Neutral Ground State Research Articles

Molecular Association and Reactivity of the Pyridine Dimer Cation.

A recent experimental report has identified the formation of the C-N hemibonded pyridine dimer cation following vacuum ultraviolet near-threshold photoionization [J. Phys. Chem. Lett., 2021, 12, 4936-4943]. Herein, the dynamics and consequent reactivity of the pyridine dimer cation were investigated employing Born-Oppenheimer molecular dynamics (BOMD) simulations. An antiparallel π-stacked pyridine dimer in the neutral ground state is transformed into a noncovalently interacting C-H···N hydrogen-bonded structure which can lead to proton transfer in the cationic state. Additionally, C-N- and N-N-bonded adducts were formed in the cationic state. Further, metastable C-H···H-C-bonded cationic species was observed, which rearranged to an N-N bonded adduct. In contrast to the experimental observation, migration of the proton to the α position was not observed in the C-N bonded adduct owing to a high barrier of about 2 eV. The observed trends in the molecular association, proton transfer, and the formation of C-N and N-N bonded adducts are a consequence of the roaming dynamics of one pyridine moiety over the other in the cationic state.

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.

Extended N-centered ensemble density functional theory of double electronic excitations.

A recent work (arXiv:2401.04685) has merged N-centered ensembles of neutral and charged electronic ground states with ensembles of neutral ground and excited states, thus providing a general and in-principle exact (so-called extended N-centered) ensemble density functional theory of neutral and charged electronic excitations. This formalism made it possible to revisit the concept of density-functional derivative discontinuity, in the particular case of single excitations from the highest occupied Kohn-Sham (KS) molecular orbital, without invoking the usual "asymptotic behavior of the density" argument. In this work, we address a broader class of excitations, with a particular focus on double excitations. An exact implementation of the theory is presented for the two-electron Hubbard dimer model. A thorough comparison of the true physical ground- and excited-state electronic structures with that of the fictitious ensemble density-functional KS system is also presented. Depending on the choice of the density-functional ensemble as well as the asymmetry of the dimer and the correlation strength, an inversion of states can be observed. In some other cases, the strong mixture of KS states within the true physical system makes the assignment "single excitation" or "double excitation" irrelevant.

High-Resolution Photoelectron Imaging of Cryogenically Cooled BiB2- and BiB3- Bismuth-Boron Clusters.

We report a high-resolution photoelectron imaging study of cryogenically cooled BiB2- and BiB3- clusters. Vibrational features are completely resolved for the ground-state detachment transitions, providing critical information about the structures of the anionic clusters and their corresponding neutrals. The electron affinities of BiB2 and BiB3 are accurately measured to be 2.174(1) and 2.121(1) eV, respectively. The B-B and Bi-B stretching frequencies are measured to be 1262 and 476 cm-1, respectively, in the ground state of BiB2. Three vibrational frequencies are measured for the ground state of BiB3: 1194 cm-1 (B-B stretching), 782 cm-1 (B-B stretching), and 339 cm-1 (Bi-B stretching). Both BiB2- and BiB3- and their neutral ground states are found to have planar C2v structures in which the Bi atom bridges two B atoms. BiB2- is found to have a triplet spin state (3B2), consistent with its complicated photoelectron spectra, whereas BiB3- is a doublet (2B1) and neutral BiB3 is closed shell (1A1). Both BiB2 and BiB3 consist of peripheral localized Bi-B and B-B σ bonds and delocalized π and σ bonds.

Impact of Cavity on Molecular Ionization Spectra.

Ionization phenomena have been widely studied for decades. With the advent of cavity technology, the question arises how quantum light affects molecular ionization. As the ionization spectrum is recorded from the neutral ground state, it is usually possible to choose cavities which exert negligible effect on the neutral ground state, but have significant impact on the ion and the ionization spectrum. Particularly interesting are cases where the ion exhibits conical intersections between close-lying electronic states, which gives rise to substantial nonadiabatic effects. Assuming single-molecule strong coupling, we demonstrate that vibrational modes irrelevant in the absence of a cavity play a decisive role when the molecule is in the cavity. Here, dynamical symmetry breaking is responsible for the ion-cavity coupling and high symmetry enables control of the coupling via molecular orientation relative to the cavity field polarization. Significant impact on the spectrum by the cavity is found and shown to even substantially increase for less symmetric molecules.

