Ab Initio Potential Energy Curves Research Articles

Energy-dependent photoion angular distributions in two-body Coulomb explosions of molecules.

We experimentally study two-body Coulomb explosions of CO2, O2, and CH3Cl molecules in intense femtosecond laser pulses. We observe an obvious variation in the ionic angular distribution of the fragments with respect to the kinetic energy releases (KERs). Using a classical model based on abinitio potential energy curves, we find that the dependence of the ionic angular distribution on the KER is relevant to the fact that the accurate potential energy deviates significantly from the value determined by applying the Coulomb interaction approximation at a relatively small internuclear distance of the molecule. We show that the KER-dependent ionic angular distribution provides an effective way to determine the critical internuclear distance at which the Coulomb interaction approximation holds or breaks down without relying on the knowledge of the accurate potential energy curves.

High Accuracy Ab Initio Potential Energy Curves and Dipole Moment Functions for the X1Σ+ and a3Π Spin States of the CF+ Diatomic Molecule.

Potential energy curves and dipole moment functions constructed using high-accuracy ab initio methods allow for an in-depth examination of the electronic structure of diatomic molecules. Ab initio computations serve as a valuable complement to experimental data, offering insights into the nature of short-lived molecules such as those encountered within the interstellar medium (ISM). While laboratory experiments provide critical groundwork, the ISM's conditions often permit longer lifetimes for lower stability molecules, enabling unique observations. The CF+ diatomic molecule is one such molecule that has been observed spectroscopically in the ISM. Previous experimental and theoretical work have examined different spectroscopic aspects of the CF+ molecule, but the development of newer, more complete potential energy curves and dipole moment functions allows for even greater insight. We constructed both potential energy curves and dipole moment functions for the ground X1Σ+ and first excited a3Π states of CF+ for both the 12C and 13C isotopologues. The potential energy curves were constructed using coupled cluster with single, double, and perturbative triple excitations (CCSD(T)) at the complete basis set limit with corrections from full triple, quadruple, quintuple, and hextuple excitations within a finite-basis coupled cluster wave function as well as corrections from full configuration interaction and relativistic effects. Rovibrational wave functions were calculated using a vibrational Hamiltonian matrix, which moves beyond the harmonic oscillator approximation. The equilibrium bond length, vibrational constant, and rotational constant were reproduced to within 0.00013 Å, 0.28 cm-1, and 0.00045 cm-1, respectively, of experimental values. Experimental transition energies from rovibrational spectra were reproduced with an error of no larger than 0.63 cm-1. The triplet excited state (a3Π) was found to have a longer equilibrium bond length at 1.21069 Å, a vibrational constant of 1611.29 cm-1, and a rotational constant of 1.56376 cm-1. Rovibrational line lists for the 12C and 13C isotopologues for both the X1Σ+ and the excited a3Π states were generated.

An ab initio diabatic study of rovibronic spectra of CN.

An ab initio study of the rovibronic spectra for the cyano radical (CN) based on a diabatic representation is presented. This work considers 17 electronic states, 59 dipole moment curves, 88 spin-orbit coupling curves, and 30 electronic angular momentum coupling curves, which are obtained using the internally contracted multireference configuration interaction method including the Davidson correction (icMRCI + Q) with the aug-cc-pwCV5Z-DK basis set. The diabatic transformations are performed based on a property-based diabatization method to remove the avoided crossings for the D2Π-H2Π and b4Π-24Π pairs. Ab initio potential energy curves of the X2Σ+, B2Σ+, E2Σ+, A2Π, D2Π, H2Π, F2Δ and J2Δ electronic states are shifted to match the experimental electronic excitation energies and the equilibrium internuclear distances. The coupled nuclear motion Schrödinger equations are then solved to obtain the rovibronic spectra of CN for wavenumbers from 0 to 80 000 cm-1. At wavenumbers of 0-30 000 cm-1, our absorption cross sections agree well with available theoretical data. For wavenumbers above 30 000 cm-1, our cross sections are larger than previous data in view of the fact that the transitions involving high-lying electronic states are considered. This work provides an overall prediction of the rovibronic spectrum of CN. Our results are suitable for temperatures below 8000 K and could be useful for the investigations of planetary exploration.

