The electronic structure of CaO is complex due to the large number of low-lying energy states and a multitude of rovibronic interactions among them. Several vibrionic bands of the A’¹Π−X¹Σ⁺ CaO have been identified in the visible region using laser induced fluorescence. Analysis of these rotationally resolved data provides more accurate band origins and rotational constants for levels that were previously determined indirectly using perturbation data. Fluorescence decay lifetime measurements were used to determine the radiative decay rate for the A’ state. A previously noted homogeneous perturbation of the C¹Σ⁺ state was examined to determine the identity of the perturbing state. Dispersed fluorescence spectra and fluorescence decay rate measurements were used to show that the perturbation results from the interaction with a state of ³Π(0+) symmetry.

The alkaline earth monohalides are molecules of interest in fields such as astrophysics, high-temperature chemistry, chemiluminescent reactions, spectroscopy, computational chemistry, and cold molecule trapping. The electronic properties of the CaX molecules are well described by Ligand Field Theory. These properties include the relative energies of the X, A, B, and C states, their spin-orbit, spin-rotation, and lambda-doubling constants, and their spin-orbit and L-uncoupling perturbation matrix elements. In this talk we will present some Ligand Field Theory-based ideas: Schmidt orthogonalization, ligand-induced 4p-3d induced polarization, ligand-to-metal charge transfer, and molecular orbital-size based on effective principal quantum number 1/n*⁽3) orbital radius scaling. This paper is dedicated to Joelle Rostas (deceased June 2019) who spent many years studying the alkaline earth monohalide molecules and discussing them with the authors of this talk.

The CaX family of diatomic molecules illustrates concepts developed by inorganic chemists to rationalize the properties of metal-centered complexes. The basic idea is that an atom or an atomic-ion is surrounded by ligands, and that the electronic properties of the complexes are dealt with in a model in which the central metal atom and the ligands are treated as retaining their separated atom or molecule properties perturbed by identifiable and quantifiable metal-ligand interactions. Ligand Field Theory is semi-empirical in the sense that it is a framework for building a systematic understanding of the properties of families of complexes from spectroscopic measurements of the properties of the separated species and the interactions between them. The electronic structures of the CaX molecules are described by atomic-ions-in-molecule ligand field models. The Ca atom is treated as Ca⁺ with a single electron in the 4sσ , 4pσ or π, or 3dσ,π, or δ orbital For X=F, Cl, Br, and I, the ligand is a closed-shell halide ion. For X=O, the ligand is an open-shell O⁻ ion with a single hole in the pπ (π⁻¹) or pσ (σ⁻¹) orbital. The building blocks of the electronic structure model are known by different names in the inorganic chemistry, small-molecule spectroscopy, and quantum chemistry communities. Fine structure (spin-orbit, spin-spin, spin-rotation, and lambda-doubling) and spectroscopic perturbation matrix elements (spin-orbit and L-uncoupling) report on the CaX electronic structure.

The alkaline earth monohalides are molecules of interest in fields such as astrophysics, high-temperature chemistry, chemiluminescent reactions, spectroscopy, computational chemistry, and cold molecule trapping. The electronic properties of the CaX molecules are well described by Ligand Field Theory. These properties include the relative energies of the X, A, B, and C states, their spin-orbit, spin-rotation, and lambda-doubling constants, and their spin-orbit and L-uncoupling perturbation matrix elements. In this talk we will present some Ligand Field Theory-based ideas: Schmidt orthogonalization, ligand-induced 4p-3d induced polarization, ligand-to-metal charge transfer, and molecular orbital-size. The lambda doublings definitely prove that the spin-orbit splitting in the C-state is 1/2 below 3/2 (i.e. regular). Finally, the simultaneous use of both laser techniques Laser Induced Fluorescence (LIF) and Cavity Ring Down Spectroscopy (CRDS), allowed the extension of previous measurements to high vibrational levels and provided accurate vibrational parameters This paper is dedicated to Joelle Rostas (deceased June 2019) who spent many years studying the alkaline earth monohalide molecules and discussing them with the authors of this talk.

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Footnotes:

This paper is dedicated to Joelle Rostas (deceased June 2019) who spent many years studying the alkaline earth monohalide molecules and discussing them with the authors of this talk..