Unveiling geometric and electronic structures of NbSi4−/0 clusters and electron detachments of the anionic cluster: A quantum chemical investigation

AbstractThis article presents computational insights into the geometric and electronic structures of NbSi4−/0 clusters using density functional theory and the CASSCF/CASPT2 method. The anionic and neutral ground states are identified as the 1A′ and 2A′ states, respectively, within a trigonal bipyramidal isomer where the Nb atom occupies the equatorial position. The adiabatic detachment energy for the transition from the anionic ground state 1A′ to the neutral ground state 2A′ is estimated to be 2.30 eV. Additionally, an evaluation of the vertical detachment energies for transitions to the neutral states 12A′, 12A″, 22A′, 22A″, 32A′, 32A″, and 42A′ yields respective values of 2.42, 2.78, 2.94, 3.34, 3.86, 4.08, and 4.28 eV. These computed electron detachment energies successfully account for all five bands observed in the photoelectron spectrum of the anionic cluster. Furthermore, the Franck–Condon factor simulations reveal extensive vibrational progressions associated with the transition to the neutral ground state 12A′, manifesting as an unresolved broad band at 2.41 eV in the spectrum.

The overlooked role of excited anion states in NiO2- photodetachment.

Photodetachment spectra of anionic species provide significant insights into the energies and nature of ground and excited states of both the anion and resultant neutral molecules. Direct detachment of the excess electron to the continuum may occur via formally allowed or forbidden transitions (perhaps as the result of intensity borrowing through vibronic coupling). However, alternate indirect pathways are also possible and often overlooked. Here, we report a two-dimensional photoelectron spectral study, combined with correlated electronic structure calculations, to elucidate the nature of photodetachment from NiO2-. The spectra are comprised of allowed and forbidden transitions, in excellent agreement with previously reported slow electron velocity mapped imaging spectra of the same system, which were interpreted in terms of direct detachment. In the current work, the contributions of indirect processes are revealed. Measured oscillations in the branching ratios of the spectral channels clearly indicate non-direct detachment processes, and the electronic structure calculations suggest that excited states of the appropriate symmetry and degeneracy lie slightly above the neutral ground state. Taken together, the results suggest that the origin of the observed forbidden transitions is the result of anion excited states mediating the electron detachment process.

Excited dipole bound electronic states of potassium iodide anions: A theoretical perspective

The information about electronic excited states of molecular anions is of pivotal importance for understanding electron attachment/detachment processes. Here, we present a high-level theoretical study on electronic states of potassium iodide anions (KI−). By the evaluation of different basis sets, we present accurate spectroscopic constants of the anionic ground electronic state using the multireference configuration interaction with Davidson correction method. The equation-of-motion electron-attachment coupled-cluster singles and doubles method is carried out to calculate electron binding energies (EBEs) of electronic states. With the addition of different s-/p-/d-diffusion functions in the basis set, we have identified possible excited dipole bound states (DBSs) of KI−. The results indicate that, owing to the large dipole moment of KI molecules, the anions can hold three excited DBSs, i.e., two σ-type DBSs and one π-type DBS, with the EBEs of 39 meV (σ-DBS1), 4.7 meV (π-DBS), and only 1.8 meV (σ-DBS2) below the neutral ground state. Molecular orbitals, potential energy curves, and spectroscopic constants of DBSs are presented. Our study would shed some light on the electronic states of potassium iodide molecular anions.

Quantum simulation of extended electron-phonon-coupling models in a hybrid Rydberg atom setup

State-of-the-art experiments using Rydberg atoms can now operate with large numbers of trapped particles with tunable geometry and long coherence time. We propose a way to utilize this in a hybrid setup involving neutral ground-state atoms to efficiently simulate condensed-matter models featuring electron-phonon coupling. Such implementation should allow for controlling the coupling strength and range as well as the band structure of both the phonons and atoms, paving the way towards studying both static and dynamic properties of extended Hubbard-Holstein models.

