Experimental and theoretical studies of the valence-shell dipole excitation spectrum of molecular fluorine

Electron energy loss measurements and concommitant RPAE calculations are reported of the valence-shell dipole excitation spectrum of molecular fluorine. The measured spectrum is dominated by a series of strong features in the 12–16 eV interval which are in accord with X¹Σ_g⁺→¹Σ_u⁺ bands assigned in a previously reported high-resolution optical study. These features are attributed on basis of the present RPAE calculations to configuration mixing between 1π_g→npπ_u Rydberg and 3σ_g→3σ_u intravalence excitations. A depleted X→V_σ charge-transfer excitation is correspondingly observed at ≈17 eV, in good accord with the calculated values. The appearance of the σ→σ* transition in F₂ below the 3σ_g^?1 threshold is in marked contrast to the situation in other light diatomic molecules, in which cases σ→σ* transitions appear as intravalence shape resonances in photoionization continua. Assignments are also provided of weak, irregularly spaced X¹Σ_g⁺→¹Π_u excitations the origins of which are attributed to configuration mixing between 1π_g→npσ_u and 1π_u→nsσ_g Rydberg series.     

Ab initio MRD CI calculations for the electron spectrum of the C3 radical

Calculations using the MRD CI method are reported for the ground and low lying excited states of C₃. Transitions from the 3σ_u, 4σ_g and 1π_u MO's into 1π_g are considered, as well as the 1π_u → 3s Rydberg species and the corresponding ionization, and good agreement with experimental data is obtained where comparison is possible. Potential curves calculated for the ground and (1π_u → 1π_g) ¹Σ⁺_u excited state are discussed.     

The structure of BO2 in its ground and low-lying excited states from ab initio SCF RHF AND CI calculations

Potential curves for the X²Π_g, A²Π_u, B²Σ⁺_u and C²Σ_g⁺ electronic states of BO₂ were calculated at ab initio SCF RHF and configuration interaction (CI) level. The results obtained are consistent with a linear molecular model for all states considered. The calculated structural parameters and transition energies are in good agreement with relevant experimental data.     

A theoretical study of the bound electronic states of the C−2 negative ion

Configuration interaction calculation are employed to study the X ²Σ⁺_g, A ²Π_u, B ²Σ⁺_u, ⁴Σ⁺_u and ⁴Δ_u states of the C^?₂ ion. The results are in good quantitative agreement with experimental findings for the Herzberg—Lagerquist (²Σ⁺_u-²Σ⁺_g) bands and predict a T_e value for the ²Π_u state of only 0.40 eV; corresponding transition moment results are obtained as a function of CC distance. The Cl electron affinity is 3.43 eV, in good agreement with the most recent experimental estimate for this quantity.     

Photoionization of carbon disulfide: theoretical studies of outer-valence-shell partial cross sections

Studies in vertical-electronic, static-exchange approximation are reported of the outer-valence-shell (2π_g⁻¹, 2π_u⁻¹, 5σ_u⁻¹, 6σ_g⁻¹) partial-channel photoionization cross sections of CS₂. The origins of strong features present in the calculated profiles, and in corresponding experimental cross sections, are attributed to the 6σ_u(σ¹) and 7σ_g (σ¹) orbitals of Mulliken on basis of Stieltjes orbital diagnostics.     

Diabatic molecular states and the shielded diatomic orbital method. Evolution of two-interacting state systems for molecular hydrogen

Within Lichten's method, shielded diatomic orbitals (SDO's) are proposed as a basis for building-up diabatic molecular states in the single configurati approximation. The determination of SDO's i.e. eigenfunctions in prolate spheroidal coordinates of the two-center problem with a parametric shielding potential is extended to mono-excited states of many-electron diatomic systems, the shielding potential being obtained from simple electrostatic considerations. Four diabatic molecular states of H₂ are investigated i.e. ¹Σ_g⁺ (2pσ², 1Σ_g⁺ (1s,σ 2sσ), ¹Σ_u⁺(1sσ,4Pσ), ¹Σ_u⁺ (1sσ, 4fσ) using a minimal basis of SDO'S. The dynamical evolution of the nuclei for the two sets ¹Σ_g and ¹Σ_u⁺ of two interacting states is described in both the diabatic and the corresponding adiabatic representation.     

O‐loss photodissociation of the OCS+ ion in the low‐lying electronic states studied using multiconfiguration second‐order perturbation theory