Infrared spectra and fragmentation dynamics of isotopologue-selective mixed-ligand complexes.

Isolated mixed-ligand complexes provide tractable model systems in which to study competitive and cooperative binding effects as well as controlled energy flow. Here, we report spectroscopic and isotopologue-selective infrared photofragmentation dynamics of mixed gas-phase Au(12/13CO)n(N2O)m+ complexes. The rich infrared action spectra, which are reproduced well using simulations of calculated lowest energy structures, clarify previous ambiguities in the assignment of vibrational bands, especially accidental coincidence of CO and N2O bands. The fragmentation dynamics exhibit the same unexpected behaviour as reported previously in which, once CO loss channels are energetically accessible, these dominate the fragmentation branching ratios, despite the much lower binding energy of N2O. We have investigated the dynamics computationally by considering anharmonic couplings between a relevant subset of normal modes involving both ligand stretch and intermolecular modes. Discrepancies between correlated and uncorrelated model fit to the ab initio potential energy curves are quantified using a Boltzmann sampled root mean squared deviation providing insight into efficiency of vibrational energy transfer between high frequency ligand stretches and the softer intermolecular modes which break during fragmentation.

Photodissociation cross sections and rates of NaO

ABSTRACT Photodissociation of NaO may be important for the sodium chemistry in various astrophysical regions. This work produces the photodissociation cross sections and rates of NaO over the temperature range from 0 to 15 000 K. First, the state-resolved cross sections for transitions from the ground and first excited states of NaO are investigated using ab initio potential energy curves and transition dipole moments. The temperature-dependent cross sections were then obtained by assuming a Boltzmann distribution to describe the population of the initial state. Detailed comparisons with experimental cross sections at 200 and 300 K reveal that the X 2Π → 1 2∆ and X 2Π → 2 2Σ− transitions may be the main photodissociation pathways for NaO in the wavelengths of about 2400–2580 Å, while the X 2Π → B 2Σ− transition may play a dominant role in the wavelengths of about 3534–4230 Å. Finally, photodissociation rates in the interstellar, solar, and blackbody radiation fields were determined. In the interstellar and solar radiation fields, the X 2Π → B 2Σ− transition dominates at low temperatures and the A 2Σ+ → 2 2Σ+ transition dominates at high temperatures. The total photodissociation rates in ultraviolet-rich and visible-rich radiation fields are almost insensitive to the temperature. The photodissociation cross sections and rates of NaO should be useful for investigating the chemical evolution of the sodium element in planetary exospheres, atmospheres of cool stars, and envelopes of evolved stars.

The elastic scattering cross sections of the N[formula omitted]-H and N-H[formula omitted] collisions for the transport theory

The transport cross sections and integrated cross sections are calculated for interactions of hydrogen and nitrogen atoms and ions (in the ground and excited states) for use in plasma science and hypersonic flows theory. The data for the transport cross sections are given in the energy range typically needed (or broader) for those applications: 10−6-37 hartree (around 2.7⋅10−5eV-1 keV). The reported cross sections are calculated from the high quality ab initio potential energy curves with additional extrapolation for highly repulsive region. The methods and energy range are discussed in the context of LXCat and IAEA (ALADDIN and CollisionDB) databases. The uncertainty of data is also estimated.