A DMRG-CASPT2 investigation on the electronic states of NiSin−/0/+ (n = 1–3) clusters

The electronic states of NiSin−/0/+ (n = 1–3) clusters were studied using the DFT, RASPT2, and DMRG-CASPT2 methods. The DMRG-CASPT2 approach increases the complete active space to 18, 20, and 23 orbitals for NiSin−/0/+ (n = 1–3) clusters to attain accurate energies. The leading configurations, bond distances, vibrational frequencies, and relative energies of the electronic states of NiSin−/0/+ (n = 1–3) clusters are presented. The electron detachment energies of the anionic and neutral clusters are determined. The simulation of the Franck-Condon factors for the transition from the anionic to the neutral ground state within the NiSi3−/0 clusters reveals intense vibrational progressions.

Accuracy of using metastable state measurements in laser-induced fluorescence diagnostics of xenon ion velocity in Hall thrusters

Laser-induced fluorescence measurements of singly-charged xenon ion velocities in Hall thrusters typically target metastable states due to lack of available laser technology for exciting the ground state. The measured velocity distribution of these metastable ions are assumed to reflect the ground state ion behavior. However, this assumption has not been experimentally verified. To investigate the accuracy of this assumption, a recently developed xenon ion (Xe II) collisional-radiative model is combined with a 1D fluid model for ions, using plasma parameters from higher fidelity simulations of each thruster, to calculate the metastable and ground state ion velocities as a function of position along the channel centerline. For the HERMeS and SPT-100 thruster channel centerlines, differences up to 0.5 km s−1 were observed between the metastable and ground state ion velocities. For the HERMeS thruster, the difference between the metastable and ground state velocities is less than 150 m s−1 within one channel length of the channel exit, but increases thereafter due to charge exchange (CEX) that reduces the mean velocity of the ground state ions. While both the ground state ions and metastable state ions experience the same acceleration by the electric field, these small velocity differences arise because ionization and CEX directly into these states from the slower neutral ground state can reduce their mean velocities by different amounts. Therefore, the velocity discrepancy may be larger for thrusters with lower propellant utilization efficiency and higher neutral density. For example, differences up to 1.7 km s−1 were calculated on the HET-P70 thruster channel centerline. Note that although the creation of slow ions can influence the mean velocity, the most probable velocity should be unaffected by these processes.

Nuclear quantum dynamics on the ground electronic state of neutral silver dimer 107Ag109Ag probed by femtosecond NeNePo spectroscopy.

The nuclear quantum dynamics on the ground electronic state of the neutral silver dimer 107Ag109Ag are studied by femtosecond (fs) pump-probe spectroscopy using the 'negative ion - to neutral - to positive ion' (NeNePo) excitation scheme. A vibrational wave packet is prepared on the X1Σ+g state of Ag2via photodetachment of mass-selected, cryogenically cooled Ag2- using a first ultrafast pump laser pulse. The temporal evolution of the wave packet is then probed by an ultrafast probe pulse via resonant multiphoton ionization to Ag2+. Frequency analysis of the fs-NeNePo spectra obtained for a single isotopologue and pump-probe delay times up to 60 ps yields the harmonic (ωe = 192.2 cm-1), quadratic anharmonic (ωexe = 0.637 cm-1) and cubic anharmonic (ωeye = 3 × 10-4 cm-1) constants for the X1Σ+g state of neutral Ag2. The fs-NeNePo spectra obtained at different pump wavelengths provide insight into the excitation mechanism. At a pump wavelength of 510 nm instead of 1010 nm, resonant excitation of a short-lived electronically excited state of the anion followed by autodetachment results in population of higher-energy vibrational levels of the neutral ground state. In contrast, at 1140 nm dynamics with a slightly shorter beating period and different relative phase are observed. The present study demonstrates that isotopologue-specific fs-NeNePo spectroscopy provides accurate vibrational constants of mass-selected neutral clusters in their electronic ground state in the terahertz spectral region, which remains difficult to obtain directly in the frequency domain with any other type of spectroscopy of comparable sensitivity.