The CASPT2 potential energy curves (PECs) for O‐loss dissociation from the X²Π, A²Π, B²Σ⁺, C²Σ⁺, 1⁴Σ^?, 1²Σ^?, and 1⁴Π states of the OCS⁺ ion were calculated. The PEC calculations indicate that X²Π, 1⁴Σ^?, 1²Σ^?, and 1⁴Π correlate with CS⁺(X²Σ⁺) + O(³P_g); A²Π and B²Σ⁺ correlate with CS⁺(A²Π) + O(³P_g); and C²Σ⁺ probably correlates with CS⁺(X²Σ⁺) + O(¹D_g). The CASSCF minimum energy crossing point (MECP) calculations were performed for the C²Σ⁺/1⁴Σ^?, C²Σ⁺/1⁴Π, A²Π/1⁴Σ^?, A²Π/1²Σ^?, A²Π/1⁴Π, and B²Σ⁺/1²Σ^? state pairs and the spin‐obit couplings were calculated at the located MECPs. A conical intersection point between the B²Σ⁺ and C²Σ⁺ potential energy surfaces was found at the CASSCF level. Based on our calculations, seven O‐loss predissociation processes of the C²Σ⁺ state are suggested and an appearance potential value of 7.13 eV for the CS⁺ + O product group is predicted. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Potential curves of Li2+ by a local potential method

Local potential calculations have been carried out for the first eight ²Σ_g, ²Σ_u and the first five ²Π_g, ²Π_u states of Li₂⁺. The results indicate the usefulness of calculating highly excited potential curves by a local potential method.     

Franck-Condon Factors for the Transitions N2, C 3πu → B 3πg and CN,B 2 Σ + → X 2 Σ +

Franck-Condon factors for the transitions N₂, C ³π_u→ B ³π_g and CN, B ² Σ ⁺ → X ² Σ ⁺ have been calculated by the method of Chang and Karplus.     

Experimental and theoretical studies of the valence-shell dipole excitation spectrum and absolute photoabsorption cross section of hydrogen fluoride

Absolute cross sectional measurements are reported of the valence-shell dipole excitation spectrum of HF obtained from suitably calibrated high impact energy, small momentum transfer, electron energy-loss scattering intensities. Detailed assignments are provided of all prominent features observed on the basis of concomitant single- and coupled-channel RPAE calculations. The measured spectrum, obtained at an energy resolution of = 0.06 eV (fwhm) in the = 9 to 21 eV interval, includes a dissociative feature centered at = 10.35 eV assigned as X¹Σ⁺ → (1π^?14σ)A¹Π, as well as numerous strong, sharp bands in the = 13 to 16 eV excitation energy region. These bands are attributed on basis of the present calculations to Rydberg (1π^?1npπ)-valence (3σ^?14σ) mixing in X¹Σ⁺ → ¹Σ⁺ excitation symmetry, which gives rise to a long conventional progression, and to strong 1π → nsσ, moderate 1π → ndσ, and weak 1π → npσ Rydberg series in X¹Σ⁺ → ¹Π excitation symmetry. A weaker 1π → ndπ Rydberg series also contributes to the spectrum in X¹Σ⁺ → ¹Σ⁺ symmetry. The calculated and measured excitation energies and f numbers, particularly for the X¹Σ → (1π^?14σ)A¹Π, → (1π^?13pπ)B¹Σ⁺, → (1π^?13sσ)C¹Π, and → (3σ^?14σ)D¹Σ⁺ transitions, are in good quantitative accord, suggesting that the overall nature of the HF spectrum is generally clarified on basis of the present studies. Finally, tentative assignments are provided of weak features observed above the 1π^?1 ionization threshold. As in previously reported joint experimental and theoretical studies of the valence-shell spectrum of F₂, high-resolution optical VUV measurements and calculated potential energy curves aid in the assignment and clarification of the HF spectrum.     

MRD-CI ground state geometry and vertical spectrum of N3

CI calculations have been carried out for the prediction of the ground state geometry and of the vertical spectrum of N₃. The first three states are ²Π_g, ⁴Π_u and ²Σ⁺_u. The C_∞v correlation diagram for the first dissociation limits is discussed by taking into account possible nonadiabatic pathways.     

Laser-induced fluorescence in N2 and N+2 by multiple-photon excitation at 266 nm

The fluorescence transitions corresponding to the second positive system of N₂ (C³Π_u → B³Π_g) for Δv = 0, 1 and the first negative system of N⁺₂(B²Σ⁺_u → X²Σ⁺_g) for Δv = 0, 1, 2 have been observed following laser-induced mul excitation of N₂.     

Geometric and electronic structure of ground and excited states of groupVA diatomics. A theoretical LCGTO-MP-LSD study

LCGTO-MP-LSD calculation was performed for the ground and several low-lying excited states of homo- (N₂, P₂, As₂, and Sb₂) and hetero-nuclear (PN, AsN, AsP, AsSb, SbN, and SbP) groupVA diatomics. For all the systems the ground state is found to be¹Σ⁺. For N₂ and P₂, the¹Σ _g ⁺ ground state is followed by the³Σ _u ⁺ ,³Π _g ,³Δ _u ,¹Π _g , and¹Δ _u low-lying exited states while for As₂ the order is found to be³Σ _u ⁺ ,³Δ _u ,³Π _g ,¹Δ _u ,¹Π _g . Finally for Sb₂ the relative stability of excited states is³Σ _u ⁺ ,³Δ _u ,¹Δ _u ,³Π _g ,¹Π _g . For the hetero-nuclear diatomics the¹Σ⁺ ground state is, in the case of PN, AsN, AsP, SbN, and SbP, followed by the³Σ⁺,³Δ,³Π,¹Π and¹Δ low-lying excited states while for the AsSb diatomic an inversion of stability of the two last singlets occurs. The calculated spectroscopic parameters (Re, ωe, andDe) are in good agreement with all the available experimental results while, theTe values are overestimated by about 0.5 eV. Mulliken population analysis shows that both homo- and hetero-nuclear groupVA diatomics are essentially triple bonded systems.     

