Lowest Dissociation Limit Research Articles

Efficiency of charge transfer in changing the dissociation dynamics of OD+ transients formed after the photo-fragmentation of D2O.

We present an investigation of the relaxation dynamics of deuterated water molecules after direct photo-double ionization at 61eV. We focus on the very rare D+ + O+ + D reaction channel in which the sequential fragmentation mechanisms were found to dominate the dynamics. Aided by theory, the state-selective formation and breakup of the transient OD+(a1Δ, b1Σ+) is traced, and the most likely dissociation path-OD+: a1Δ or b1Σ+ → A 3Π → X 3Σ- → B 3Σ--involving a combination of spin-orbit and non-adiabatic charge transfer transitions is determined. The multi-step transition probability of this complex transition sequence in the intermediate fragment ion is directly evaluated as a function of the energy of the transient OD+ above its lowest dissociation limit from the measured ratio of the D+ + O+ + D and competing D+ + D+ + O sequential fragmentation channels, which are measured simultaneously. Our coupled-channel time-dependent dynamics calculations reproduce the general trends of these multi-state relative transition rates toward the three-body fragmentation channels.

Exploring the low-lying electronic states of AsO including spin–orbit interaction

In this paper, the multireference configuration interaction method (MRCI) combined with the Davidson correction (+Q) is used to study the high-level ab initio calculation of AsO electronic states. The potential energy curves (PECs) of 27 Λ-S states and 18 Ω states related to the three lowest dissociation limits are obtained. According to these PECs, the spectroscopic constants of Λ-S and Ω states can be calculated, which is little different from the previous experimental values. The dipole moments (DMs) of the Λ-S state is also calculated. In addition, the transition properties of several transitions are calculated, including the transition dipole moments (TDMs), Franck-Condon factors (FCFs) and spontaneous radiative lifetimes of Λ-S state. This work should improve our understanding of the electronic structure and spectroscopic information of AsO.

Exploring the excited states of the GeH+ radical cation including spin-orbit interaction: A revisited study

In this paper, we have carried out high-level ab initio calculations on the electronic states of GeH+ with the configuration interaction method. The spin-orbit coupling (SOC), core-valence correlation (CV), scalar relativistic effects and Davidson correction (+Q) are included. The potential energy curves (PECs) of 13 Λ-S correlated with the four lowest dissociation limits and 32 Ω electronic states generated from those Λ-S states are obtained. Our results indicate that the first 3Σ− and second 3Π states are adiabatically correlated with the dissociation limit Ge(3Pg) + H+(1S), which is different from the previously reported Ge+(4Pg) + H(2Sg). From the computed PECs, the spectroscopic constants of the Λ-S and Ω states are determined, which are in good agreement with previous experiments. The dipole moments (DMs) for Λ-S electronic states are also investigated. With the help of spin-orbit coupling matrix involving the 13Σ− and 23Π states, the intricate couplings related with the crossing states are revealed, and the weak predissociation for ν' ≥0 vibrational levels of 13Σ− and the perturbations for vibrational levels of 21Σ+ (ν' ≥1) and 11Δ (ν' ≥0) states are analyzed. Finally, the transition properties of five transitions are predicted, including the Franck-Condon Factors (FCFs), transition dipole moments (TDMs), and the spontaneous radiative lifetimes of lower vibrational states. This study will improve our comprehension on the detailed electronic structure and spectroscopy of GeH+ radical cation.

Ab initio calculation on spectroscopic properties and radiative lifetimes of low-lying excited states of NaK

We performed high-level ab initio calculations on electronic structure of NaK. The potential energy curves (PECs) of 10 Λ-S states correlated with the three lowest dissociation limits have been calculated. On the basis of the calculated PECs, the spectroscopic constants of the bound Λ-S states are obtained, which are in good agreement with experimental results. The maximum vibrational quantum numbers of the singlet ground state X1Σ+ and the triplet ground state a3Σ+ have been analyzed with the semiclassical scattering theory. Transition properties including transition dipole moments, Franck-Condon factors, and radiative lifetimes have been investigated. The research results indicate that such calculations can provide fairly reliable estimation of parameters for the ultracold alkali diatomic molecular experiment.