Interference effects in the photoelectron spectrum of the NeKr dimer and vibrationally selected interatomic Coulombic decay

In our work we study the interatomic Coulombic decay (ICD) in the NeKr dimer, where after $2s$ ionization of the Ne, the system relaxes and the excess energy is utilized to ionize the Kr. The temporal evolution of the ICD process in NeKr has been recently measured and theoretically explained by Trinter et al. [Chem. Sci. 13, 1789 (2022)]. Here we focus on two other main goals. The first goal regards the found interference effects in the photoemission (PE) spectrum, which are unusual phenomena in noble gas dimers. They result from the coherently excited vibrational energy levels and substantial dependence of the large ICD decay width on the internuclear distance. The PE spectrum reacts sensitively to changes in the potential energy curve (PEC) of the $2s$ ionized state, and we modified the available ab initio PEC in such a way that satisfactory agreement between theoretical and experimental data is achieved. The impact of isotope masses on the PE spectrum is briefly discussed and used in the determination of the PEC. Our second main goal concerns the nuclear motion during the ICD process. Here we investigate the impact of different vibrationally excited states of the electronic ground state on the ICD-electron and kinetic energy release (KER) spectra. To transfer our vibrationally selected ICD model to a realizable experiment, we also present the impact of temperature on the ICD-electron spectrum. Finally, our studies are complemented by comparing the directly computed KER spectrum to the mirror image of the ICD-electron spectrum, which coincide under certain conditions.

The ab initio potential energy curves of atom pairs and transport properties of high-temperature vapors of Cu and Si and their mixtures with He, Ar, and Xe gases.

The potential energy curves (PECs) for the homonuclear He-He, Ar-Ar, Cu-Cu, and Si-Si dimers, as well as heteronuclear Cu-He, Cu-Ar, Cu-Xe, Si-He, Si-Ar, and Si-Xe dimers, are obtained in quantum Monte Carlo (QMC) calculations. It is shown that the QMC method provides the PECs with an accuracy comparable with that of the state-of-the-art coupled cluster singles and doubles with perturbative triples corrections [CCSD(T)] calculations. The QMC data are approximated by the Morse long range (MLR) and (12-6) Lennard-Jones (LJ) potentials. The MLR and LJ potentials are used to calculate the deflection angles in binary collisions of corresponding atom pairs and transport coefficients of Cu and Si vapors and their mixtures with He, Ar, and Xe gases in the range of temperature from 100 K to 10 000 K. It is shown that the use of the LJ potentials introduces significant errors in the transport coefficients of high-temperature vapors and gas mixtures. The mixtures with heavy noble gases demonstrate anomalous behavior when the viscosity and thermal conductivity can be larger than that of the corresponding pure substances. In the mixtures with helium, the thermal diffusion factor is found to be unusually large. The calculated viscosity and diffusivity are used to determine parameters of the variable hard sphere and variable soft sphere molecular models as well as parameters of the power-law approximations for the transport coefficients. The results obtained in the present work include all information required for kinetic or continuum simulations of dilute Cu and Si vapors and their mixtures with He, Ar, and Xe gases.

Formation of CO through C (2s 22p 2 3P) and O (2s 22p 4 3P) Radiative Association

The formation of CO through the radiative association of the carbon (C, 2s 22p 2 3P) and oxygen (O, 2s 22p 4 3P) atoms is investigated. The corresponding cross sections and rate coefficients for temperatures T = 10–10,000 K are calculated using the quantum-mechanical approach based on ab initio potential energy curves, permanent dipole moments, and transition dipole moments, which are obtained by the internally contracted multi-reference configuration interaction method with the Davidson correction and aug-cc-pwCV5Z-DK basis set. All dipole-allowed transitions between singlet, triplet, and quintet states converging to the C (2s 22p 2 3P) + O (2s 22p 4 3P) dissociation limit are considered. Compared to the previous results that only contain the X1Σ+ → X1Σ+, A1Π → X1Σ+, and B1Σ+ → X1Σ+ transitions, our results suggest that the a′3Σ+ → a3Π and d3Δ → a3Π transitions make significant contributions to the radiative association for T = 10–30 K. The total rate coefficient at low temperatures is estimated to be about 10−18 cm3 s−1, which shows significant deviation from the previous results, where only three transitions were considered. New rate coefficients may improve the chemical modeling of CO in the low-density region of the interstellar medium.

