We present an experimental investigation of the high-resolution electronic spectrum of the A²Π → X²Σ⁺ band of CaF isotopologues via laser-induced fluorescence. The CaF molecules were produced via laser ablation of a calcium rod in presence of an Ar/SF₆ gas mixture, then supersonically expanded to form a cold molecular beam. Different isotopologues (⁴⁰CaF, ⁴²CaF, ⁴⁴CaF) were identified in the spectrum, and the hyperfine splitting due to fluorine was resolved in a field-free environment¹. The spectra were fitted with PGOPHER to determine constants for each isotopologue. The fitted constants were then compared with those extrapolated from the most abundant isotopologue, ⁴⁰CaF. (1) Devlin, J.; Tarbutt, M. R.; Kokkin, D. L.; Steimle, T. C. Measurements of the Zeeman Effect in the A²Π and B²Σ⁺ States of Calcium Fluoride. J. Mol. Spectrosc. 2015, 317, 1–9. https://doi.org/10.1016/j.jms.2015.07.009.

In the era of modern chemistry, we have achieved significant advancements in designing and implementing switchable systems which can be reversibly interchanged between two distinct stationary states on the potential energy surface (PES). The mechanism behind the switchable molecules lies in the multiple-well potential bistability concept that allows the system to move from one potential minimum to another under an external stimulation, shedding light on a number of flexible materials, responsive agents, and controlled devices. Recently, the application of external oriented electric field has been proposed as an effective perturbation to modify the electronic configurations and PES of chemical systems, making it possible to trigger a potential switching mechanism observed from the oriented rotational PES for non-switchable molecules. In this study, we investigate the electric field-perturbed PES of CO and OCS molecules on ground state as well as low excited states. The field-induced PES of both molecules explicitly exhibit two stable configurations when the field vector is parallel or anti-parallel to the molecular principal axis, separated by a potential energy barrier. In terms of molecular responses to an external electric field, the dipole moment plays a secondary role in these systems for describing the total external field effect, while the anisotropic polarizability determines the majority of the distinctive response to different orientations of the external field. We also find that the external field induces vibronic mixing of excited states that features some interesting behaviors relevant to photochemistry, molecular dynamics, etc. Finally, we propose the use of a polynomial expansion to quantitatively describe the field-driven PES from the molecular electrical properties such as static moment, polarizability, and hyper-polarizabilities.

The super ccCA (s-ccCA) composite approach has been applied to many 3d and 4d diatomic molecules. s-ccCA has obtained excellent results against state-of-the-art Resonant Two-Photon Ionization energies. Many of these species have challenging electronic structure and require considering spin-orbit effects, energetic contribution beyond CCSD(T), and at least 1-electron relativistic methods (DKH, x2c). These challenges are only more magnified as the 5d species are considered. Herein the s-ccCA composite scheme is applied to a large set of 5d diatomics with known, accurate R2PI dissociation energies. Molecular ionization energies, only recently determined, are also compared with the s-ccCA composite scheme.

Diffusion Monte Carlo (DMC) is a useful computational method for investigating the vibrational landscape of small molecular systems that undergo large amplitude vibrational motions, but is limited by both the availability of potential energy surfaces and the computational cost of evaluating electronic structure energies. In a typical production run DMC simulation, hundreds of millions of potential energy evaluations are required to reach convergence, which is the major computational cost of this method. To address these limitations, a generalized protocol was developed for training GPU-accelerated neural net potentials to be evaluated during large scale DMC simulations. In the development of this protocol, we focused on efficient collection of the training dataset from on-the-fly ab initio calculations accelerated by machine learning approaches. In this work, neural net potentials were developed for small clusters of hydroxide or hydronium ions with several water molecules. Production run DMC simulations utilizing these neural net potentials resulted in converged ground state wave functions that are in agreement with previous DMC studies that utilized previously developed potential energy surfaces for these systems. This work demonstrates that machine learning approaches can be utilized to significantly reduce the computational cost of potential development and evaluation, providing a step forward in extending DMC as a viable method for studying larger, more complex systems.