Spectroscopic investigations of transition properties for the electronic states of PN+ correlating to two lowest dissociation limits

The PN+ cation is regarded as a candidate of ions which may exist in the interstellar medium. Accurate transition properties are desired to identify the spectra from astronomical observation. However, transition data of PN+ are relatively limited. In this work, the state-of-the-art ab initio calculations are carried out to investigate the low-lying electronic states of PN+ correlating to two lowest dissociation limits, its transition properties and its spin-orbit couplings. For the X2Σ+ and A2Π states, the calculated spectroscopic constants agree well with previous experimental data. For the other electronic states, the accuracy of the spectroscopic parameters is also ensured by comparing with previous theoretical results. Then, transition probabilities of dipole allowed transition systems are calculated. The radiative lifetimes of vibrational levels for the A2Π, 14Π, 14Δ, 14Σ−, and 24Π states are also evaluated. Such transition data can be used in the identification of PN+ in the laboratory or the interstellar space. Finally, the predissociation of 14Σ− (υ' = 1)→16Σ+, 14Σ− (υ' = 15)→16Π and 14Δ (υ'= 19)→16Π are analyzed with the help of spin-orbit coupling integrals.

Radiative transition probabilities between low-lying electronic states of N2

ABSTRACTThis work mainly investigates the transition dipole moments (TDMs) and radiative transition probabilities of dipole-allowed transitions between the , , , , , , , , and states of N2. Many of these transition properties are previously unknown. For completeness, another 14 electronic states that correlate to four lowest dissociation limits are also calculated. The potential energy curves (PECs) are calculated at the valence internally contracted multireference configuration-interaction (icMRCI) level of theory, along with the Davidson correction, the core-valence (CV) correction and the scalar relativistic correction, as well as the basis-set extrapolation. These corrections, especially the CV correction, greatly improve the accuracy of the PECs, as shown by the excellent agreement of the fitted spectroscopic parameters with the available experimental data. In order to verify the accuracy of transition properties, we calculate the Einstein coefficients of the extensively studied band transition systems and compute the radiative lifetimes of states, which are in good agreement with the experimental data. Similar accuracy can be assumed for the previously undetermined band transition systems. The large Einstein coefficients of these band systems can provide guidelines for observing such newly predicted band transitions in the appropriate spectroscopy experiments.

Electronic and vibrational spectroscopy of the low-lying states of potassium mono-sulphide KS, and comparison in the series of MS (M = Li, Na, K, Rb, Cs)

ABSTRACTPotential energy curves (PECs) of the lowest electronic states of the potassium mono-sulphide KS have been determined with highly correlated ab initio calculations, using internally contracted multi reference interaction configuration methods including Davidson correction (MRCI + Q) with and without considering spin-orbit effects. For the three low-lying bound states, we report a set of spectroscopic parameters including equilibrium distances, dissociation energies, vibrational and rotational constants. The effects of spin-orbit-induced changes on these parameters are also discussed. An analysis of the properties of the three bound states, X2Π, 12Σ+ and 22Π, illustrates the common characteristics of the whole series of compounds in the MS family (M = Li, Na, K, Rb, Cs). Indeed, the shapes of the PECs of these bound states are strongly affected by the interactions between the two ionic states, 2Σ+ and 2Π, correlating at large internuclear separations (RMS) to the first ionic dissociation limit [M+ + S−] and the electronic states correlating to the three/four lowest dissociation limits. The spectroscopy of these low-lying electronic states and the lifetime of their vibrational levels are thus affected by the spin-orbit interactions which are mainly related to the S atom and consequently common to all alkali-metal mono-sulphides.

Multi reference studies of gas phase vanadium nitride di- and trications

Using ab initio configuration interaction methodologies, we compute the potentials of the doubly and triply charged vanadium nitride. By performing benchmark calculations, we show that the 4s and 3d AOs are enough for the accurate description of VN multiple ionizations. In VN2+, several deep potential wells are located below the lowest dissociation limit. Within these potential wells, long-lived rovibrational levels may be found. Specifically, we find an unconventional shape for the ground state potential of VN2+ that differs from the volcanic form typical of dications. For VN3+, four shallow potential wells were characterized that may support rovibrational levels, where metastable VN3+ can be found. Accordingly, we predict the existence of VN2+ and VN3+ species in the gas phase as already noticed experimentally for the dication VN2+. For the bound states, we provide a set of accurate spectroscopic parameters. Double and triple ionization energies of VN are computed to be 23.56 and 51.05 eV, respectively.