A quantum mechanical calculation of the CN radiative association

ABSTRACT Radiative association of CN is investigated through the quantum mechanical method, including the cross sections and rate coefficients. The ab initio potential energy curves, transition dipole moments, and permanent dipole moments of CN are obtained by the internally contracted multireference configuration interaction method with Davidson correction and aug-cc-pwCV5Z-DK basis set. For the collision of the ground state C (3Pg) and N (4Su) atoms, except for the four previously studied processes including the A2Π → X2Σ+, X2Σ+ → A2Π, A2Π → A2Π, and X2Σ+ → X2Σ+ transitions, four other radiative association processes including b4Π → a4Σ+, a4Σ+ → b4Π, b4Π → b4Π, and a4Σ+ → a4Σ+ transitions are considered. We also considered the collision of the excited C (1Dg) and the ground N (4Su) atoms including the 24Π → 14Σ− process and the collision of the ground C (3Pg) and the excited N (2Du) atoms including 22Π → B2Σ+, 32Π → B2Σ+, and 42Π → B2Σ+ transitions. The temperature population factor is considered to describe the thermal population of the three different dissociation asymptotic energies. The results show that the contribution of the A2Π → X2Σ+ and b4Π → a4Σ+ transitions to the total rate coefficients is significant over the entire temperature range. While considering the collision of C and N involving excited states, the contribution of the 22Π → B2Σ+, 32Π → B2Σ+, and 42Π → B2Σ+ transitions to the total rate coefficients cannot be ignored at the temperature range larger than 10 000 K. Finally, the rate coefficients are fitted to an analytical function for astrochemical reaction modelling.

ExoMol line lists – XLVI. Empirical rovibronic spectra of silicon mononitrate (SiN) covering the six lowest electronic states and four isotopologues

ABSTRACT Silicon mononitride (28Si14N, 29Si14N, 30Si14N, 28Si15N) line lists covering infrared, visible, and ultraviolet regions are presented. The SiNful line lists produced by ExoMol include rovibronic transitions between six electronic states: $X\, {}^{2}\Sigma ^{+}$, $A\, {}^{2}\Pi$, $B\, {}^{2}\Sigma ^{+}$, $D\, {}^{2}\Delta$, $a\, {}^{4}\Sigma ^{+}$, $b\, {}^{4}\Pi$. The ab initio potential energy and coupling curves, computed at the multireference configuration interaction (MRCI/aug-cc-pVQZ) level of theory, are refined for the observed states by fitting their analytical representations to 1052 experimentally derived SiN energy levels determined from rovibronic bands belonging to the X–X, A–X, and B–X electronic systems through the MARVEL procedure. The SiNful line lists are compared to previously observed spectra, recorded and calculated lifetimes, and previously calculated partition functions. SiNful is available via the www.exomol.com database.

Rydberg states of ZnAr complex

Ab initio potential energy curves (PECs) of electronic states of ZnAr complex are calculated up to Rydberg state correlating with the asymptote of Zn atom. Analysis of various sources of errors of presented calculations is performed including finite basis set size, deficiencies in inclusion of the electron correlation and relativistic effects, and size-inconsistency errors of the multireference second-order perturbation theory. In particular, it is emphasised that the inclusion of the midbond bases substantially improves the convergence rate of the binding energies with respect to the basis set size not only for the van der Waals ground state of ZnAr complex, but also it is the case for the excited states including the Rydberg ones. Wherever it is possible, the comparison with the experimental data and other theoretical results is presented. Properties of the double-well PECs of the Σ Rydberg states are interpreted within the simplified theory, in which, the appearance of the energy barrier and, partially, of the outer well originates from the low-energy scattering of the Rydberg electron on the Ar atom.

Quantum state-dependent anion-neutral detachment processes.

