Low-lying Electronic States Research Articles

OCS2 Isomers in the Venusian Atmospheric Chemistry: Spectroscopic Characterization and Photochemistry

The atmosphere of Venus exhibits absorption in the 300–500 nm wavelength range, which is driven by unknown chemical processes. In our study, we explore electronic transitions in molecules that may exist in the Venusian atmosphere, specifically focusing on the photoabsorption cross sections and the lowest singlet and triplet electronic states of the OCS2, SSCO, and OSCS isomers using highly accurate ab initio methods. Our analysis suggests that the SSCO isomer is a strong candidate for explaining the unknown UV absorption. Furthermore, these isomers may serve as significant astrochemical reservoirs in the atmosphere of Venus, where photodissociation could produce atomic sulfur in both its ground and excited states along with OCS and CS2, offering a plausible mechanism for the sulfur cycle dynamics and the formation of S x species. This study provides valuable insights into the complex sulfur chemistry within the atmosphere of Venus.

Comment on “Relativistic spinless energies and thermodynamic properties of sodium dimer molecule”

In this Comment we point out two incorrect results in a paper published recently in this journal. In the first place, the fact that the maximum number of vibrational energy levels is not a variable parameter of the model because it is determined by the potential-energy function and, in particular, by the dissociation energy. In the second place, we argue that the vibrational thermodynamic functions for an excited electronic state of a diatomic molecule are of no physical utility because any physical application requires the more relevant contribution of the lower electronic states to the canonical partition function. To illustrate this point we show the calculation of the equilibrium constant for the dimerization of sodium using only spectroscopic information about the ground electronic state. The theoretical expression agrees remarkably well with the available experimental data.

Electronic structure, spectroscopic constants, and transition properties of NaC [formula omitted] diatomic species: An ab initio investigation

Diatomic species play a pivotal role in complex media, such as the interstellar and circumstellar media. Furthermore, there is a notable lack of information regarding NaC and its ions, NaC+ and NaC−, despite the pervasiveness of the carbon element (C) and the validated role of Na-bearing molecules as organic reagents or in catalytic processes. In this study, ab initio multi-reference calculations, including core valence and complete basis set schemes, were employed to accurately determine the potential energy curves (PECs), dipole moment functions (DMFs) and transition dipole moments (TDMs) of ten, three and ten low-lying electronic states energies, respectively, for NaC, NaC+ and NaC− diatomics species. The obtained results permitted the accurate determination of spectroscopic constants and vibrational transition properties, including vibrational level energies, Franck–Condon factors (FCFs), Einstein coefficients, absorption band oscillator strengths, vibrational transition energies and radiative lifetimes of allowed transitions. The predicted adiabatic ionisation energy value for NaC is 6.659 eV, while the vertical ionisation energy value is 6.676 eV. Similarly, the predicted adiabatic electron affinity value for NaC is 0.614 eV, while the vertical electron affinity value is 0.613 eV.

The characterization of electronic structures and spin-orbit couplings in the diatomic sodium arsenide cation

A high-level ab initio computation has been performed in order to determine the molecular structures, electronic characters and transition properties of a hitherto unknown diatomic cation of NaAs+. The electronic states associated with five dissociation channels, including both the Na++As and Na+As+ limits, are subjected to detailed analysis. The results of the potential energy functions, spectroscopic constants and vibrational energy levels of the relevant states are presented herewith. The impact of the spin-orbit coupling effect on the low-lying electronic states is evaluated as well. Additionally, predictions of transition features, such as Einstein coefficients, Frank-Condon factors, and radiative lifetimes, are considered. It is concluded that the direct laser cooling of NaAs+ is infeasible.

Autodetachment of Diatomic Carbon Anions from Long-Lived High-Rotation Quartet States.

We show that strong molecular rotation drastically modifies the autodetachment of C_{2}^{-} ions in the lowest quartet electronic state a^{4}Σ_{u}^{+}. In the strong-rotation regime, levels of this state only decay by a process termed "rotationally assisted" autodetachment, whose theoretical description is worked out based on the nonlocal resonance model. For autodetachment linked with the exchange of six rotational quanta, the results reproduce a prominent, hitherto unexplained electron emission signal with a mean decay time near 3ms, observed on stored C_{2}^{-} ions from a hot ion source.

