A theoretical study of the spectroscopic states of the CF molecule

LCAO-MO SCF calculations on the ground and excited states of CF are described. Close agreement with observed term values is obtained. The calculation of some molecular properties is discussed, and a very good result is obtained for the spin-orbit coupling in the ground state. The positive spin-orbit coupling in the ²Δ state is not explained. It is shown that the observed predissociation in the A state must be attributed to a curve crossing at a smaller internuclear distance than equilibrium, and that the ⁴Σ^- state from 5σ 2π² is responsible. Calculations show that a second-order treatment of the A ²Σ⁺-⁴∑^- interaction is necessary and sufficient to account for the predissociation.     

MRCI studies on ground and excited states of CBr

Potential curves and spectroscopic constants for a large number of doublet and quartet states of CBr were obtained by multireference configuration interaction calculations, using valence triple-zeta basis sets with polarization and diffuse functions. Besides the X²Π ground state, 1⁴Σ^?, 1²Δ and 2²Σ⁺ have been found to be stable. Spectroscopic constants calculated for 1²Δ are in excellent agreement with experimental values obtained by Dixon and Kroto in 1963. Their observed predissociation of one component of 1²Δ can be explained by the crossing of the 1²Δ potential near equilibrium by 1²Σ⁺. The 1²Σ⁺ state is calculated to have a shallow long-range minimum at 2.31?Å. The dissociation energy of X²Π is calculated to be 3.43?eV. An observed T _e of 4.97?eV for 2²Σ⁺ agrees with the theoretical value. Several Rydberg states of the 2π→Ryd and 3σ→Ryd series, starting at T _e ?=?5.25?eV, were identified. Photodissociation of CBr by sunlight, important in the ozone cycle, can occur via direct dissociation of the ground state, or by excitation to 1²Δ followed by predissociation. Most dissocative repulsive states lie at higher energies, and are not expected to participate in the photodisscociation of CBr.     

Accurate ab initio potential energy curves and spectroscopic properties of the low-lying electronic states of OH− and SH− molecular anions

ABSTRACT

Multireference configuration interaction method was used in order to generate accurate potential energy curves of the OH, SH, OH^? and SH^? electronic states correlating to the three lowest dissociation limits. These curves were used in addition with core–valence correlation and scalar relativistic corrections for the calculations of accurate spectroscopic constants of bound states, which generally are found in excellent agreement with best available experimental and theoretical values in the literature. The spin–orbit interactions between electronic states have been calculated for the cases in which the couplings were assumed to be responsible for perturbations and used to explain the predissociation of A²Σ⁺ state of OH and SH by dissociative states 1⁴Σ^?, 1²Σ^? and 1 ⁴Π. Dipole moment functions were also computed along internuclear distances and used to explain polarity of these molecules in different calculated electronic states. In addition, stability and metastability of electronic states (X ¹Σ⁺, A¹Π and a³Π) of OH^? and SH^? molecular anions have been studied relatively to curves of neutral parent electronic states. Finally, we have computed adiabatic electron affinity of OH and SH and these values have been found in very good agreement with the best experimental values and resort as among the best achieved values.     

Ab initio configuration interaction study on the electronic structure of the X2Σ+, B2Σ+ and 32Σ+ states of SiO+

The energy levels and electronic structure of the X²Σ⁺, B²Σ⁺ and 3²Σ⁺ states of SiO⁺ are studied using ab initio configuration interaction (CI) calculations at and around their equilibrium internuclear distances R _e. Spectroscopic constants and the vertical excitation energy from the SiO⁺ X²Σ⁺ state are predicted for the 3²Σ⁺ state. Based on the calculated CI wavefunctions, avoided crossings of the potential energy curve for the 3²Σ⁺ state and a near-degeneracy effect in the avoided crossing region are examined. The effects of the mixing of excited configuration state functions in the total electronic wavefunctions for the 1–3 ²Σ⁺ states are investigated by analysing correlation energies in terms of the contributions from classes of excited configurations. The importance of both the near-degeneracy effect and the correlation energy effect in describing correctly the electronic structure of the 3 ²Σ⁺ state in the neighbourhood of its R _e is discussed.     