Spectroscopic constants of molecules characterize the unique fingerprints of molecules and atoms. In particular, for diatomic molecules, spectroscopic constants encapsulate essential information about different applications in atomic, molecular, and optical physics, such as laser cooling or quantum information. To help find molecules well-suited for a given application, we have developed a database linked to an interactive website.X. Liu, S. Truppe, G. Meijer and J. Pérez-Ríos, J. Cheminform. 12, 31 (2020)he spectroscopic constants and Franck-Condon factors can be retrieved in useful formats from the website or via an application programming interface. New data can also be uploaded upon acceptance by the web managers. By applying machine-learning approaches to this dataset, we show that the spectroscopic constants, including the equilibrium distance, harmonic vibrational frequency, and binding energy, are correlated and depend solely on the constituent's groups and periods. Based on these relationships, ground-state spectroscopic constants of polar diatomic molecules can be predicted with a relative error of < 5%, whereas the same properties for the A-excited electron state are predicted with an error of < 11%.X. Liu, G. Meijer and J. Pérez-Ríos, arXiv:2005.07913 (2020)nyone can make these predictions with access to our database, and it does not require any quantum chemistry knowledge. Separately, we construct a database consisting of experimental electric dipole moments of 162 diatomic molecules. We show that the dipole moment of diatomic molecules is related to the constituent atoms' atomic properties, including electron affinity and ionization potential, and molecular properties that describe the force on the electrons at the equilibrium distance.X. Liu, G. Meijer and J. Pérez-Ríos, Phys. Chem. Chem. Phys. 22, 24191 (2020)html:<hr /><h3>Footnotes: X. Liu, S. Truppe, G. Meijer and J. Pérez-Ríos, J. Cheminform. 12, 31 (2020)T X. Liu, G. Meijer and J. Pérez-Ríos, arXiv:2005.07913 (2020)A X. Liu, G. Meijer and J. Pérez-Ríos, Phys. Chem. Chem. Phys. 22, 24191 (2020)

Plasmonic metal nanoparticles exhibit strong and tunable light-matter interactions which have enabled a variety of applications. Alongside their strong linear extinction, plasmonic nanoparticles have also been noted for their nonlinear optical properties. Here we present a study of multiphoton absorption in plasmonic gold nanorods using Fourier-transform spectroscopy with nonlinear optical detection. The excitation spectra of single gold nanorods is retrieved from an interferometric multiphoton photoluminescence (MPPL) signal which is generated by a pair of phase-locked pulses with attosecond interpulse delay precision. The resulting Fourier-transform spectra reveal the two- and three-photon excitation pathways that lead to emission from gold nanorods. Investigation of the spectra demonstrates the connection between resonance frequency and nonlinear absorption cross-section in gold nanorods, which is attributed to the influence of gold’s electronic structure.This work was supported by the Air Force Office of Scientific Research, grant number FA-9550-18-1-0347, the National Science Foundation Graduate Research Fellowship Program under grant no. DGE1255832 and the National Science Foundation, award number CHE-1807999html:<hr /><h3>Footnotes: This work was supported by the Air Force Office of Scientific Research, grant number FA-9550-18-1-0347, the National Science Foundation Graduate Research Fellowship Program under grant no. DGE1255832 and the National Science Foundation, award number CHE-1807999

Badger’s rule originally described a non-linear relationship between the equilibrium bond length and force constant for diatomic molecules. This rule has since become synonymous with many different relationships between molecular structure and the frequency of a corresponding molecular vibration. In this work we provide an intuitive and general rule describing the relationship between the OH equilibrium bond lengths, R_e, and the harmonic OH stretch frequencies, ω_e, as well as the vibrationally averaged OH bond lengths, R₀, and the anharmonic OH stretch frequencies, ν₀. We show that the same rule applies for hydrogen bonds of varying strengths and corresponding red shifts in the OH vibrations such as the ones in neutral water clusters, protonated water clusters, and aqueous halide clusters. Remarkably, we find a simple linear correlation between the changes in the covalent OH bond length and corresponding stretching frequency of -19 cm⁻¹/ 0.001 Å  that holds for both the (ω_e vs R_e) and (ν₀ vs R₀) pairs. We provide physical insights regarding the origin of this linear correlation by modeling the covalent, hydrogen bonded OH bond via a Morse oscillator. In particular, we show that the electric-field-dependent frequency shift for the harmonic and anharmonic frequencies lies on the same line. More importantly, using this simple linear relationship and scaling factor obtained from the aqueous clusters, we are able to reproduce both the structure and the position of the OH vibrational band in liquid water using the OH bond length distribution obtained from an ab initio molecular dynamics simulation.

