may be predissociated. A search for unperturbed lower energy electronic states is in progress. This work is being guided by our ab initio calculations (MRSDCI and MRCC) for the ground and low energy electronically excited states.

Alkaline-earth-metal-containing free radicals, like calcium alkoxides are promising candidates for direct laser cooling with potential applications in precision measurements, cold chemistry, and quantum information science. Our group has studied calcium alkoxide radicals such as CaOCH₃, CaOC₂H₅, and CaOCH(CH₃)₂ using laser-spectroscopic techniques under jet-cooled conditions, revealing their vibronic transition frequencies and relative intensities. The present study focuses on the electronic and vibrational structures of the Ã2/Ã1-~X system of Ca-1-propoxide radicals, particularly examining the effects of gauche (G) and trans (T) conformations on the vibrational branching ratios and the role of C-H stretch modes. In the laser-induced fluorescence (LIF) and dispersed-fluorescence (DF) spectra of Ca-1-propoxide, we identified the Ã2/Ã1-~X origin bands, the spin-orbit (SO) splitting, and distinctive vibrational signatures of the G and T conformers. Vibronic bands of both conformers are assigned based on density functional theory (DFT) and time-dependent (TD-DFT) calculations. Experimentally determined vibronic transitions intensities and the derived Franck-Condon factors reveal the practicality of laser-cooling the Ca-1-propoxide radicals. Notably, transitions to C-H stretch levels, absent in monoalkoxide radicals studied previously, were observed in the LIF and DF spectra of the G conformer C₁, underlining the critical role of molecular symmetry. Rotationally resolved LIF spectroscopic measurement using narrow-linewidth continuous-wave (CW) ring dye lasers is in process.

The electronic absorption spectrum of CuO between 14,000 and 16,000 cm⁻¹ is quite complex due to the five low-lying electronic states found in this region. Recent calculations have encouraged the reexamination and reassignment of several spectral features in this region. The previously reported electronic states γ²Π_3/2, γ²Π_1/2, and αΣ states at 15,166, 15,470, and 15424 cm⁻¹, respectively, are now reassigned as spin-orbit components of a ⁴Π state. The high resolution intracavity laser absorption spectra of these transitions in CuO have been analyzed and results will be presented.

Recent experiments in polariton chemistry indicate that reaction rates can be significantly enhanced or suppressed inside an optical cavity. One possible explanation for the rate modulation involves the cavity mode altering the intramolecular vibrational energy redistribution (IVR) pathways in the vibrational strong coupling (VSC) regime. In this study, motivated by a recent work of Ahn et al (Science, 380, 1165 (2023)), we present our classical and quantum dynamical IVR results on a model effective Hamiltonian. We show that tuning the cavity frequency to a key reactant stretching mode results in a strong perturbation of the cavity-free IVR pathways. Thus, in the VSC regime, an appropriately tuned cavity mode can efficiently scramble the initial zero-photon state over the molecular quantum number space. Further support comes from the behaviour of the Shannon entropy and participation ratio distributions. The extent of IVR, however, is strongly mode-specific and dependent on the cavity frequency. Interestingly, good classical-quantum correspondence is seen even for low total cavity-molecule excitations.

Free radicals like NO₃ and CH₃O often exhibit significant vibronic couplings. Past experimental and theoretical works on both these molecules have indicated the need for higher order potential energy surfaces than those available at the time of study, to enable a better understanding of the experimental results e.g. studying the vibronic structure of the à  ² E" state of NO₃ radical¹ and the spin-vibronic structure in the C-H stretch region of the X̃  ²E state of CH₃O radical² . A typical problem with generating such potentials is the extremely high number of parameters required to represent them accurately. In this presentation, we talk about generating up to sixth order potential energy surfaces for molecules with three-fold symmetry. Molecular symmetry properties have been used to significantly reduce the number of independent parameters in the potential part of the corresponding spin-vibronic Hamiltonian matrices. Machine-learning-based methods have been developed to fit adiabatic potential energy surface data that have been obtained using electronic structure calculations. The improved quality of the new potentials is evidenced by comparing the predicted and observed values of spectral parameters. Improvements to the SOCJT3 software³ used for solving the vibronic and spin-vibronic problems will also be discussed.

The presence of a conical intersection at the molecular geometry of three-fold symmetry has led to significant interest in studying the methoxy radical. Many studies in the past have analysed both spin-vibronic¹ and rotationally resolved spectra of the radical² . However, analysis of the spectra of the X̃ state of the CH₃O radical in the CH-stretch region has remained a challenge, as it has not only required a better potential energy surface (PES) than available, but also the development and improvement in efficiency of computational codes that solve spin-vibronic problems. In this talk, we present our efforts to understand and assign the spin-vibronic levels of the X̃ ²E state up to the 3000 cm⁻¹ region. For this work, a PES has been calculated at the EOM-CCSDT/ANO1 level of theory. Subsequently this PES was fitted to a quartic power series expansion in all 9 vibrational normal coordinates (determined at the conical intersection) by the use of a machine-learning-based algorithm. After the addition of spin-orbit coupling to this PES, the spin-vibronic problem was solved using both Krylov-Schur and Lanczos algorithms using the SOCJT3 software ³ to converge eigenvalues and eigenvectors up to 3500 cm⁻¹. These eigenvectors were used, in conjunction with the dipole moment and its derivatives (calculated using finite differences at EOM-CCSDT/ANO1 level), to determine intensities for the spin-vibronic spectra. The calculated transition frequencies and intensities were used to assign the observed transitions of the spin-vibronic spectra of the radical ^{1, 2, 4} .

The preceding talk by Vasilyev et al. focuses on the spin-vibronic structure of the CH-stretch region of the X̃² E ground electronic state of the methoxy radical (CH₃O). To extract a rotation-free spin-vibronic energy level structure from experimental spectra, one must simulate and fit the rotational and fine structure. The CH-stretch region of the X̃² E-state CH₃O is a labyrinth of spin-ro-vibronic energy levels. Experimental, computational, and theoretical techniques have been developed to understand the energy level structure. Experimentally, the fluorescence-depletion infrared (IR) spectroscopy technique was employed to simplify the ro-vibrational spectra. For each vibrational transition within the X̃² E state, only a few transitions were observed, all originating from the same lower rotational level. Theoretically, a spectroscopic model incorporating an effective rotational Hamiltonian was used to account for the spin-orbit interaction coupled with the Jahn-Teller interaction (SOcJT) and simulate the rotational and fine structure. The fit B rotational constants significantly deviate from that obtained using the undistorted geometry, which may be attributed to strong vibronic interactions. Therefore, the analysis of rotational and fine structures not only provides rotation-free transition frequencies but also illuminates the mechanisms and strengths of spin-vibronic interactions.

Beryllium molecules are known to provide a challenge to even the most high level computations. There is much interest in pure Be clusters in the metallic community to determine the onset of 'bulk' behavior in metallic clusters of increasing size. These clusters of increasing size often have large contributions from many body effects and suffer from difficult to compute multi-coordinate potential surfaces. Relatedly, Jahn-Teller distortions are a difficult and interesting subsection of theoretical chemistry research. These distortions shift molecules away from their high symmetry forms and into lower symmetry geometries, presenting potential energy surfaces with a conical intersection that can often provide difficulty in computational prediction and questions in experiment due to the potential for intense vibronic coupling. In this talk, we present our joint theoretical and experimental study on the Be₅ molecule and its anion, which suffer from the Jahn Teller effect, and present several open questions with the nature of these distortions as well as the molecule itself.