Low-lying electronic states of LiF molecule with inner electrons correlation

The potential energy curves and dipole moments of the low-lying electronic states of LiF molecule are performed by using highly accurate multi-reference configuration interaction with Awcv5z basis sets. 1s, the inner shell of Li is considered as the closed orbit, which is used to characterise the spectroscopic properties of a manifold of singlet and triplet states. 16 electronic states correlate with two lowest dissociation channels Li(²S)+F(²P) and Li(²P)+F(²P) are investigated. Spectroscopic parameters of the ground state X¹Σ⁺ have been evaluated and critically compared with the available experimental values and the other theoretical data. However, spectroscopic parameters of 1³Π, 1¹Δ, 1¹Σ^?, 1¹Π, 1³Σ⁺, 2³Σ⁺, 1³Δ, 1³Σ^?, 2³Π, 2¹Π, 3³Π, 3¹Π and 3³Σ⁺ states are studied for the first time. These 13 excited states have shallow potential wells, and the dispersion coefficients of these excited states are predicted. In additional, oscillator strengths of excited states at equilibrium distances are also predicted.     

Is there a stable B2Π state for the CNO molecule?

We report MRD-CI calculations on the ground state X²Π and the excited states A²Σ⁺ and B²Π of the CNO molecule in linear geometry. The surfaces for oxygen and carbon extraction are calculated using a limited CI expansion of 47 configuration state functions; in the vicinity of the minima obtained with this procedure large-scale CI calculations are carried out including deter-mination of the spin-orbit splitting of the ²Π states of the minima. We find that the B²Π state will be difficult to detect spectroscopically due to an avoided crossing just at the equilibrium geometry of the ground state at R_CN = 2.25 a.u., R_NO = 2.30 a.u. Accordingly we find two shallow minima for B²Π at R_CN = 2.33 a.u., R_NO = 2.91 a.u. and R_CN = 2.78 a.u., R_NO = 2.28 a.u., respectively.     

Quasi-relativistic treatment of the low-lying KCs states

The potential energy curves, permanent and transition dipole moments as well as spin-orbit and angular coupling matrix elements between the KCs electronic states converging to the lowest three dissociation limits were evaluated in the basis of the spin-averaged wavefunctions corresponding to pure Hund’s coupling case (a). The quasi-relativistic matrix elements have been obtained for a wide range of internuclear distance by using of small (9-electrons) effective core pseudopotentials of both atoms. The core-valence correlation has been accounted for a large scale multi-reference configuration interaction method combined with semi-empirical core polarization potentials. The static dipole polarizabilities of the ground X¹Σ⁺ and a³Σ⁺ states were extracted from the closed-shell coupled-cluster energies by the finite-field method. Among the singlet and triplet Σ⁺ states manifold the pronounced avoided crossing effect between repulsive walls of the (2,3)³Σ⁺ states has been discovered and analyzed by finite-difference calculation of radial coupling matrix elements. The resulting transition dipole moments and potentials were used to predict radiative lifetimes and emission branching ratios of excited vibronic states while the calculated angular coupling matrix elements were transformed to Λ-doubling constants of the (1,2)¹Π states and magnetic g-factor of the ground state. The accuracies of the present results are discussed by comparing with experimental data and preceding calculations.     

Identification of New Low-Lying Electronic States of the FeF Radical

Dispersed fluorescence studies on the ⁶Π-X⁶Δ and ⁶Φ−X⁶Δ systems of the FeF radical have resulted in the observation of vibrational progressions for transitions to the X⁶Δ state as well as at least two previously unobserved electronic states about 5000 cm⁻¹ above the ground state. The states are assigned as the A⁶Π and B⁶Σ⁺ electronic states. The spin components of both electronic states were found to be heavily perturbed resulting in uneven splittings between them. A third, weak series was also observed but could not be assigned. The (0,0) band of the ⁶Π_7/2−B⁶Σ⁺_5/2 transition at 398 nm was observed in absorption by laser induced fluorescence and its rotational structure was assigned. The spectra obtained were weak because of a poor population of the B⁶Σ⁺ state by the reaction used to form FeF. The levels were found to be markedly perturbed at high J values. Attempts were made to fit the data on the ⁶Π_7/2-B⁶Σ_5/2⁺ system to an effective Hamiltonian, but the presence of perturbations meant that the system is not well described by such a model.     