MRCI+Q study of the low-lying electronic states of CdF including spin–orbit coupling**Project supported by the National Natural Science Foundation of China (Grant Nos. 11604052, 11404180, and 11574114 ), the Natural Science Foundation of Heilongjiang Province, China (Grant No. A2015010), the Natural Science Foundation of Anhui Province, China (Grant No. 1608085MA10), the International Science & Technology Cooperation Program of Anhui Province,

CdF molecule, which plays an important role in a great variety of research fields, has long been subject to numerous researchers. Due to the unstable nature and heavy atom Cd containing in the CdF molecule, electronic states of the molecule have not been well studied. In this paper, high accurate ab initio calculations on the CdF molecule have been performed at the multi-reference configuration interaction level including Davidson correction (MRCI+Q). Adiabatic potential energy curves (PECs) of the 14 low-lying Λ–S states correlating with the two lowest dissociation limits and have been constructed. For the bound Λ–S and Ω states, the dominant electronic configurations and spectroscopic constants are obtained, and the calculated spectroscopic constants of bound states are consistent with previous experimental results. The dipole moments (DMs) of are determined, and the spin–orbit (SO) matrix elements between each pair of are obtained. The results indicate that the sudden changes of DMs and SO matrix elements arise from the variation of the electronic configurations around the avoided crossing region. Moreover, the Franck–Condon factors (FCFs), the transition dipole moments (TDMs), and radiative lifetimes of low-lying states-the ground state are determined. Finally, the transitional properties of and are studied. Based on our computed spectroscopic information of CdF, the feasibility and challenge for laser cooling of CdF molecule are discussed.

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

ABSTRACTMultireference 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.

Configuration interaction study on electronic structures and transitional properties of excited states of GeO molecule

GeO molecule, which plays an important role in fabricating integrated optics and semiconductor components, has received much attention. However, the electronic state density of the molecule is very large, and the electric structures and transitional properties of the molecule have not been well investigated. In this work, the 18 Λ -S states correlated to the lowest dissociation limit (Ge(3Pg)+O(3Pg)) are calculated by a complete active space self-consistent field (CASSCF) method, through using the previous Hatree-Fock molecular orbitals as the starting orbitals. Furthermore, we take all configurations in the configuration interaction expansions of the CASSCF wave functions as a reference configuration, and calculate the energies of the 18Λ-S states by a high-level multireference configuration interaction method. The core-valence correlation effect of the 3d orbit of Ge atom, the scalar relativistic effect, and the Davidson correction are taken into consideration in the calculations. On the basis of the calculated potential energy curves of the bound and quasibound electronic states, the spectroscopic constants (Re, Te, ωe, ωeχe, and Be), vibrational energy levels, vibrational wave functions, and Franck-Condon factors (FCFs) are obtained by solving the radical Schrödinger equation. The computed spectroscopic constants of these electronic states are well consistent with previously available experimental results. We calculate the electric dipole moments of electronic states with different bound lengths, and analyze the influences of the variation of electron configuration on the electric dipole moment. The calculated potential energy curves indicate that the adiabatic transition energies of A1Π, 11Σ-, D1Δ, a3Π, a’3Σ+, d3Δ, and e3Σ- sates are located in a range of 26000-37000 cm-1, and the spin-orbit coupling of the states can obviously affect the corresponding vibrational wave functions. With the help of calculated spin-orbit coupling matrix elements, the perturbations of the nearby states to a3Π and A1Π are discussed in detail. Our calculation results indicate that the spin-orbit coupling between A1Π and e3Σ- states has an evident perturbation on the v’> 4 vibrational levels of A1Π, and the v’≥ 0 vibrational levels of a3Π state are perturbed by the crossing states a’3Σ+, d3Δ, e3Σ-, 11Σ-, and D1Δ. On the basis of computed transition dipole moments and FCFs of A1Π-X1Σ+ and A’1Σ+-X1Σ+ transitions, the radiative lifetimes of the six lowest vibrational levels of the two singlet excited states are computed.

Theoretical calculations on the NaS and NaS+ radicals: electronic structure, spectroscopy and spin-orbit couplings

We used an ab initio methodology for the computation of the potential energy curves of the lowest electronic states of NaS and NaS+ diatomics. Using these highly correlated wavefunctions, we calculated their spin-orbit couplings. The X2Π and the A2Σ+ electronic states of NaS are found to possess deep potential wells. The upper bound electronic states have either shallow potential wells located in the molecular region or potentials of ∼ 1 eV depth located for large NaS distances. For NaS+, our calculations reveal the existence of shallow potentials for all states correlating to the four lowest dissociation limits. Using our potentials, we calculated an accurate set of spectroscopic constants for NaS and NaS+. Most of the data relative to the electronic excited states represent predictions. The spin-orbit induced predissociation of NaS(A2Σ+ is discussed. Finally, our cationic potentials and the NaS(X2Π) potential were used for the prediction of the single ionization spectrum of NaS.