The interactions between hydroxide and hydronium and water molecules have been of long-standing spectroscopic interest due to their importance in proton migration and the autoionization of water. While there have been numerous experimental and computational studies of protonated water clusters, less work has focused on their hydroxide counterparts. In this talk, we present the results of recent diffusion Monte Carlo calculations that explore the structure and zero-point motions of the OH⁻·(H₂O)₂ complex. This work was enabled by the development of a new potential surface using machine learning approaches both for the electronic structure calculations and the fit of the potential. Of particular interest is the vibrations of the water OH bonds that are bound to the hydroxide ion. These motions have been assigned to peaks at 1730 and 1810 cm⁻¹.O. Gorlova, J. W. DePalma, C. T. Wolke, A. Brathwaite, T. T. Odbadrakh, K. D. Jordan, A. B. McCoy and M. A. Johnson, "Characterization of the primary hydration shell of the hydroxide ion with H2 tagging vibrational spectroscopy of the OH⁻·(H₂O)_n=2,3 and OD⁻· (D₂O)_n=2,3 clusters," J. Chem. Phys. 145, 134304 (2016)he large red-shift of these frequencies compared to isolated water molecules indicates that these protons are highly delocalized. Interestingly, the positions of these peaks are close to the analogous transitions in the H₃O⁺·(H₂O)₂ complex, although the peaks in the H₃O⁺·(H₂O)₂ spectrum of are found to be significantly broadened compared to the OH⁻·(H₂O)₂ system. Deuteration shifts these transitions to 1403 and 1469 cm⁻¹. Insights into the spectral signatures of large amplitude motions in this hydroxide/water complex will also be discussed.

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Footnotes:

O. Gorlova, J. W. DePalma, C. T. Wolke, A. Brathwaite, T. T. Odbadrakh, K. D. Jordan, A. B. McCoy and M. A. Johnson, "Characterization of the primary hydration shell of the hydroxide ion with H2 tagging vibrational spectroscopy of the OH⁻·(H₂O)_n=2,3 and OD⁻· (D₂O)_n=2,3 clusters," J. Chem. Phys. 145, 134304 (2016)T

Fluorophenide anions C₆H_5−xF_X (2 ≤ x ≤ 4) are formed by electron-enabled photodissociation of fluorobenzene neutrals. The photoelectron (PE) spectra revealed each isomer has a unique electron affinity (EA), which aligned with the calculations. This finding indicates the impact the positioning and symmetry of the F atoms on the electronic structure, and therefore, the chemical properties. The di-, tri-, and tetrafluorophenide molecules with the lowest EA had F atoms in ortho- and para- positions and have less electron density at the C atom opposite the C atom bearing the radical center. The molecules with the highest EAs had two F atoms in ortho positions to the radical C. This shows that withdrawal of the electron density away from the radical C increases the stability of the anion.

Contrary to Hund’s rule, many members of the azaphenalene family of molecules are predicted to have inverted singlet-triplet gaps (STGs) wherein their first excited singlet state (S₁) lies energetically below their first excited triplet (T₁).^[1–3] In the context of light emitting devices, inverted STG emitters have the potential to significantly enhance photon yield by eliminating the triplet loss channel that ordinarily resides below the singlet. They may also have a unique advantage as photocatalysts since their lowest lying, catalytically active state would be a singlet rather than a triplet, making them insensitive to ambient oxygen. Recent studies utilizing optical absorption- and emission-based techniques have provided evidence that STG inversion is possible for some substituted azaphenalene species.^[4,5] However, direct measurement of the T₁ energy is precluded in those cases by the spin selection rule. Here, using a combination of high-resolution cryogenic anion photoelectron spectroscopy (which circumvents the spin selection rule) and matrix isolation absorption spectroscopy, we report the direct measurement of the inverted STG in 1,3,4,6,9b-pentaazaphenalene as -47(5) meV. We find that EOM-CCSD, which is widely considered to be the "gold standard" for prediction of small or inverted STGs accurately predicts the STG in the vertical excitation framework but predicts a positive STG when geometry relaxation and zero-point energy (ZPE) are accounted for. We also find that less expensive methods, including ADC(2) and CC2 perform better than EOM-CCSD at predicting transition energies and the STG when geometry relaxation and ZPE are considered.^[2] [1] R. Pollice, P. Friederich, C. Lavigne, G. dos P. Gomes, A. Aspuru-Guzik, Matter 2021, 4, 1654–1682. [2] L. Tučková, M. Straka, R. R. Valiev, D. Sundholm, Phys. Chem. Chem. Phys. 2022, 24, 18713–18721. [3] P.-F. Loos, F. Lipparini, D. Jacquemin, J. Phys. Chem. Lett. 2023, 14, 11069–11075. [4] N. Aizawa, Y.-J. Pu, Y. Harabuchi, A. Nihonyanagi, R. Ibuka, H. Inuzuka, B. Dhara, Y. Koyama, K. Nakayama, S. Maeda, F. Araoka, D. Miyajima, Nature 2022, 609, 502–506. [5] J. Ehrmaier, E. J. Rabe, S. R. Pristash, K. L. Corp, C. W. Schlenker, A. L. Sobolewski, W. Domcke, J. Phys. Chem. A 2019, 123, 8099–8108.