A high resolution visible spectrum of iridium monophosphide

Laser induced fluorescence spectra of iridium monophosphide, IrP, have been obtained at low and high resolution in the blue region of the visible spectrum. Two electronic transitions were observed with origins near 459.6 and 471.9 nm. Three vibronic bands in each of these transitions have been observed at high resolution allowing for full characterization of the states. A J-independent doubling of the rotational lines has been ascribed to nuclear electric quadrupole coupling in the ground state. Multireference configuration interaction (MRCI) calculations have been performed in order to confirm the nature of the ground state and aid in the assignment of the excited states. The two observed transitions have been assigned as the [21.7]¹Σ⁺-X¹Σ⁺ and the [21.2] ³Σ⁺-X¹Σ⁺ electronic systems based on comparison with the theoretical calculations. The v + 2 level of each of these electronic transitions was found to be heavily perturbed and a successful deperturbation analysis was performed allowing for a complete global fit of the data.     

Accurate potential energy curves and spectroscopic properties of the 27 Λ-S states and 73 Ω states of the PO radical

The potential energy curves (PECs) were calculated for the 27 Λ-S states and 73 Ω states of PO radical. The calculations were done using the CASSCF method, which was followed by the internally contracted multireference configuration interaction (icMRCI) approach. To improve the quality of PECs, core-valence correlation and scalar relativistic corrections as well as Davidson correction were included. Of the 27 Λ-S states, the 1⁶Σ⁺ state was repulsive at any case. The 1⁴Φ and 1⁶Π states were bound, but they became repulsive with the spin-orbit coupling (SOC) effect accounted for. The 3⁴Σ⁺, a⁴Π, C′²Δ, D′²Π, 1⁴Δ, 1²Φ, 1⁶Σ⁺ and 1⁶Π states were inverted with the SOC effect included. The F²Σ⁺ state had double wells. The avoided crossings existed between the B²Σ⁺ and F²Σ⁺ states, the F²Σ⁺ and 3²Σ⁺ states, the C′²Δ and 2²Δ states, the 1⁴Δ and 2⁴Δ states, the 2⁴Δ and 3⁴Δ states, the 2⁴Π and 3⁴Π states and the 3⁴Π and 4⁴Π states. The c⁴Σ⁺, 2⁴Σ⁺, 3⁴Σ⁺, 3⁴Π, 4⁴Π, 5⁴Π, 3⁴Δ, 1⁴Φ and 1⁶Π states were weakly bound, which well depths were within several hundred cm^?1. The spectroscopic parameters were derived. The SOC effect on the spectroscopic properties was evaluated. The spectroscopic results obtained here could be expected to be reliably predicted ones.     

Ab initio study of the feasibility of laser cooling of ScO molecule

ABSTRACT

Using ab initio quantum chemistry method, the feasibility of laser cooling ScO was investigated. The ground state Χ²Σ⁺ and low-lying excited states Α²Π, Α′²Δ are calculated at the multi-reference configuration interaction (MRCI) level of theory. The calculated spectroscopic constants are in good agreement with available theoretical and experimental results. At the MRCI level of theory with Davidson correction, the permanent dipole moments of the Χ²Σ⁺ and Α²Π states of ScO are also calculated. The highly diagonally distributed Franck–Condon factors and shorter radiative lifetime for the Α²Π→Χ²Σ⁺ transition are calculated with the corresponding potential energy curves and transition dipole moment. Although there is an intermediate state Α′²Δ, the loss will be dominated by branching to the intermediate electronic state Α′²Δ at a level of η < 1.4 × 10^?4. These results demonstrate the probability of laser cooling of ScO, and we provide a promising laser-cooling scheme for ScO molecule.     

Spectroscopic Investigation of the Electronic Structure of YN: The XΣ Ground State and the New B1, C1 and D1 Excited Electronic States

YN molecules were produced in a free jet molecular beam apparatus by a laser vaporizing yttrium metal in the presence of He doped with NH₃. Laser excitation spectra were observed in the range 18 250-19 850 cm⁻¹. The ground state was confirmed to have ¹Σ⁺ symmetry. The fundamental vibration in the ground state was measured to be 650.6(1) cm⁻¹. Three new electronic states, B1, C1, and D1, were observed at 18 974.7(1), 19 023.3(1), and 19 824.0(1) cm⁻¹, respectively. The fundamental vibrations and equilibrium internuclear distances were found to be 718.3(1) cm⁻¹ and 1.939(8) for the B1 state and 723.5(1) cm⁻¹ and 1.9194(3) for the C1 state. Two additional electronic states were identified with the help of a deperturbation procedure, one of which is either the ¹Σ⁺ or the ³Σ₀⁻ state. The newly observed electronic states cannot be accounted for based on the existing ab initio results. We expect that these states correlate with the excited asymptote Y(4d¹5s²²D)+N(²D).     