Theoretical investigations of the MgO+ cation: spectroscopy, spin–orbit coupling and single ionization spectrum

We used ab initio methodology for the computation of the potential energy curves of the doublet and quartet electronic states of the MgO+ cation correlating with the four lowest dissociation limits. Using these highly correlated wavefunctions, we calculated their spin–orbit coupling and transition moments. We deduced an accurate set of spectroscopic constants for this cation. Most of the data relative to the electronic excited states of this cation are predictive in nature. Moreover, our potentials, transition moments and spin–orbit coupling evolutions are incorporated into Fermi golden rule calculations to deduce the natural lifetimes of the lowest MgO+(12Σ+) ro-vibrational levels. Finally, our potentials were used for the prediction of the single ionization spectrum of MgO.

Anharmonic analysis of the PFI-ZEKE photoelectron spectrum of CH2F2 from the ionization potential to the onset of fragmentation

The complete vibrational fine structure of the pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of difluoromethane is analysed in terms of fully coupled anharmonic calculations. Nine-dimensional ab initio potential functions complete up to five-body terms are constructed directly from ab initio calculations using Møller–Plesset and coupled cluster theory together with up to quadruple-ζ basis sets. The onset of dissociation is accompanied by strongly anharmonic vibrational dynamics involving resonant interactions between both symmetric and non-symmetric modes. The detailed analysis leads to a partial revision of earlier interpretations and provides improved theoretical values for the adiabatic ionization potential IP=12.740 eV and for the lowest dissociation limit both within about 1 kJ mol–1 of experimental estimates.

Theoretical spectroscopy and metastability of BeS and its cation

Multiconfiguration self-consistent field and multiconfiguration reference interaction including the Davidson’s correction techniques were employed to calculate the potential energy curves (PECs) of the BeS/BeS + electronic states correlating to the 4/5 lowest dissociation limits. After nuclear motion treatment, we deduced reliable spectroscopic data for the neutral and cationic bound states. For BeS, the transition moments and spin-orbit couplings were also evaluated and used later with the PECs to deduce the rovibronic transition probabilities and the radiative lifetimes in the low-lying states, and to investigate the unimolecular decomposition processes of BeS (X 1Σ +, A 1Π, 3Σ + and B 1Σ +) leading to Be( 1S g) + S( 3P g). The prominent mechanism is a spin-orbit induced predissociation via the repulsive BeS(1 3Σ −) state. Finally, we give the single ionization spectrum of BeS (X 1Σ +) populating the BeS + (X 2Π, 1 2Σ −, 1 2Σ +, 1 2Δ, 2 2Σ +, 2 2Π and 3 2Π) electronic states. The adiabatic ionisation energy of BeS is estimated to be ∼9.15 eV.

Velocity map imaging studies of the photodissociation of H2O+ cations

The near-threshold photofragmentation dynamics of state-selected H2O+ cations have been investigated using velocity map ion imaging methods. The cations were prepared in the v = 0 level of the ground ( 2B1) electronic state, by 2 + 1 resonance enhanced multiphoton ionization via selected rotational levels of the H2O, 1B1, v = 0 state. Subsequent two photon excitation of the resulting H2O+ cations to 2B2 state levels lying above the lowest dissociation limit (i.e. at total energies in the range 46000–50600 cm−1) results in O–H bond fission and OH+ fragment ion formation. These fragments display isotropic recoil velocity distributions, which peak at low kinetic energy but extend to the highest speeds allowed by energy conservation. Ab initio calculations of key sections through the potential energy surfaces (PESs) for the ground and first few excited states of H2O+ suggest two possible mechanisms for the observed rotational and (when energetically allowed) vibrational excitation of the OH+ fragments. Both require initial non-adiabatic (vibronic coupling) from the photo-prepared state level to high levels of the 2A1 state, but involve different subsequent HO+−H bond fission mechanisms. One involves Renner–Teller coupling to the ground state PES, while the alternative requires spin-orbit induced coupling to the repulsive 4B1(4A″) state PES.