The pseudodiabatic basis of Köppel, Domcke, and Cederbaum has proven its power in the prediction and interpretation of vibronic spectra. While most diabatization techniques rely on rotation of CAS-like states (differing only in the objective function), the diabatization method of Ichino et al. operates within the highly accurate and systematically improvable EOM-CC framework; however, this ansatz is limited to diabatic states of differing symmetry. Presented in this work is a new diabatic ansatz inspired by the work of Ichino et al. with no symmetry restrictions, instead exploiting the correspondence between the chemist’s intuition and the diabatic picture.

Exploring how charge-changing affects the photoluminescence of small organic dyes presents challenges simply due to the inherent difficulty of selectively preparing well-defined samples of charged chromophores for spectroscopy. Here, we used helium tagging photodissociation action spectroscopy in gas phase and dispersed laser-induced fluorescence spectroscopy in solid Ne matrix to compare the intrinsic photophysical properties of pyronin Y cation [PY]⁺ and its one electron-reduced neutral radical [PY]^· at 5 K. Whereas the cation shows efficient visible photoluminescence, no emission from the neutral, in line with theoretical predictions was detected. B3LYP/aug-cc-pVDZ calculations based on the TD-DFT/FCHT method allow for unambiguous assignment of recorded vibrationally resolved absorption and emission spectra. Surprisingly, our experimental sensitivity was high enough to also observe electronic pre-resonance Raman (ePR-Raman) spectra of [PY]⁺, with a significant efficiency factor. These characteristics of the [PY]^·/[PY]⁺ pair suggest that appropriately functionalized derivatives may open new perspectives in the area of in-vivo bio-imagining microscopy and find applications in various sophisticated stimulated-Raman spectroscopies.

The nature of the lowest-lying electronic transitions in simple alcohols has been debated for decades, especially the valence-shell or Rydberg character of their excited states. Here, we report vacuum ultraviolet absorption spectra of subcritical and supercritical methanol thin fluid films combined with TD-DFT computations, gaining new insight into the nature of the two lowest-lying absorption bands and the impact of intermolecular forces on the absorption spectrum. We suggest that the known lowest-lying, weak-intensity, broad absorption band at 6.757 eV in the gas phase that demonstrates mixed σ*(OH){- 2a" valence shell character and 3s{-2a" Rydberg character loses a large part of the later upon condensation. This band is blue-shifted by 0.65 eV in the liquid at 25^°C due to hydrogen bonding and changes to the excited-state potential energy surface, such that it is buried under the more intense, higher-energy transition (assigned to 3p{- 2a" Rydberg character in the gas phase). With increasing temperature up to 250^°C, above the critical temperature (Tc = 240^°C), this feature gradually redshifts by 0.41 eV as thermal energy breaks down the hydrogen-bond network, likely affecting both ground and excited states. At 250^°C, incrementally decreasing the pressure from 150 to 14.5 bar (density from 0.477-0.011 g cm⁻³) causes narrowing and sharpening of the intense Rydberg transitions, revealing the broad, weak, lowest-lying band, as in the gas phase. From these data, we extract the fractions of methanol monomer vs. dimer/oligomer present as a function of pressure/density in supercritical methanol and the extent of monomer electronic perturbation due to Rydberg effects. Using known methanol dimer interaction potentials, we estimate the critical distance between molecules (ca. 4.7 Å) needed to explain the observed dimerization.