Electronic states and spectroscopic properties of MgH in absence and presence of spin–orbit coupling − a configuration interaction study

Electronic spectrum of astrophysically important molecule magnesium hydride (MgH) has been studied using configuration interaction methodology excluding and including spin–orbit coupling. Potential energy curves of several spin-independent (Λ?S) electronic states have been constructed and spectroscopic constants of low-lying bound Λ?S states within 8.2 eV of term energy are reported in the first stage of calculations. The X²Σ⁺ is identified as the ground state in the Λ?S level. In the subsequent stage, the spin–orbit interaction has been incorporated and its effects on the potential energy curves and spectroscopic features of different electronic states of the species have been investigated. The X²Σ⁺_1/2 is identified as the spin–orbit (Ω) ground state of the species. Transition moments of several dipole-allowed transitions are computed in both the stages and radiative lifetimes of the corresponding excited states are computed. Electric dipole moments (µ) for a number of low-lying bound Λ?S states as well as several low-lying Ω-states are also calculated in the present study.     

Theoretical investigation of the molecular structure and transition dipole moments of the NaK low lying electronic states

The electronic structure and the spectroscopic constants of the low lying electronic states of the NaK⁺ ionic molecule have been determined through using an ab initio approach involving a non-empirical pseudopotential for the Na and K cores and core valence correlation correction. The potential energy of nearly 26 electronic states of ²Σ⁺, ²Π, and ²Δ symmetries has been calculated up to their dissociation limit Na(4d) + K⁺ and Na⁺ + K(6s). Their spectroscopic constants (R_e, D_e, T_e, ω_e, ω_eχ_e, and B_e) are derived and compared with the few available theoretical studies. A good agreement has been found for the ground state and few excited states with previous works. New potential energy curves were presented, for the first time, for the higher excited states. Numerous avoided crossing between electronic states of ²Σ⁺, ²Π symmetries have been localized and analyzed. Their existences are related to the charge transfer between the two ionic molecules Na⁺K and NaK⁺. Furthermore, we have determined the transition dipole moments for several states and analyzed the avoided crossings related to charge transfer between alkaline atoms.     

A theoretical study of potential energy curves and spectroscopic constants of VC

Potential energy curves for the various low-lying electronic states of VC have been studied using complete active space multi-configuration self-consistent field (CASMCSCF) followed by first-order and multireference singles and doubles configuration interaction (FOCI, MRSDCI) calculations. The MRSDCI calculations included up to 6 million configurations. Two very low-lying electronic states are found as candidates for the ground state of VC, namely a high spin state ⁴Δ and a low-spin ²Δ state, which is favoured at higher levels. A number of low-lying excited electronic states of VC are predicted, which are yet to be observed. The low-lying electronic states of VC are found to be ionic as inferred from the dipole moments and the charge density calculations. Electron donation and the back-donation process are suggested to be operative in the V-C bond formation.     

The ion-molecule reaction O+ (4S) + N2(X1Σ+) → NO+ (X1Σ +, v′) + N(4S) and the predissociation of the A2Σ+ and B2Π states of N2O+

The potential energy surfaces (PESs) for several electronic states involved in the reaction O⁺ (⁴S) + N₂(X¹Σ⁺) → NO⁺ (X¹Σ ⁺, v′) + N(⁴S) and the role of the ionic N₂O⁺ intermediate have been investigated by ab initio calculations. The ⁴A″ PES, which correlates with the ground state educts, has a barrier of about 1 eV, and therefore at low collision energies the reaction cannot take place adiabatically on this surface. However, the spin-orbit coupling in the entrance channel allows the system to pass into the Renner-Teller system of the X² Π electronic ground state of the N₂O⁺ intermediate. The reaction then proceeds on these surfaces up to the region in the exit channel where a similar coupling allows it to reach the product quartet asymptote. At collision energies higher than about 1 eV, the reaction proceeds mainly on the adiabatic PES of the ⁴A″ state. The A²Σ⁺ state of N₂O⁺ predissociates via a vibronic coupling with the B²Π state, and in bent structures via a spin-orbit coupling with the ⁴A″ component of the ⁴II state. The electronic structure of the B²Π state is found to be of crucial importance for the understanding of the reactive processes in low lying electronic states of N₂O⁺.     