Unimolecular processes in diatomic carbon anions at high rotational excitation

On the millisecond to second time scale, stored beams of diatomic carbon anions C2− from a sputter ion source feature unimolecular decay of yet unexplained origin by electron emission and fragmentation. To account for the magnitude and time dependence of the experimental rates, levels with high rotational and vibrational excitation are modeled for the lowest electronic states of C2−, also including the lowest quartet potential. Energies, spontaneous radiative decay rates (including spin-forbidden quartet-level decay), and tunneling dissociation rates are determined for a large number of highly excited C2− levels and their population in sputter-type ion sources is considered. For the quartet levels, the stability against autodetachment is addressed and recently calculated rates of rotationally assisted autodetachment are applied. Nonadiabatic vibrational autodetachment rates of high vibrational levels in the doublet C2− ground potential are also calculated. The results are combined to model the experimental unimolecular decay signals. Comparison of the modeled to the experimental rates measured at the cryogenic storage ring facility CSR gives strong evidence that C2− ions in quasistable levels of the quartet electronic states are the so far unidentified source of unimolecular decay. Published by the American Physical Society 2024

Theoretical spin-orbit laser cooling for AlZn molecule.

A spin-orbit coupling electronic structure study of the AlZn molecule is conducted to investigate the molecular properties of the low-lying electronic states and their feasibility toward direct laser cooling. This study uses the complete active-space self-consistent field level of theory, followed by the multireference configuration interaction method with Davidson correction (+Q). The potential energy and dipole moment curves and the spectroscopic constants are computed for the low-lying doublet and quartet electronic states in the 2S+1Λ± and Ω(±) representations. The transition dipole moments, the Franck-Condon factors, the Einstein coefficient, the radiative lifetimes, the vibrational branching ratio, and the slowing distance are determined between the lowest spin-orbit bound electronic states. These results show that the molecule AlZn has a high potential for laser cooling through the X2Π1/2 → (2)2Π1/2 transition by utilizing four lasers at a wavelength in the ultraviolet region, reaching a sub-microkelvin temperature limit.

Vibronic coupling in the energetically six lowest electronic states of oxirane radical cation.

Multi-dimensional quantum mechanical simulations are carried out to understand the multi-state and multi-mode vibronic interactions in the first six low-lying viz., X̃2B1, Ã2A1, B̃2B2, C̃2A2, D̃2A1, and Ẽ2B1 electronic states of c-C2H4O·+. Vibronic coupling theory is applied to study interactions among electronic states using symmetry selection rules. A model 6 × 6 diabatic electronic Hamiltonian is constructed. The parameters of the diabatic Hamiltonian are estimated by performing extensive abinitio electronic structure calculations, using the EOM-IP-CCSD method. The nuclear dynamics calculations are performed with both time-independent and time-dependent quantum mechanical methods. The calculated vibronic structures of six electronic states are found to be in excellent agreement with the available experimental findings. Progressions found in the theoretical spectrum are assigned in terms of vibrational modes. It is found that extremely strong vibronic interactions among the X̃2B1-Ã2A1, B̃2B2-C̃2A2, and D̃2A1-Ẽ2B1 electronic states results into highly overlapping vibronic bands due to multiple multi-state conical intersections. The impact of associated nonadiabatic effects on the vibronic structure and dynamics of the mentioned electronic states is examined at length. Interesting comparison is made with the results obtained for the isomeric acetaldehyde radical cation.

Topological photon pumping in quantum optical systems

We establish the concept of topological pumping in one-dimensional systems with long-range couplings and apply it to the transport of a photon in quantum optical systems. In our theoretical investigation, we introduce an extended version of the Rice-Mele model with all-to-all couplings. By analyzing its properties, we identify the general conditions for topological pumping and theoretically and numerically demonstrate topologically protected and dispersionless transport of a photon on a one-dimensional emitter chain. As concrete examples, we investigate three different popular quantum optics platforms, namely Ryd-berg atom lattices, dense lattices of atoms excited to low-lying electronic states, and atoms coupled to waveguides, using experimentally relevant parameters. We observe that despite the long-ranged character of the dipole-dipole interactions, topological pumping facilitates the transport of a photon with a fidelity per cycle which can reach 99.9%. Moreover, we find that the photon pumping process remains topologically protected against local disorder in the coupling parameters.