A CI non-empirical pseudopotential calculation of the vertical valence excited states of the iodine molecule

The non-empirical atomic pseudopotential proposed by Durand and Barthelat has been used, together with the CIPSI algorithm for large scale CI, to calculate the vertical transition energies of the iodine molecule, in a valence extended (double-zeta + d) basis set. All the valence excited states were considered. The mixing of configurations is very important especially for the Σ⁺_g, Π_g and Π_u symmetries. The experimentally known transition energies are calculated within a 1 eV error, despite the lack of diffuse orbitals and spin-orbit interaction. Some qualitative Mulliken's estimates are discussed. A new ³Σ⁺_g state from the 10 σ_u → 11 σ_u single excitation is predicted in the 9 eV region.     

The kinetics of free radicals generated by IR laser photolysis. II. Reactions of C2(X 1Σ+g), C2(a 3Πu), C3(X̄ 1Σ+g) and CN(X 2Σ+) with O2

The 300 K reactions of O₂ with C₂(X ¹Σ⁺_g), C₂(a ³ Π_u), C₃(X? ¹Σ⁺_g) and CN(X ²Σ⁺), which are generated via IR multiple photon dissociation (MPD), are reported. From the spectrally resolved chemiluminescence produced via the IR MPD of C₂H₃CN in the presence of O₂, CO molecules in the a ³Σ⁺, d ³Δ_i, and e ³Σ^? states were identified, as well as CH(A ²Δ) and CN(B ²Σ⁺) radicals. Observation of time resolved chemiluminescence reveals that the electronically excited CO molecules are formed via the single-step reactions C₂(X ¹Σ⁺_g, a ³Π_u) + O₂ → CO(X ¹Σ⁺ + CO(T), where T denotes are electronically excited triplet state of CO. The rate coefficients for the removal of C₂(X ¹Σ⁺_g) and C₂(a ³Π_u) by O₂ were determined both from laser induced fluorescence of C₂(X ¹Σ⁺_g) and C₂(a ³Π_u), and from the time resolved chemiluminescence from excited CO molecules, and are both (3.0 ± 0.2)10^?12 cm³ molec^?1 s^?1. The rate coefficient of the reaction of C₃ with O₂, which was determined using the IR MPD of allene as the source of C₃ molecules, is <2 × 10^?14 cm³ molec^?1 s^?1. In addition, we find that rate coefficients for C₃ reactions with N₂, NO, CH₄, and C₃H₆ are all < × 10^?14 cm³ molec^?1 s^?1. Excited CH molecules are produced in a reaction which proceeds with a rate coefficient of (2.6 ± 0.2)10^?11 cm³ molec^?1 s^?1. Possible reactions which may be the source of these radicals are discussed. The reaction of CN with O₂ produces NCO in vibrationally excited states. Radiative lifetime of the ā ²Σ state of NCo and the ā ¹Π_u(000) state of C₃ are reported.     

Theoretical studies of molecular ions. Vertical ionization potentials of the nitrogen molecule

The ²Σ⁺_g, ²Π_u, and ²Σ⁺_u vertical ionization energies of nitrogen are obtained by using our theory of molecular electron affinities and ionization potentials, which permits the direct calculation of the ion-molecule energy differences. The contributions of charge redistribution and correlation energy change to the calculated ionization potentials are evaluated. The computational efficiency of the method is illustrated and comparisons are made with recent experimental results.     

Electronic assignments of the violet bands of sodium

The puzzling violet bands of sodium ( ≈ 425-460 nm), known since 1932, are shown conclusively to arise from the superposition of two distinct continuum emission bands - one singlet (2 ¹Σ⁺_u → X ¹Σ⁺_g) and one triplet (primarily 2 ³Π_g → 1 ³Π_u⁺). Each continuum emission system shows complex interference structure arising from multiple branches of the Mulliken difference potential.     

Theoretical study of the vertical electron affinity and ionization potentials of C3

The electron affinity and first three ionization potentials of C₃ are calculated using the multiconfigurational SCF and configuration interaction methods and by Möller-Plesset perturbation theory. Whereas Koopmans' theorem and SCF calculations indicate that the first cation state is ²Π_u, upon inclusion of correlation effects both the ²Σ_u and ²Σ_g cation states are found to lie lower in energy. CI calculations indicate that the ground state (²Π_g) anion is stable by 1.74 eV. Allowing for the error in the calculated electron affinity of the carbon atom, C₃^? is estimated to be stable by 2.0 eV, in excellent agreement with the 2.05 eV value determined from recent photodetachment measurements. No excited anion states are found to be bound at the equilibrium geometry of the neutral molecule.