Silicates represent an understudied class of molecules in astrochemistry despite surfaces having been shown to have important chemical and physical effects. Clinoenstatite has been detected in interstellar space\footnote{Jones, A. P. (2007). \emph{EJM}, 19(6), 771-782.} and is a major component of meteorites.\footnote{Mason, B. (1968). \emph{Lithos}, 1(1), 1-11.} Clinoenstatite is also interesting because it has chiral faces.\footnote{Hazen, R.M. (2006). \emph{Am Min}, 91(11/12), 1715.} Structural and spectral studies utilizing density functional theory (DFT) and time-dependent (TD-) DFT were conducted on the monomer and a nanosilicate dimer of enstatite first described in Valencia et al.\footnote{Valencia, E. M., et al. (2020). \emph{MNRAS}, 492(1), 276-282.}; in addition, a 2D chiral conformer of the dimer was also studied. The HSE06/aug-cc-pVQZ level of theory was chosen for optimization, frequency, and energy calculations. CAM-B3LYP and $\omega$B97X-D3 functionals were used with the cc-pVQZ basis set in the TD-DFT calculations to obtain the UV-Vis and ECD spectra. We modeled each structure in its bare silicate form but also with H, H$^{+}$, and H$_{2}$. Placing the neutral H on the 2D chiral enstatite conformer produces a 3D structure that makes it optically active. This provides a symmetry breaking mechanism for chiral silicates in the ISM.

We report on the characterization of the rovibrational structure of the ground and first excited electronic states of MgO⁺ by high-resolution pulsed-field ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. Rotationally cold (T_rot=5 K) MgO molecules in the X¹Σ⁺ (v = 0-2) levels are generated in a supersonic expansion of a 0.1% N₂O/He carrier gas following laser ablation off an magnesium (Mg) rod[1].The rovibrational ionization thresholds corresponding to both spin-orbit components (Ω= \frac12,\frac32) of the X^{+ 2}Π_Ω (v⁺ = 0-10) states and to the lowest vibrational levels of the A^{+ 2}Σ_\frac12⁺ state are reached in a resonant 1+1^′ two-photon excitation sequence via the F¹Π, E¹Σ⁺, and G¹Π rovibrational intermediate levels of MgO studied previously by Breckenridge and coworkers [2]. Our new results include accurate values for the adiabatic ionization energy of MgO and for the dissociation energies of the MgO X ¹Σ⁺ and MgO⁺ X^{+ 2}Π_\frac12 states. This work is carried out in the context of our studies of the rovibrational structure of doubly charge dications by high-resolution PFI-ZEKE spectroscopy of singly charged cations following a similar approach as recently taken to characterize the ground state of the thermodynamically stable dication MgAr²⁺ [3]. The talk will present a roadmap towards characterizing the ground state of MgO²⁺ by resonant multiphoton excitation via electronically excited states of MgO⁺. The experiments will reveal whether MgO²⁺ is thermodynamically stable as predicted in Ref. [4] or metastable as predicted in Ref. [5]. [1] T.D. Persinger, D.J. Frohman, W.M. Fawzy and M.C. Heaven, J. Phys. Chem. 153 (5), 054308 (2020). [2] J. Wang and W.H. Breckenridge, J. Chem. Phys. 124 (12), 124309 (2006). [3] D. Wehrli, M. Génévriez and F. Merkt, Phys. Chem. Chem. Phys. 23 (18), 10978-10987 (2021). [4] R. Linguerri, M. Hochlaf, M.C. Bacchus-Montabonel and M. Desouter-Lecomte, Phys. Chem. Chem. Phys. 15, 824-831 (2013). [5] M. Kolbuszewski and J.S. Wright, Chemical Physics Letters 218 (4), 338-342 (1994).

Transition metal nitrides are of growing interest due to their catalytic, energy storage, sensing, superconducting, and mechanical properties. The (0,0) band of the A ⁴Π - X ⁴Σ⁻ transition of MoN was recorded at Doppler-limited resolution using intracavity laser spectroscopy (ILS) integrated with a Fourier-transform spectrometer used for detection (ILS-FTS). The target MoN molecules were produced in the plasma discharge of a molybdenum-lined copper hollow cathode, using a gas mixture of Ar with about 1% N₂ in a reaction chamber with about 1 Torr total pressure. Isotopologue structure in the spectrum is clearly visible and analysis is underway for the five abundant isotopologues with no nuclear spin (I_Mo=0): ⁹²MoN (14.6%), ⁹⁴MoN (9.2%), ⁹⁶MoN (16.7%), ⁹⁸MoN (24.3%), and ¹⁰⁰MoN (9.7%). Hyperfine structure in the spectrum is not resolved, but clearly adds to the profile of each feature. To perform a comprehensive fit, the line positions ⁹⁵MoN (15.9%) and ⁹⁷MoN (9.6%) from Cheung et al. [JMS 202, (2000)] were included in the fit. PGOPHER was used with mass-constrained parameters based on ⁹⁸MoN, using a Hund's case(a) approach for both states. Although the constrained approach worked well for spectral analysis, several parameters needed to be fit individually for each isotopologue.