Recent experiments have successfully laser cooled a variety of molecules, including diatomic, linear triatomic, and symmetric top species [1-3]. Laser cooling and trapping can require repeatedly scattering more than 10,000 photons per molecule, so all potential losses above the level of 1 part in 10⁵ must be identified and repumped to mitigate losses. Here, we report on the use of optical cycling to measure vibrational branching ratios of laser-coolable polyatomic molecules. We achieve relative intensity sensitivities at the 10⁻⁵ level, approximately a factor of 100 more sensitive than previous dispersed fluorescence studies [4-6]. The apparatus described can be adapted to probe any molecule with a nearly-closed cycling transition by tuning two laser wavelengths. In addition, we discuss how these high-precision branching ratio measurements have allowed us to infer values for Renner-Teller parameters in CaOH and YbOH, and for pseudo-Jahn-Teller parameters in CaOCH₃. [1] J. Barry, et al., Nature 512, 286 (2014). [2] I. Kozyryev, et al., Phys. Rev. Lett. 118, 173201 (2017). [3] D. Mitra, N. B. Vilas, et al., Science 369, 1366 (2020). [4] I. Kozyryev, et al., New J. Phys. 21, 052002 (2019). [5] A. C. Paul, et al., J. Chem. Phys. 151, 134303 (2019). [6] E. T. Mengesha, et al., J. Phys. Chem. A 124, 3135 (2020).

The electronic excited states of polycyclic aromatic hydrocarbons (PAHs) are especially interesting and high-resolution laser spectroscopy is very powerful to elucidate molecular structure and excited-state dynamics. We observed and analyzed the high-resolution and high-precision spectrum of the S₁ ← S₀ 0-0 band of jet-cooled a-benzanthracene using a single-mode laser system precisely controlled by optical frequency comb A. Nishiyama, K. Nakashima, A. Matsuba, and M. Misono, J. Mol. Spectrosc 318, 40 (2010). There are two candidates for the S₁ state of planar PAHs with high symmetry such as naphthalene and anthracene, Ψ_A ( HOMO → LUMO : strong transition and short fluorescence lifetime ) and Ψ_B ( HOMO → LUMO+1 and HOMO−1 → LUMO : weak transition and long fluorescence lifetime ) M. Baba, T. Katori, M. Kawabata, S. Kunishige, and T. Yamanaka, J. Phys. Chem. A 117, 13524 (2013). The S₁ states of naphthalene and zigzag catacondenced PAHs are well expressed by Ψ_B, but The S₁ states of anthracene and linear catacondenced PAHs show typical properties of Ψ_A, srong fluorescence and short-lived. It is concluded that the S₁ state of a-benzanthracene is the mixture of Ψ_A and Ψ_B and shows an intermediate property because of its low symmetry. A. Nishiyama, K. Nakashima, A. Matsuba, and M. Misono, J. Mol. Spectrosc 318, 40 (2010).. M. Baba, T. Katori, M. Kawabata, S. Kunishige, and T. Yamanaka, J. Phys. Chem. A 117, 13524 (2013)..

Trans-stilbene os og great interest in the excited-state dynamics such as cis-trans isomerization in the electronic excited state. Zewail at al. reported the results of time-resolved spectroscopy and suggested its nonplanar structure in the ground S₀ state J. A. Syage, P. M. Felker, and A. H. Zewail, J. Chem. Phys. 81, 4685 (1984). In contrast, Pratt et al. concluded that the molecule is essentially planar both in the S₀ and S₁ states by analyzing the rotationally resolved high-resolution speoctrum of the S₁ ← S₀ 0-0 band D. W. Pratt, W. L. Meerts et al., J. Phys. Chem. 94, 6 (1990). We observed the spectrum with much higher accuracy and quality, and re-determined the rotational constants. Although it is impossible to accurate determine the abosolute value of A for the a-type transition, We could conclude that trans-stilbene is nonplanar in the S₀ state. Theoretical calculation using WB97XD functional provided the nonplanar structure in which the phenyl rings are rotated around the C-C bond axis and take the C₂ symmetry. It suggests that steric repulsion between H atoms of ortho-position in a phenyl ring and in an ethylene part surpasses stabilization by π conjugation.

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Footnotes:

J. A. Syage, P. M. Felker, and A. H. Zewail, J. Chem. Phys. 81, 4685 (1984).. D. W. Pratt, W. L. Meerts et al., J. Phys. Chem. 94, 6 (1990)..