Ab initio study of the ground and excited states of hydrogen sulphide using SCF and CI calculations

Results from ab initio SCF and CI calculations on the ground state and low-lying valence and Rydberg states of H₂S are reported. A double ξ basis of contracted gaussian functions augmented by polarization and diffuse 3d, 4s and 4p functions is used for the calculations. The geometries of various excited states are studied by means of SCF calculations. The first observed band in the absorption spectrum is predicted to arise from the overlapping of transitions from the 2b₁ orbital to a Rydberg 4s and strongly bent valence upper state. The calculations support the assignment of other spectral features to transitions from the 2b₁ to components of the Rydberg 3d and 4p orbitals.     

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.     

Two-photon vibronic spectroscopy of allene at 7.0–10.5 eV: experiment and theory

2?+?1 resonance-enhanced multiphoton ionization (REMPI) spectra of allene at 7.0–10.5?eV have been observed. The excited vibronic symmetry has been determined from polarization-ratio measurements. Based on the vibronic energies and peak intensities calculated using ab initio MO and time-dependent density functional theory, the very congested REMPI spectra have been assigned as due to π*?←?π, 3p?←?π, 4s?←?π, 4p?←?π, and 4d?←?π transitions. Vibrational progressions related to the CH₂ twisting (ν₄ ～770?cm^?1) have been observed for several excited electronic states. Calculated Franck–Condon factors also confirm that CH₂ twisting is the most active mode in the vibronic spectra of allene. In this study, theoretical calculations of two-photon intensities and polarization ratios have been made through the ab initio computed one-photon transition dipole moments to various electronic states as intermediates. As a starting point to interpret the complicated vibronic spectrum of allene, the theoretical approach, without vibronic couplings, has been applied to predict the peak positions, spectral intensities, and polarization ratios of Rydberg states, and qualitatively shows a considerable agreement with experimental observations.     

A numerical study of the convergency of second and approximate second-order multiconfiguration Hartree-Fock procedures

A detailed numerical study has been made of the convergency of second-and approximate second-order multiconfigurational Hartree-Fock procedures. Calculations were performed on the excited 2p ^{2 1} S state of Be and on the lowest states of ³Σ _g ^-, ¹δ _g , ¹Σ _g ⁺, ¹Δ _g and ³Δ _g symmetry in O₂. The O₂ calculations included all configurations that could be formed from doubly occupied core orbitals with eight electrons in the valence orbitals, 3σ _g , 1π _u , 1π _g and 3σ _u . All second-order calculations converged in between 4 and 6 iterations even for a case where approximate second-order procedures did not converge.     

Theoretical study of nonadiabatic interactions,radiative lifetimes and predissociation lifetimes of excited states of BH

Ab initio multireference configuration interaction calculations have been carried out on the lower-lying electronic states of BH and their nonadiabatic interactions. The ab initio data have been included in subsequent calculations involving solution of the complex eigenvalue Schrödinger equation to determine predissociation widths and lifetimes of vibrational-rotational levels of these states. Secondly, previously calculated ab initio data on the Rydberg states and their nonadiabatic interactions have been included in multi-state vibrational calculations on the 3p and 3d complexes in BH and BD. The results are in good agreement with the experimental analysis of the 3p and 3d spectra in BH and BD. Furthermore, interaction of the 3d states with the neighbouring 4s state is also found to be important.     

Theoretical study of low lying electronic states of GdO

Low lying electronic states of GdO have been investigated by complete active space SCF (CASSCF) and multireference singly and doubly excited configuration interaction (MRSDCI) calculations using the model core potential (MCP) method. The 4f electrons of Gd were included explicitly in the valence space. Relativistic effects were incorporated in the MCP and basis sets for Gd at the level of Cowan and Griffin's quasirelativistic Hartree—Fock method. The ⁹Σ^? state (f⁷σ) was the ground state, and excited states, ⁹Δ, ⁹Π, 2⁹Σ^?, ⁷Σ^?, ⁷Δ, ⁷Π, and 2⁷E^?, lay between 0 ～ 22 300 cm^?1. The energy separations for these states agreed well with available experimental values. Calculated GdO bond lengths and vibrational frequencies for these states are in the ranges of 1.81–1.85 Å and of 800–880 cm^?1, respectively. Mulliken population analysis showed that the gross population of the 4f orbitals was 7.1 e for all these states, and that the 4f electrons were strongly localized on Gd atom. The effective charge distribution was approximated to be Gd⁺O^?. The σ and π bonding orbitals were mainly formed by Gd 5d and O 2p orbitals.     

On the ultraviolet absorption of nitrous oxide and its van der Waals complexes

The possibility of simultaneous characterization of N₂O and N₂O van der Waals complexes in supersonic jet expansions is demonstrated for neat N₂O samples. Room temperature and jet-cooled spectra of the Rydberg transitions 3pπ¹Δ ←← X¹Σ⁺ and 3pσ¹Π ←← X¹Σ⁺ are presented. Theoretical calculations support the assignment of the Rydberg transitions used. For stagnation pressures above 2 bar, a hitherto unreported broad spectral feature at 84 650 cm⁻¹ is observed, where calculations predict absorption of the (N₂O)₂ dimer essentially due to excitation of the 4sσ and 3pπ Rydberg orbitals. Consequences for the use of N₂O as a precursor for O(¹D) atoms in reactive scattering experiments are discussed.     