MRCI+Q calculations on electronic structure and spectroscopy of low-lying electronic states of silicon monobromide including spin-orbit coupling effect

The electronic structures of silicon monobromide (SiBr) correlating with the lowest three dissociation channels are studied using high-level configuration interaction method. The spin-orbit coupling (SOC) effect and core-valence (CV) correlations effect are taken into account to improve the accuracy of electronic structures. Based on the calculated electronic structures of the lowest three dissociation channels of SiBr, the spectroscopic constants of quasibound and bound electronic states are fitted, which are coincided with the results of experiment. The dipole moment curves (DMs) of the lowest three dissociation channels of SiBr are obtained, and the abrupt change of DMs nearby the avoided crossing point are explained by the variation of electronic configurations of the corresponding states. With the help of the calculated SOC matrix elements, the predissociation channels of the low-lying vibrational states of 2Δ(Ⅱ) and 2Π(Ⅲ) sates are analyzed. The complicated interaction between crossing states is investigated. The ν'≥0 vibrational states of 2Δ(Ⅱ) and ν′≥2 vibrational states of 2Π(Ⅲ) would predissociate rapidly through predissociation channels of 2Δ(Ⅱ)-2Π(Ⅱ) and 2Π(Ⅲ)-2Σ+(Ⅱ). Finally, the transition properties of A2Σ+-X2Π, 2∆(Ⅱ)-X2Π, 2Σ+(Ⅱ)-X2Π, 2Π(Ⅲ)-X2Π, 1/2(Ⅱ)-X2Π1/2, 1/2(Ⅲ)-X2Π1/2 and 3/2(Ⅱ)-X2Π1/2 transitions are investigated, and radiative lifetime of bound states are evaluated.

An energy-modified quantum defect method for the analysis of Rydberg spectra: Application to 2-butyne.

The high resolution Rydberg absorption spectrum of 2-butyne C4H6 recorded previously at the SOLEIL synchrotron facility has been interpreted using multichannel quantum defect theory (MQDT). The calculations are based on the continuum scattering calculations of Xu et al., J. Chem. Phys. 136, 154303 (2012) and of Jacovella et al., J. Phys. Chem. A 119, 12339 (2015) pertaining to the dipole-allowed excited state symmetries in absorption from the ground state. In contrast to the traditional approach of calculating low-lying electronic states first and then attempting to extend the calculations to ever higher energy, here the analysis proceeds through the extension of these detailed calculations of the electronic continuum scattering down into the discrete region of the spectrum. The continuum reaction matrices and dipole transition moments are adapted to the discrete Rydberg region via the use of an energy-modified formulation of MQDT theory and associated energy dependences of the quantum defects. The analysis reproduces more than 40 Rydberg states from n ≈ 10 down to the 3d and 4s levels with an rms error of better than 20 cm-1. These belong to five Rydberg series with three different molecular symmetries. While the approach predicts many additional series, most of these are calculated and observed to carry only little oscillator strength. The analysis shows that the Rydberg spectrum is dominated by the excitation of an e″ symmetry electron of fδ and gπ type, in line with what previous studies of the above-threshold shape resonance of 2-butyne have shown. The present study is intended to serve as an example showing how first principles continuum calculations may be useful for the interpretation of highly bound discrete states in a range that poses problems for the standard abinitio techniques. The quantitative treatment of the dipole absorption cross sections is deferred to a future paper.

Photoionization of aziridine: Nonadiabatic dynamics of the first six low-lying electronic states of the aziridine radical cation.

In this article, the theoretical photoionization spectroscopy of the aziridine (C2H5N) molecule is investigated. To start with, we have optimized the geometry of this molecule at the neutral electronic ground state at the density functional theory/augmented correlation-consistent polarized valence triple zeta level of theory using the G09 program. The electronic structure calculations were restricted to the first six low-lying electronic states in order to account for the experimental photoelectron spectrum of the C2H5N molecule. The first six low-lying electronic states (X̃2A', Ã2A', B̃2A″, C̃2A″, D̃2A', and Ẽ2A') of the potential energy surfaces (PESs) are calculated by both equation of motion-ionization potential-coupled cluster singles and doubles and multi-configuration quasi-degenerate perturbation theory abinitio quantum chemistry methods along the dimensionless normal displacement coordinates in which multiple conical intersections were established among the considered electronic states. A (6 × 6) model vibronic Hamiltonian is constructed on a diabatic electronic basis, using the symmetry selection rules and Taylor series expansion. The Cs symmetry point group of the aziridine molecule leads to electronic states symmetry of either A' or A″, and these states are close in energy, due to which the same symmetry electronic states avoid each other. To get a smooth diabatic PES, a fourfold diabatization scheme is used, which is implemented in the General Atomic and Molecular Electronic Structure Systems suite of programs. All the parameters used in the diabatic vibronic coupling model Hamiltonian are calculated in terms of the normal modes of vibrational coordinates. Finally, the vibronic model Hamiltonian constructed for the coupled six electronic states is used to solve both time-independent and time-dependent Schrödinger equations using the multi-configuration time-dependent Hartree program module to obtain the dynamical observables. The theoretical vibronic band structure is found to be in good accord with the available experimental results.