The complex electronic structure of transition metal diatomic molecules, such as tungsten monoxide (WO), makes them intriguing targets for high level spectroscopic analysis. A plethora of electrons and accessible valence orbitals make WO a difficult molecule to model computationally due to the large number of possible electronic interactions. The (0,0) and (0,1) vibrational bands of the A1 - X0⁺ and B1 - X0⁺ transitions of WO were recorded in absorption at Doppler-limited resolution using intracavity laser spectroscopy integrated with a Fourier-transform spectrometer [for the (0,1) band] and a 1-m monochromator with CCD detector [for the (0,0) band]. The target WO molecules were produced in the plasma discharge of a tungsten-lined copper hollow cathode, using a gas mixture of approximately 70% Ar and 30% H₂ giving a reaction chamber pressure of about 1 torr total. All 4 stable WO isotopologues were observed and analyzed. Initial fits using mass-constained parameters based on the most abundant isotope were not successful, and hint at strong interactions between the two excited electronic states.

Electronic spectra of gas-phase polyatomic actinide species such as UO₂ Han, J.; Goncharov, V.; Kaledin, L. A.; Komissarov, A. V.; Heaven, M. C., Electronic spectroscopy and ionization potential of UO₂ in the gas phase. Journal of Chemical Physics 2004, 120, 5155-5163.nd UF₂ have yielded electronic spectra where the rotational structure cannot be resolved at a resolution of 0.06 cm⁻¹. The molecules are predicted to be linear, so the lack of resolvable rotational structure appears to be a consequence of high electronic state densities. To probe this notion we have examined OThF, where the electronic complexity is reduced as there is only one unpaired electron in the metal-centered orbitals. Gas phase OThF has been generated by laser ablating Th metal in the presence of trace amounts of SF₆ and O₂. Six prominent vibronic bands were observed in the range 27700-28900 cm⁻¹. These bands were detected using both REMPI and LIF techniques. An additional intense band was observed at 22430 cm⁻¹. TD-DFT calculations indicate that OThF is a bent molecule in the ground and low-lying electronically excited states. Attempts to obtain rotationally resolved data and computational studies of OThF are in progress.

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Footnotes:

Han, J.; Goncharov, V.; Kaledin, L. A.; Komissarov, A. V.; Heaven, M. C., Electronic spectroscopy and ionization potential of UO₂ in the gas phase. Journal of Chemical Physics 2004, 120, 5155-5163.a

High-level ab initio approaches are often necessary in detailing the many excited states and spin multiplicities of lanthanide and actinide species, while offering reliable routes towards calculating accurate spectroscopic properties. In this work, multi-reference methodologies including the complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI) methods were utilized to calculate the ground and excited state properties of lanthanide and actinide oxides and fluorides. Potential energy curves for the ground state, several excited states, and different dissociation channels were determined. Spin-orbit corrections were performed by diagonalizing the MRCI wavefunction on the basis of the Breit-Pauli Hamiltonian. Bond dissociation energies (BDEs) of the lanthanide and actinide oxides and fluorides species were also determined, and the impact of core-valence, relativistic, and spin-orbit contributions to the ground state were considered. Density functional theory (DFT) and wavefunction method predictions were compared.

Cyanide-containing d⁶ transition metal complexes are known to exhibit strong solvatochromism of their MLCT band, but also a solvent-lifetime dependence. These effects are understood to be modulated by second-shell donor-acceptor interactions at the cyanide nitrogen, the strength of which are dictated by solvent acceptor number, a measure of relative Lewis acidity. These modulations can be understood as altering the σ-donor π-acceptor character of the cyanide ligands. A recently reported long-lived Fe^II complex, Fe(HMTI)(CN)₂, exhibits a biphasic solvent-lifetime relationship, with a rising and falling edge as acceptor number increases, making it difficult to pin down the nature of the relationship. In this work, a variant of the Fe(HMTI)(CN)₂, Fe(TIM)(CN)₂ (TIM=2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) and its bound lewis acid counterpart, Fe(TIM)(CNBPh₃)₂, are investigated. Fe(TIM)(CN)₂ exhibits a largely monophasic solvent-lifetime relationship, with lifetime falling nearly exponentially as solvent acceptor number rises. This relationship is probed more closely by UV-Vis transient absorption experiments, and DFT calculations, to elucidate the effects of modulating σ-donor π-acceptor character of ligands bound to Fe^II chromophores.