Approximate calculation of anharmonic densities of vibrational states for very large molecules

A fast linear-scaling method for calculating the density of vibrational states for a system of coupled anharmonic oscillators, applicable to very large molecules, is presented. Instead of discrete counting of molecular eigenstates, convolution of one-dimensional densities of states is performed in order to obtain the numbers and densities of states as a function of total internal energy. An efficient numerical integration scheme over an n -simplex is used. The density of states is given as the harmonic density multiplied by a correction factor that exclusively depends on dissociation energies and the bonding topology of the molecule. While the harmonic contribution to the density of states relies on the usual normal-mode picture, local modes are employed to account for anharmonicity and intermode coupling. Mode assignment problems encountered in previous models are absent due to permutational invariance of the integrand in the numerical integration. One-dimensional densities of states for coupled systems are defined as partial derivatives. Stretch-bend coupling is accounted for through the empirical model proposed by Troe [J. Troe, Chem. Phys. 90 (1995) 381]. The proposed method reproduces very well the results obtained from discrete counting for triatomic molecules. Extension to energies beyond the first dissociation limit is possible. Calculations on very large alkanes indicate anharmonic corrections to the number and density of states in the order of 2 at the lowest dissociation limit, while explicit consideration of hindered rotations increases this value to about 4. These large corrections have far-reaching consequences for all processes that directly depend on the density of vibrational states.

A study of the isomerization and dissociation of formal [acetone–methanol]+· ion–molecule complexes

The energy barrier for the keto–enol isomerization of the isolated acetone ion to its distonic (enol) isomer lies above its lowest dissociation limit and so the spontaneous isomerization can never be observed. Keto–enol isomerizations can be catalyzed within appropriate ion–molecule complexes. The present study involved two systems, [(CH3)2C=O···H+···O(H)CH2·] (ion 1) and [(CH3)2C=O···H+····OCH3] (ion 2), in both stable and metastable adducts. When acetone is bound to ·CH2OH though a proton bridge, shown as ion 1, an enol acetone ion is produced. This reaction results from a proton attaching to the acetone, which then gives an H· atom back to the radical site by a 1,6-H transfer, involving a transition state of low energy requirement. In contrast, when the acetone is protonated and bound to the radical CH3O· (ion 2), the above rearrangement does not take place. The metastable complex ion 2 loses a methyl radical, producing a new [C3H7O2]+ isomer of structure [CH3C+(O)···(H)OCH3]. Tandem mass spectrometry combined with ab initio calculations were used to investigate the two systems. Potential energy surface diagrams were obtained by calculations at the MP2/6-31+G(d) level of theory to aid further elucidation of the reaction mechanisms. Key words: ion–molecule complexes, keto–enol mechanisms, ion rearrangements and structures.

Theoretical studies of the electronic spectrum of SnSe

Multireference configuration interaction calculations are performed on the SnSe molecule to study its electronic spectrum within 5 eV of energy. Potential energy curves of the ground and low-lying electronic states, which correlate with the lowest dissociation limit, are constructed. Spectroscopic constants ( T e, r e, ω e) of nine bound states of SnSe are reported. The ground-state dissociation energy is estimated to be 4.38 eV. The spin–orbit interaction has also been included in the calculation. Transition dipole moments of several dipole-allowed and spin-forbidden transitions are computed. Transitions such as E 1 Σ 0 + + – X 1 Σ 0 + + and D 1 Π 1 – X 1 Σ 0 + + are found to be more probable. The radiative lifetimes of A 0 + – X 1 Σ 0 + + and B 1 – X 1 Σ 0 + + are predicted to be about 11 and 130 μs, respectively. Dipole moments of the ground and low-lying states are estimated and compared with observed data.

Exploring the potential energy surface of ion–molecule pairs by experiment and by theory: acetaldehyde and methanol

The energy barrier for the keto–enol isomerization of the isolated acetaldehyde ion to vinyl alcohol lies above its lowest dissociation limit and so the spontaneous isomerization is never observed. However, it has been shown that the distonic methanol ion can ionize the acetaldehyde molecule and greatly lower the energy barrier of the keto–enol isomerization. The present study involved two systems in which the acetaldehyde was associated with the distonic methanol cation, CH 2OH 2 +, and with its conventional isomer, CH 3OH + , in stable and metastable adducts. The isomerization barrier for acetaldehyde was lowered by association with CH 2OH 2 +; in contrast, the CH 3OH + ion did not facilitate the rearrangement of acetaldehyde to its enol ion. Tandem mass spectrometry combined with ab initio calculations was applied to investigate the two systems. Potential energy surface diagrams were obtained by MP2/6-31+g(d) and B3-LYP/6-31+g(d) calculations to further elucidate the reaction mechanisms.