Low‐lying states of FeSin−/0/+ (n = 1‐2) clusters from DMRG‐CASPT2 calculations

AbstractThe electronic states of FeSin−/0/+ (n = 1‐2) clusters have been investigated with DFT, CASPT2, and DMRG‐CASPT2 methods. By using relatively large active spaces, the DMRG‐CASPT2 method is found to provide highly accurate relative energies for the various relevant electronic states. Leading configurations, bond distances, harmonic vibrational frequencies, and relative energies for the low‐lying states of the title clusters are reported. Electron detachment energies for the ground states of the anionic and neutral clusters were estimated at the DMRG‐CASPT2 level. Franck‐Condon factor simulations were performed for transitions from the anionic ground state to the neutral ground state and from the neutral ground state to the cationic ground state with the purpose to produce the vibrational progressions.

On the correlation between the TiN thin film properties and the energy flux of neutral sputtered atoms in direct current magnetron discharge

In this study, the energy flux of sputtered atoms on a substrate was correlated to the properties of titanium nitride (TiN) films deposited using direct current magnetron sputtering (dcMS) under mixed Ar and N2 atmospheres. The neutral titanium sputtered atoms velocity distribution functions (AVDFs) were measured by tunable diode-laser induced fluorescence (TD-LIF), and the flux of particles and their energy were derived. Mass spectrometry was used to characterize the energy-resolved flux of the ions. It was found that the neutral sputtered atoms flux and deposition rate were in good agreement, indicating that the flux of the neutral titanium ground state represents the number of deposited atoms. Moreover, TiN films were deposited at different gas pressures and at various Ar/N2 gas mixtures close to the conditions where stoichiometric TiN was formed, without bias voltage and heating of the substrates. The energy flux of the sputtered neutral Ti into the substrate was calculated from TD-LIF measurements. At a relatively low magnetron discharge pressure of 0.4 Pa, we demonstrated that the energy of sputtered neutral Ti impinging on the substrate is higher than the energy flux of ionized particles corresponding mainly to Ar+. Thus, the influence of the energy flux of the sputtered atoms on the texture and microstructure of the films is revealed. The (200) texture was obtained at 0.4 Pa when the energy flux of the sputtered atoms was higher than the ion energy flux. At 1.3 Pa where the sputtered atoms energy flux is one order lower compared to 0.4 Pa the (111) texture was obtained. The high-energy flux of the ground state of Ti sputtered atoms seems to allow stress removal in the films.

Charge Delocalization and Vibronic Couplings in Quadrupolar Squaraine Dyes.

Squaraines are prototypical quadrupolar charge-transfer chromophores that have recently attracted much attention as building blocks for solution-processed photovoltaics, fluorescent probes with large two-photon absorption cross sections, and aggregates with large circular dichroism. Their optical properties are often rationalized in terms of phenomenological essential state models, considering the coupling of two zwitterionic excited states to a neutral ground state. As a result, optical transitions to the lowest S1 excited state are one-photon allowed, whereas the next higher S2 state can only be accessed by two-photon transitions. A further implication of these models is a substantial reduction of vibronic coupling to the ubiquitous high-frequency vinyl-stretching modes of organic materials. Here, we combine time-resolved vibrational spectroscopy, two-dimensional electronic spectroscopy, and quantum-chemical simulations to test and rationalize these predictions for nonaggregated molecules. We find small Huang-Rhys factors below 0.01 for the high-frequency, 1500 cm-1 modes in particular, as well as a noticeable reduction for those of lower frequency modes in general for the electronic S0 → S1 transition. The two-photon allowed state S2 is well separated energetically from S1 and has weak vibronic signatures as well. Thus, the resulting pronounced concentration of the oscillator strength in a narrow region relevant to the lowest electronic transition makes squaraines and their aggregates exceptionally interesting for strong and ultrastrong coupling of excitons to localized light modes in external resonators with chiral properties that can largely be controlled by the molecular architecture.

Electron-impact ionization cross sections of small molecules containing Fe and Cr ∗ ∗ This work is dedicated to Enrico Clementi who pioneered the interdisciplinary use of quantum chemistry and computational modelling.