Molecular structure and spectra of ytterbium dihalides from first principles

The molecular structure, vibrational frequencies, infrared intensities and electric dipole moments of ytterbium dihalides YbX2 (X = F, Cl, Br, I) in their ground electronic state are studied at the coupled-cluster singles, doubles and perturbative triples CCSD(T) level using a series of large all-electron basis sets together with the complete basis set (CBS) extrapolation procedure, and with both scalar-relativistic and spin–orbit coupling effects taken into account. Excitation energies of low-lying electronic states are calculated at the multireference configuration interaction, equation-of-motion CCSD and single-reference CCSD(T) levels of theory. For all of the molecules under study, the CCSD(T)/CBS equilibrium geometries are found to be non-linear (C2v symmetry). There is a rapid decrease in heights of the barriers to linearity on passing through the YbX2 molecular series from fluoride to iodide: 1587 → 261 → 110 → 7 (all in cm−1) for YbF2 → YbCl2 → YbBr2 → YbI2, respectively. The uncertainties in the molecular structure data published to date are discussed and resolved in the light of the present results.

Al2O Photochemistry

The chemistry within the interstellar medium (ISM) is notably influenced by the interplay between kinetics and photochemical processes, which play significant roles in both the formation and destruction of molecular species. This study focuses on theoretical investigations of Al2O photochemistry, aiming to elucidate the mechanisms underlying the production of AlO and Al in the VY-CMa star. Utilizing advanced theoretical methodologies, we explore the lowest electronic states with singlet and triplet spin multiplicities in linear Al2O. We investigated the photostability of Al2O in the near UV‒Vis region, revealing the low likelihood of photodissociation and photoconversion while suggesting the plausibility of fluorescence and phosphorescence phenomena. Calculations also identify three prominent peaks in the UV range at 261.5, 206.2, and 199 nm. Finally, Al2O is predicted to be photostable and cannot be the parent molecule of the diatomic AlO or even the astrochemical reservoir of atomic aluminum. These results contribute to improving the astronomical models in simulating aluminum chemistry in the ISM.

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells.

Molecular oxygen, O2, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O2(X3Σg-), has garnered much attention, the lowest excited electronic state, O2(a1Δg), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O2(a1Δg) can be produced and deactivated in processes that involve light, the photophysics of O2(a1Δg) are equally important. Moreover, pathways for O2(a1Δg) deactivation that regenerate O2(X3Σg-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O2(a1Δg) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O2(a1Δg) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O2(a1Δg). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M+•O2-• charge-transfer state in both the formation and deactivation of O2(a1Δg).

Theoretical investigation of the electronic structure of the Rhodium Halides molecules RhF and RhCl with dipole moment calculation

The current study involves an ab initio exploration of the ground and low-lying excited electronic states of the rhodium halide molecules RhF and RhCl using the complete active space self-consistent field (CASSCF) with multireference configuration interaction (MRCI+Q) method including single and double excitations and with Davidson corrections. We investigated the potential energy curves, the transition and permanent electric dipole moments, the electronic energy relative to the ground state Te, the harmonic frequency ωe, the internuclear distance Re, and the rotational constant Be corresponding to each of the bounded states. Our findings demonstrate good agreement with the available experimental data. Notably, this work represents the inaugural theoretical investigation of the excited states of RhF and RhCl molecules, identifying the ground state of both to be X3Π, as observed in the sole two experimental investigations.

Electronic structure and spectroscopic properties of some low-lying electronic states of neutral and ionic species CN and CN−

ABSTRACT The present work provides Potential Energy Curves (PECs) of both cyano radical CN and cyanide anion CN − up to the third asymptote of dissociation out of ab initio calculations. The [MRCI+Q] method is the level of theory used here and we have associated it with the Dunning basis set AV6Z (Augmented correlation-consistent polarised Valence sextet Zeta) for geometry optimisation of our systems. Electronic configurations of the states of the two species were explored and based on these configurations and the relative positions of the PECs of CN − against those of the neutral radical CN, stable and metastable states have been found and exhibited. The calculated electron affinity here that proves itself to be positive and the position of the X 1 Σ + state against its corresponding neutral parent state ( X 2 Σ + ) from which it derives led to confirm that this state is a long-lived and stable ground state of the cyanide anion CN − . Spectroscopic constants of both ground and low-lying excited states are calculated and exhibited in this work.