Low resolution VUV absorption spectrum of boron monoxide

The absorption spectrum of BO has been recorded between 2400 and 300 Å on the SA61 beam line of SUPERACO in Orsay. A new electronic transition has been observed and vibrationally analysed. Nine Rydberg states have been observed and classified into Rydberg series giving two ionization limits at 12.849eV for the X ¹Σ⁺ ionic state and at 13.117eV for the A ¹II state of BO⁺. Two ionization limits have been observed at 754 and 374 Å. The first of these corresponds to the B ¹Σ⁺ state and the last one to the C ¹Σ⁺ state of BO⁺. The new results are compared with the ab initio calculations of Karna and Grein concerning BO and BO⁺. The agreement is satisfactory.     

The electronic states of carbon monofluoride: Rydberg states

Hartree-Fock and multiconfiguration Hartree-Fock calculations are reported on some low-lying Rydberg states of CF and the ground state of CF⁺. For the CF⁺ ground state, ¹Σ⁺, the calculations give a bond length of 1.55 Å, a fundamental frequency of 1821 cm^?1, and a dissociation energy of 6.9 eV. Many interactions between the valence and Rydberg state manifolds are revealed. Also a strong mixing of the 3dσ and 4sσ components due to an accidental degeneracy is described.     

Rydberg states of the HeH+ ion calculated by ab initio methods: potential energies and effective principal quantum numbers

More than 80 excited electronic states of the hydrohelium ion HeH⁺ of ^{1, 3}Σ⁺, ^{1, 3}Π, ^{1, 3}Δ, ^{1, 3}Φ and ^{1, 3}Γ symmetry have been calculated ab initio up to n = 6 for internuclear distances ranging from 0.5 to 100 bohr. The computations involve a configuration interaction (CI) treatment based on a home-made suite of programs that uses special basis sets designed for the representation of molecular Rydberg states. The results are compared with previous computations where these are available (up to n = 4), and it is found that except for the very lowest excited states, the present energies are consistently lower than those obtained previously, with an average lowering corresponding to several hundred cm^?1. It is shown that with the exception of its ground state, HeH⁺ is an effective one-electron system having an overall electronic structure similar to H⁺₂. The interaction of the excited electron with the He⁺ 1s core electron causes small singlet–triplet splittings to appear and ?-mixing interactions to occur, that are not present in H⁺₂.     

Electronic structure and reactivity of the CNO/NCO/CON isomers

Three-dimensional potential energy surfaces (PESs) have been calculated for the lowest electronic states of NCO, CNO and CON isomers, using internally contracted Multi Reference Configuration Interaction (MRCI) and Coupled-Clusters RCCSD(T) ab initio methods. For the low lying doublet and quartet excited states of the three isomers, the N–CO, O–CN, C–NO and C–ON collinear dissociation paths were mapped by the Complete Active Space SCF (CASSCF) approach and the energy variations with the bending coordinate have been explored. Several regions of conical intersections have been located and the spin–orbit interactions between states of different spin symmetry have been evaluated in the region of intersections of these states. The analysis of the PESs allows one to identify the main interactions governing the reactivity of the lowest electronic states. The NCO and CNO isomers have stable X²Π electronic ground states, for CON the X²Π ground state is separated from the dissociative [CO?+?N] asymptote by a barrier of 0.11 eV and crosses the dissociative ⁴Σ ^- state close to its minimum. At their equilibrium ground state geometries the spin–orbit interactions A _SO between the two electronic components of the X²Π states were calculated to be -95.6, -109.6 and -57.1 cm^?1 for NCO, CNO and CON, respectively. The predissociation of the vibrational levels of the A²Σ⁺ and B²Π states of NCO has been explained.     

CASPT2 and CCSD(T) calculations of dipole moments and polarizabilities of acetone in excited states

The acetone molecule is investigated in its ground state and valence ^1,3n-π*, ^1,3π-π*, and ^1,3σ-π* excited states and Rydberg ^1,3n-3s, ^1,3π-3?, ^1,3n-3p_y and ^1,3π-3p_y states using the CASSCF, CASPT2, and CCSD(T) methods. Equilibrium geometries of excited states are obtained and their changes with respect to the ground state are discussed. For most excited states the C_2v symmetry of the ground state is lowered to the C_s symmetry. A series of valence vertical and adiabatic excitation energies is presented along with excitation energies for Rydberg states. The main body of the paper contains Finite-Field Perturbation Theory (FFPT) calculations of electric properties of the vertically as well as geometry relaxed excited states. Dipole moments of valence excited states decrease significantly upon excitation, being about one half of the ground state dipole moment. Polarizabilities usually change upon excitation much less (increase by about 30%) but hyperpolarizabilities are enhanced up to one or two orders of magnitude. The orientation of the dipole moment is reversed in some vertically excited Rydberg states. Properties of the ground and excited states are discussed considering alterations of the electronic structure and shifts in the geometry.     