The detachment loss dynamics between rubidium atoms (Rb) and oxygen anions (O-) are studied in a hybrid atom-ion trap. The amount of excited rubidium present in the atomic ensemble is actively controlled, providing a tool to tune the electronic quantum state of the system and, thus, the anion-neutral interaction dynamics. For a ground state Rb interacting with O-, the detachment induced loss rate is consistent with zero, while the excited state Rb yields a significantly higher loss rate. The results are interpreted via ab initio potential energy curves and compared to the previously studied Rb-OH- system, where an associative electronic detachment reactive loss process hinders the sympathetic cooling of the anion. This implies that with the loss channels closed for ground-state Rb and O- anion, this system provides a platform to observe sympathetic cooling of an anion with an ultracold heavy buffer gas.

ExoMol molecular line lists – XLIII. Rovibronic transitions corresponding to the close-lying X 2Π and A 2Σ+ states of NaO

ABSTRACT The sodium monoxide radical (NaO) is observed in night-glow in the Earth’s mesosphere and likely has astronomical importance. This study concerns the optical transitions within the ground X 2Π state and to the very low-lying (Te ≈ 2000 cm−1) excited A 2Σ+ state. A line list consisting of rovibronic term values, allowed electric dipole transitions, Einstein coefficients, and partition functions for varying temperature are produced using a variational solution of the coupled-channel Schrödinger equations using the program duo. multi-reference configuration interaction (MRCI) ab initio calculations characterizing the potential energy curves of the two states, spin-orbit and L-uncoupling non-adiabatic matrix elements, as well as permanent and transition dipole moments were integral in the formation of the final deperturbation model. Ab initio potential energy curves are represented in the analytical Extended Morse Oscillator form and refined, along with the spin-orbit and L-uncoupling functions, by least-squares fitting to the available spectroscopic data. The input experimental data consisted of pure rotational transitions within the fine-structure components of the X 2Π state for v″ ∈ [0, 3] vibrational levels as well as the rovibronic A 2Σ+(v′ = 0) ← X 2Π(v″ = 0) transitions, both with limited coverage over rotational excitation. The lack of data detailing the vibrational structure of the X and A states points to the need for further experimental study of higher excited levels, which would provide a more robust spectroscopic model. The NaO NaOUCMe line list is available via www.exomol.com and the CDS data base.

Relating Binding Energy and Scattering Length of Cold Hybrid Ion‐Atom Systems

AbstractThe universal properties of the loosely bound vibrational states of cold hybrid ion‐atom systems are probed using a “scaling” approach. This approach is based on the use of numerical relations (a versus Dlast) between the binding energy (Dlast) of the highest vibrational state and the s‐wave scattering length (a) obtained by solving the appropriate Schrödinger equations with scaled interaction potentials. The actual probing is accomplished by evaluating the “a versus Dlast” relations for 6Li and 7Li in the ground electronic state using a set of ab initio potential energy curves from the literature. Although these curves provide strongly differing values of a and Dlast, the corresponding “a versus Dlast” relations closely coincide, thus enabling an accurate determination of scattering lengths directly from experimental binding energies and vice versa.

Temperature-dependent direct photodissociation cross sections and rates of AlCl

ABSTRACT The photodissociation process of aluminium monochloride (AlCl) plays an important role in modelling the chemistry of the circumstellar envelope. In this work, direct photodissociation cross sections of AlCl have been computed for transitions from the ground X1Σ+ state to six low-lying excited electronic states by using ab initio potential energy curves and transition dipole moments, which are obtained by the internally contracted multireference configuration-interaction method with Davidson correction and the aug-cc-pV6Z basis set. State-resolved cross sections for transitions from 38 958 rovibrational levels (υ″ ≤ 100, J″ ≤ 400) of the ground X1Σ+ state have been obtained for photon wavelengths from 500 Å to the dissociation threshold. Photodissociation cross sections in local thermal equilibrium are evaluated for gas temperatures from 500 to 10 000 K. Using the computed cross sections, temperature-dependent photodissociation rates of AlCl in the interstellar and blackbody radiation fields are determined. The results can be applied to the investigation of the chemical evolution of Al in the envelope of carbon-rich and oxygen-rich stars.