Ab initio exploration of low-lying electronic states of linear and bent MNX+ (M = Ca, Sr, Ba, Ra; X = O, S, Se, Te, Po) and their origins.

High-level multireference and coupled cluster quantum calculations were employed to analyze low-lying electronic states of linear-MNX+ and side-bonded-M[NX]+ (M = Ca, Sr, Ba, Ra; X = O, S, Se, Te, Po) species. Their full potential energy curves (PECs), dissociation energies (Des), geometric parameters, excitation energies (Tes), and harmonic vibrational frequencies (ωes) are reported. The first three chemically bound electronic states of MNX+ and M[NX]+ are 3∑-, 1Δ, 1∑+ and 3A″, 1A', 1A″, respectively. The 3∑-, 1Δ, 1∑+ of MNX+ originate from the M+(2D) + NX(2Π) fragments, whereas the 3A″, 1A', 1A″ states of M[NX]+ dissociate to M+(2S) + NX(2Π) as a result of avoided crossings. The MNX+ and M[NX]+ are real minima on the potential energy surface and their interconversions are possible. The M2+NX-/M2+[NX]- ionic structure is an accurate representation for their low-lying electronic states. The Des of MNX+ species were found to depend on the dipole moment (μ) of the corresponding NX ligands and a linear relationship between these two parameters was observed.

An Exhaustive Quantum-Classical Study of C6F6+ Using the Newly Formulated Parallel TDDVR Method.

We recently implemented our parallelized quantum-classical dynamical approach, known as the Time-Dependent Discrete Variable Representation (TDDVR) method, which is applied to the spectroscopically important hexafluorobenzene (HFBz) radical cation, where several conical intersections exist in their seven lowest excited electronic states (S11B2u, S21E1g, S31B1u, S41E1u, and S51A2u) considering degeneracy among potential energy surfaces (PESs), to demonstrate their various dynamical aspects. This new parallel version shows almost linear scalability with increasing number of computing processors. To get photoelectron (PE) spectra, Mass-Analyzed Threshold Ionization (MATI) spectra, population dynamics, and many other dynamical observables, the first-principles dynamics is applied at the state-of-the-art level to the corresponding Hamiltonian, where the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) type interactions are involved in those coupled seven electronic states. The quantum-classical method is used to thoroughly analyze the effects of these couplings on the nuclear dynamics of the involved electronic states, and the findings are compared with those observables obtained from experiments. Intrinsic dynamical properties are explained using the reduced densities of the wave packet (WP) in a coupled electronic manifold. The PE and MATI spectra of HFBz computed using TDDVR are found to be in good agreement with earlier experimental data and other theoretically simulated spectra.

Approximate Functionals for Multistate Density Functional Theory.

Recently Lu and Gao [J. Phys. Chem. Lett. 2022, 13 (33), 7762-7769] published a new, rigorous density functional theory for excited states and proved that the projection of the kinetic and electron-repulsion operators into the subspace of the lowest electronic states are a universal functional of the matrix density D(r). This is the first attempt to find an approximation to the multistate universal functional . It is shown that (i) does not explicitly depend on the number of electronic states and (ii) is an analytic matrix functional. The Thomas-Fermi-Dirac-von Weizsäcker model and the correlation energy of the homogeneous electron gas are turned into matrix functionals guided by two principles: that each matrix functional should transform properly under basis set transformations and that the ground state functional should be recovered for a single electronic state. Lieb-Oxford-like bounds on the average kinetic and electron-repulsion energies in the subspace are given. When evaluated on the numerically exact matrix density of LiF, this simple approximation reproduces the matrix elements of the electron-repulsion operator in the basis of the exact eigenstates accurately for all bond lengths. In particular the off-diagonal elements of the effective Hamiltonian that come from the interactions of different electronic states can be calculated with the same or better accuracy than the diagonal elements. Unsurprisingly, the largest error comes from the kinetic energy functional. More exact conditions that constrain the functional form of are needed to go beyond the local density approximation.