The electronic spectrum of PN. A configuration interaction study

Potential energy curves were calculated for the ground state of PN and for all excited singlet and triplet states resulting from the 2π → 3π, 7σ → 3π, 2π → 8σ, and 7σ → 8σ orbital excitations. CI studies at 4 Å served to establish dissociation energies. Spectroscopic constants were calculated, and are in good agreement with those of the known X¹Σ⁺ and A¹Π states. Overall, their similarity with those observed for N₂ is striking. Various states considered to perturb the known excitations are discussed. The recently discovered second ¹Σ⁺ state is included.     

Rydberg states of SiBr

New uv absorption spectra have been observed for SiBr. Five Rydberg states are identified to the states $\text{(4sσ)}{}^2\text{Σ}{}^{\mathrm{+}}$, $\text{(5sσ)}{}^2\text{Σ}{}^{\mathrm{+}}$, $\text{(4pπ)}{}^2\text{Π}$, $\text{(3dπ)}{}^2\text{Π}$, and $\text{(3dδ)}{}^2\text{Δ}$ by comparison with SiF and SiCl. The ionization potentials of SiCl and SiBr have been determined for the first time, and were 6.82 and 6.67 eV, respectively.     

The electronic structure of the low lying sextet and quartet states of CrF and CrCl

The electronic structure of CrF and CrCl in X ⁶Σ⁺, ⁶Π, ⁶Δ, A⁶Σ⁺, ⁴Σ⁺, ⁴Π, and ⁴Δ states that correlate with the low lying ⁶S, ⁶D, and ⁴D states of Cr⁺ have been studied, using large atomic natural orbital (ANO) basis sets and a variety of ab initio methods, including multi-reference configuration interaction (MRCI) and coupled cluster with perturbative triples (RCCSD(T)). We include scalar relativistic effects perturbatively and also explore the consequence of correlating the 3s and 3p electrons on the transition metal. We report T _e, R _e,ω_e, as well as dipole moments, bond energies, and charge distributions and compare with the available experimental data as well as previous theoretical results.     

The ground and low-lying excited potential curves of SO

A method is presented for approximating the effect of core electrons by a pseudopotential which is an extension of one previously presented by Dixon and Hugo. The pseudopotential is constructed in a fully ab initio manner from atomic SCF calculations. It is non-local in both radial and angular coordinates, but its matrix elements are none the less easy to evaluate. The method has been implemented within a multi-structure valence-bond framework. The approximations arising from the use of finite basis sets, both for the pseudopotential and for the valence wavefunction, inevitably lead to errors in calculated energies. However, these errors are largely atomic in origin. Thus, in addition to ab initio calculations we also use empirical atomsin-molecules corrections to minimize both basis set errors and atomic correlation errors. These method are applied to potential curves for 21 electronic states of the SO molecule. Comparison is made of the curves calculated using the ab initio multi-structure valence-bond method without and with the atoms-in-molecules corrections. The potential energy curves of three, previously unobserved, bound electronic states of SO are calculated. We estimate that these states, ¹Σ^-, ³Δ and ³Σ⁺, lie in the region of 3·2 to 3·4 eV above the ground state.     

Anomalous isotope effects in indirect predissociations: An example in the spectrum of BeH

Measurements of the linewidths in the (3pπ) B²Π state of BeH and BeD which is predissociated by a ²Σ⁺ state show an anomalous isotope effect. Ab initio calculations of the five lowest excited ²Σ⁺ states of BeH show that the first excited ²Σ⁺ state is stable at a large inter-nuclear distance and is responsible for the observed predissociation. The anomalous isotope effect is explained by an indirect mechanism of predissociation involving this ²Σ⁺ state and the (3pσ) ²Σ⁺ state which is separated from the (3pπ) B²Π state by an energy of the order of magnitude of ω_e. A new method to calculate the widths and the shifts of predissociated levels is presented and calculations in the case of BeH and BeD give good agreement with the experimental data.     

Absorption spectrum of CO in the Hopfield helium continuum region: Rydberg bands converging to the X2Σ+ state of CO+ in the region 960–1080 Å

The high resolution absorption spectrum of CO has been reinvestigated in the Hopfield helium continuum region, particularly from 960 to 1080 Å. The Rydberg state 3pπE¹Π was extended to v = 2, and other Rydberg states, $\text{3dσ}{}^3\text{Σ}{}^{\mathrm{+}}$ and F¹Σ⁺, $\text{3dπ}{}^1\text{Π}$, and $\text{4sσ}{}^3\text{Σ}{}^{\mathrm{+}}$ and ¹Σ⁺, which are converging to the X²Σ⁺ ground state of CO⁺, have been identified. The rotational structures of only five bands among the observed ten Rydberg bands have been analyzed.