Theoretical study of the LiNa molecule beyond the Born–Oppenheimer approximation: adiabatic and diabatic potential energy curves,radial coupling,adiabatic correction,dipole moments and vibrational levels

ABSTRACT

The adiabatic potential energy curves for ground and many excited states of ^{1, 3}Σ⁺, ^1,3Π, ^1,3Δ symmetries of the LiNa molecule have been performed. We have used an ab initio approach based on non-empirical pseudopotentials, parameterised l-dependent polarisation potentials and full configuration interaction calculations. In addition, the adiabatic potential energy curves determined in our previous work [Mabrouk and Berriche, J. Phys. B: At. Mol. Opt. Phys. 41, 155101 (2008).] are corrected by using a diabatisation procedure, based on the effective Hamiltonian theory and an effective metric. The diabatic permanent moments for first 10 ¹Σ⁺ electronic states show linear behaviours, especially at intermediate and large distance. The transition dipole moment between neighbour states has revealed many peaks located around the avoided crossing positions. The radial coupling between the adiabatic states was calculated using the Hellmann-Feynman formula and numerical differentiation of the rotation matrix. The first and the second derivatives revealed many peaks, associated to neutral-neutral and ionic-neutral crossings. Furthermore, the radial coupling is used to evaluate the adiabatic correction, which is found to be of an order of tens and hundreds of cm^?1, especially of higher excited states. In addition, we have determined the vibrational level spacing for all studied states.     

Investigation of the electronic structure of Be2+He and Be+He,and static dipole polarisabilities of the helium atom

The potential energy and spectroscopic constants of the ground and many excited states of the Be⁺He van der Waals system have been investigated using a one-electron pseudo-potential approach, which is used to replace the effect of the Be²⁺ core and the electron-He interactions by effective potentials. Furthermore, the core–core interactions are incorporated. This permits the reduction of the number of active electrons of the Be⁺He van der Waals system to only one electron. Therefore, the potential energy of the ground state as well as the excited states is performed at the SCF level and considering the spin–orbit interaction. The core–core interaction for Be²⁺He ground state is included using accurate CCSD (T) calculations. Then, the spectroscopic properties of the Be⁺He electronic states are extracted and compared with the previous theoretical and experimental studies. This comparison has shown a very good agreement for the ground and the first excited states. Moreover, the transition dipole moment has been determined for a large and dense grid of internuclear distances including the spin orbit effect. In addition, a vibrational spacing analysis for the Be²⁺He and Be⁺He ground states is performed to extract the He atomic polarisability.     

Fourier transform spectroscopy of CrO: Rotational analysis of the A5Σ-X5Π (0,0) band near 8000 cm−1

The emission spectrum of CrO has been investigated by Fourier transform spectroscopy in the near infrared. New weak electronic bands have been found in the 6000- to 10 000-cm^?1 region, the strongest of which, near 8000 cm^?1, is shown to be the (0,0) band of a ⁵Σ-⁵Π transition where the ⁵Π lower state is the ground state. Fifty branches have been assigned in this band, which have permitted the first detailed characterizations of quintet electronic states in the gas phase. Accurate values have now been obtained for the spin-orbit coupling and Λ-doubling intervals in the ground state (which could only be estimated in the previous laser-induced fluorescence work in the visible region by Hocking et al. [Canad. J. Phys.58, 516–533 (1980)]). The relative branch intensities are not consistent with those calculated for a pure ⁵Σ-⁵Π(a) transition, and indicate considerable spin-orbit contamination such that there are interference effects between two or more competing transition moments. It is not known whether the ⁵Σ excited state is ⁵Σ⁺ or ⁵Σ^?.     

Calculation of potential energy curves for Rb2 including relativistic effects

All-electron relativistic calculations have been performed on the Rb₂ molecule. The molecular orbitals are optimized within a spin-free no-pair Hamiltonian formalism and spin-orbit coupling is treated using quasi-degenerate perturbation theory. Potential curves of the ground state and several excited states are calculated, and the spectroscopic constants T _e, D _e, R _e and ω_e are in good agreement with experimental values. The spin-orbit splittings at the 5p and 6p asymptotic limits are found to be underestimated by about 30%. Large perturbations in the spectra from the 1¹Σ⁺ _u(A) state are predicted due to an avoided crossing with a 1 ³Π_ub state caused by spin-orbit interaction. The predissociation dynamics of the 2 ¹Π_uC and 3 ¹Π_uD states is discussed. The calculations support the observation that a (1) ³ Δ_u state causes the fast predissociation of the ^{3 1}Π_uD state but rule out the (2)³Σ⁺ _u state as causing the slow predissociation at the lower part of the 3 ¹Π_uD potential energy curve.