Intracavity Laser Absorption Spectroscopy (ICAS) provides an excellent solution for ultrasensitive, multiplexed, quantitative detection of reactive species, but has traditionally been limited to the visible and infrared spectral regions by the requirement for direct broadband lasing media. We report the first realization of this technique in the ultraviolet (UV-ICAS), based on a home-built Ce:LiCAF laser that operates in the 280-316 nm range. Three key species were investigated using our prototype UV-ICAS spectrometer: formaldehyde, sulfur dioxide, and hydroxyl radical. Successful initial measurements of static gases (the formaldehyde à¹A₂−X̃ ¹A₁ electronic transition and the sulfur dioxide X̃ ¹B₂−B̃ ¹A₁ electronic transition) led us to record in situ the A ²Σ⁺−X ²Π spectrum of hydroxyl radical in a butane flame. Comparison of the latter to a LIFBASE simulation allowed single shot extraction of the temperature of hydroxyl radicals in the flame, demonstrating the data acquisition efficiency of UV-ICAS. We will also discuss the technique’s potential for novel ultrasensitive, high resolution, broadband spectroscopy.

Indigo is a highly photostable molecule used in many ancient civilizations such as the Roman and Mayan empires. This brilliant and deep color survived for centuries in many pieces of art. Its photostability is explained by its excited state dynamics. In a previous study, we measured two decay channels upon excitation of the S1 and we concluded that this peculiar molecule undergoes an excited state hydrogen transfer (ESHT) and an excited state proton transfer (ESPT). In our recent work, we studied this de-excitation through selective deuteration of the two target transfer sites on indigo. We monitored the photo-reaction pathways of this molecule using Resonance Enhanced Multi-Photon Ionization (REMPI) coupled with a time-of-flight mass spectrometer and measured the lifetimes of the excited states using pump-probe spectroscopy with mass spectrometric isotopomer selection. We revealed that the slower decay was unaffected by deuteration. The calculated excited state potential energy surfaces show trajectories with a different pathway to the ground state: a sequential double proton or hydrogen transfer.

Diatomic molecules are fundamental in shaping our intuitive understanding of electronic structure theory. Despite their structural simplicity, chemical binding mechanisms (i.e., from R_e to dissociation) are well understood only for a few lowest energy electronic states, even for the C/N/O diatomic species. Pervasive configuration interactions and lumpy adiabatic potentials are prominent features of most electronically excited states. Interpretation of energy level structure and spectroscopic patterns becomes increasingly challenging for these higher energy states, because of the reduced utility of simple concepts such as bond order and correlation diagrams. In this work, a global diabatization scheme, based on the valence-hole concept, is used to model the extensive web of avoided-crossing patterns in the electronically excited states of the CN (²Σ), N₂ (¹Π_u, ³Π_u), Si₂ (³Π_g), and SiC (³Π) molecules. The validity of this diabatization scheme will be further demonstrated by its ability to reproduce the unusual energy level structure and predissociation dynamics of the extensively studied CN ²Σ and N₂ ³Π_u electronic states. As we have previously demonstrated with C₂ (¹Π_g, ³Π_g, ¹Σ_u⁺, ³Σ_u⁺), the key concept of the model is the existence of valence-hole configurations that derive from an electron promotion from the nominally anti-bonding 2σ_u molecular orbital, i.e., 3σ_g(5σ)← 2σ_u (4σ) for C₂, Si₂, CN, and SiC, and 1π_g ← 2σ_u for N₂. These valence-hole configurations have a nominal bond order of 3 or higher and correlate with separated-atom configurations with a 2p←2s promotion in one of the atomic constituents. The strongly-bound diabatic valence-hole state crosses multiple weakly-bound or repulsive states that are composed of electron configurations with a 2σ_g²2σ_u² valence-core. These curve-crossings of diabatic potential curves result in an interconnected network of many avoided-crossings among multiple electronic states. Considering their systematic, disruptive impact on the global electronic structure and unimolecular dynamics, the valence-hole states should be treated as an integral part of our intuitive electronic structure model, along with familiar concepts such as Rydberg and ionic states.