We present the electron-impact ionization cross sections (EICSs) of iron and chromium hydrides, nitrides, and oxides. The motivation of this work stems from the fact that chemical sputtering from a steel surface exposed to a hot plasma can create these molecules which in turn influence the composition and energy balance of the plasma. The latter influence is quantified by the EICS which we derive by using two semi-empirical methods which can be employed in the relevant energy range of 10–1000 eV. They are important molecular properties for plasma- and materials science. We discuss the foundations of the methods and present the cross sections of the high- and low-spin states of the species in their neutral ground states and of their cations.

Photoionization spectroscopy of the SiH free radical in the vacuum-ultraviolet range.

The first measurement of the photoelectron spectrum of the silylidyne free radical, SiH, is reported between 7 and 10.5eV. Two main photoionizing transitions involving the neutral ground state, X+1Σ+ ← X2Π and a+3Π ← X2Π, are assigned by using ab initio calculations. The corresponding adiabatic ionization energies are derived, IEad(X+1Σ+) = 7.934(5)eV and IEad(a+3Π) = 10.205(5)eV, in good agreement with our calculated values and the previous determination by Berkowitz et al. [J. Chem. Phys. 86, 1235 (1987)] from a photoionization mass spectrometric study. The photoion yield of SiH recorded in this work exhibits a dense autoionization landscape similar to that observed in the case of the CH free radical [Gans et al., J. Chem. Phys. 144, 204307 (2016)].

Complex energies and transition dipoles for shape-type resonances of uracil anion from stabilization curves via Padé.

Absorption of slow moving electrons by neutral ground state nucleobases has been known to produce resonance metastable states. There are indications that such metastable states may play a key role in DNA/RNA damage. Therefore, herein, we present an ab initio non-Hermitian investigation of the resonance positions and decay rates for the low lying shape-type states of the uracil anion. In addition, we calculate the complex transition dipoles between these resonance states. We employ the resonance via Padé (RVP) method to calculate these complex properties from real stabilization curves by analytical dilation into the complex plane. This method has already been successfully applied to many small molecular systems, and herein, we present the first application of RVP to a medium-sized system. The presented resonance energies are optimized with respect to the size of the basis set and compared with previous theoretical studies and experimental findings. Complex transition dipoles between the shape-type resonances are computed using the optimal basis set. The ability to calculate ab initio energies and lifetimes of biologically relevant systems paves the way for studying reactions of such systems in which autoionization takes place, while the ability to also calculate their complex transition dipoles opens the door for studying photo-induced dynamics of such biological molecules.

High-mobility semiconducting polymers with different spin ground states

Organic semiconductors with high-spin ground states are fascinating because they could enable fundamental understanding on the spin-related phenomenon in light element and provide opportunities for organic magnetic and quantum materials. Although high-spin ground states have been observed in some quinoidal type small molecules or doped organic semiconductors, semiconducting polymers with high-spin at their neutral ground state are rarely reported. Here we report three high-mobility semiconducting polymers with different spin ground states. We show that polymer building blocks with small singlet-triplet energy gap (ΔES-T) could enable small ΔES-T gap and increase the diradical character in copolymers. We demonstrate that the electronic structure, spin density, and solid-state interchain interactions in the high-spin polymers are crucial for their ground states. Polymers with a triplet ground state (S = 1) could exhibit doublet (S = 1/2) behavior due to different spin distributions and solid-state interchain spin-spin interactions. Besides, these polymers showed outstanding charge transport properties with high hole/electron mobilities and can be both n- and p-doped with superior conductivities. Our results demonstrate a rational approach to obtain high-mobility semiconducting polymers with different spin ground states.

Dyson-Schwinger equations and the muon g-2

We present a brief introduction to the Dyson-Schwinger equations (DSEs) approach to hadron and high-energy physics. In particular, how this formalism is applied to calculate the electromagnetic form factors $\gamma^* \gamma^* \to \textbf{P}^0$ and $\gamma^* \textbf{P}^\pm \to \textbf{P}^\pm$ (with $\textbf{P}^\pm$ and $\textbf{P}^0$ charged and neutral ground-state pseudoscalar mesons, respectively) is discussed. Subsequently, the corresponding contributions of those form factors to the muon anomalous magnetic moment ($g-2$) are estimated. We look forward to promoting the DSE approach to address theoretical aspects of the muon $g-2$, highlighting some calculations that could be carried out in the future.