Radiative association of atomic and ionic nitrogen

ABSTRACT Radiative association for the formation of molecular nitrogen cation ${\rm{N}}_2^ + $ during the collision of an N(4Su) atom and an N+(3Pg) ion is investigated. The corresponding cross-sections and rate coefficients are computed by the quantum mechanical method based on ab initio potential energy curves and transition dipole moments, which are obtained by the internally contracted multireference configuration interaction method with the Davidson correction and aug-cc-pCV5Z-DK basis set. A number of low-lying doublet, quartet, and sextet states correlating to the N(4Su) + N+(3Pg) dissociation limit are considered. Hence, we investigate a number of dipole-allowed transitions and determine their contributions to the radiative association. The results show that transitions originating in the f4Πu, D2Πg, ${{\rm{B}}^2}\Sigma _{\rm{u}}^ + $, ${{\rm{1}}^4}\Sigma _{\rm{g}}^{\rm{ + }}$, and ${{\rm{1}}^6}\Sigma _{\rm{u}}^{\rm{ + }}$states are the main contributors for the radiative association process. The calculated rate coefficients are valid for temperatures from 100 to 10 000 K and fitted to the analytical function suitable for astrochemical reaction applications.

Radial molecular property functions of CH in its ground electronic state

Effective potential energy curves of the ground electronic states of isotopomers of the methylidyne radical CH are constructed by morphing accurate ab initio potential energy curves within the framework of Jenč’s reduced potential energy curve (RPC) approach. The morphing is performed by fitting the RPC parameters to available experimental ro-vibrational data. In turn, the resulting potential energy curves are used to determine the radial property functions describing the spin-orbit and fine and hyperfine energy patterns of the spin-ro-vibrational energy levels by morphing their ab initio approximants or by a direct fitting to experiment. The resulting functions enable reliable predicting outside prospected energy regions, and a quantitative probing of the impacts of external perturbations on the observed spectra.

A spectroscopic model for the low-lying electronic states of NO.

The rovibronic structure of A2Σ+, B2Π, and C2Π states of nitric oxide (NO) is studied with the aim of producing comprehensive line lists for its near ultraviolet spectrum. Empirical energy levels for the three electronic states are determined using a combination of the empirical measured active rotation-vibration energy level (MARVEL) procedure and ab initio calculations, and the available experimental data are critically evaluated. Ab initio methods that deal simultaneously with the Rydberg-like A2Σ+ and C2Π and the valence B2Π state are tested. Methods of modeling the sharp avoided crossing between the B2Π and C2Π states are tested. A rovibronic Hamiltonian matrix is constructed using the variational nuclear motion program Duo whose eigenvalues are fitted to the MARVEL. The matrix also includes coupling terms obtained from the refinement of the ab initio potential energy and spin-orbit coupling curves. Calculated and observed energy levels agree well with each other, validating the applicability of our method and providing a useful model for this open shell system.

Radial molecular property functions of the [formula omitted] state of hydroxyl

Effective (mass-dependent) potential energy curves of the A2Σ+ electronic states of the 16OH and 16OD isotopomers of the hydroxyl radical OH are constructed by morphing reliable ab initio potential energy curves within the framework of the Jenč reduced potential energy curve (RPC) approach. In turn, the resulting potential energy curves are used to determine the radial functions describing the spin-rotation and hyperfine energy patterns of the spin-rovibrational energy levels by morphing/fitting their ab initio/empirical approximants. The resulting functions allow for reliable predictions for untested spectral regions, provide a base for testing the impacts of external fields on the measured spectral transitions, and allow for accurate estimations of the mass sensitivity of the hydroxyl spectral transitions appearing as suitable probes of a cosmological variability of the electron-to-proton mass ratio.