Adiabatic calculations of the 2Σ+g excited states of He+2

The first six ²Σ⁺_g adiabatic potential energy curves of He⁺₂ are calculated with the MRD CI method employing configuration selection (T = 10 μ hartree) and perturbative energy corrections and using two basis sets differing in the number of diffuse functions. The non-adiabatic matrix elements at the numerous avoided crossings are also calculated and approximate diabatic curves are constructed. Various aspects of the results are discussed.     

Accurate potentials for the X1Σ+g states of H2 and D2 molecules

Starting from the spectroscopic constants, the electronic potential for the ground state of H₂ and D₂ molecules has been calculated. Numerical integration of the radial wave equation gives accurate self-consistent eigenvalues (mean deviation of about 0.05 cm⁻¹). A comparison between different potentials is reported. New revised spectroscopic constants are calculated.     

Decay of the A2Σ+ and B2II quasibound states of HeH

The X²Σ⁺, A²Σ⁺ and B²II wavefunctions of the HeH molecule are calculated as single configurations state functions built from molecular orbitals generated by the Fock operator of the HeH⁺ ion. Dipole transition moments and non-adiabatic coupling matrix elements are evaluated in order to study the competition between dissociative photoemission and predissociation of the A and B states. Particular attention is paid to the X-A (2σ–3σ) avoided crossing and comparisons are made between its characteristics and those of the corresponding 2¹Σ⁺–3¹Σ⁺ (2σ–3σ) avoided crossing in the HeH⁺ ion. The predissociation lifetime of the A state is found to be of the order of 10⁻¹³ s which precludes its decay via dissociative photoemission in agreement with the analysis of recent translational spectroscopy experiments. The predissociation lifetime of the B state is found to be of the same order of magnitude as its radiative lifetime (i.e. of the order of a few 10⁻⁸ s). It is still not understood why this state would radiate rather than predissociate as has been inferred from experiment.     

Electric dipole moments of the MgO B1Σ+ and X1Σ+ states

The electric dipole moment of the excited B¹∑⁺ (υ′ = 0) state of MgO produced in a gas-phase reaction of Mg(¹S) atoms and N₂O is measured using the technique of Stark quantum-beat spectroscopy. It is shown to be |μ_υ′=0| = 5.94(24) D. The lifetime of the excited B¹∑⁺ (υ′ = 0) state is determined as τ = 22.5(1.5) ns. Using laser excitation spectroscopy and by direct observation of the P(1) line splitting at high electric field strengths in the B¹∑⁺−X¹∑⁺ (0,0) system the ground state electric dipole moment is measured yielding |μ_υ=0| = 6.2(6) D. Furthermore an electric-field-induced Q branch is observed.     

New self-consistent potentials for the X1Σ+ and A1Σ+ states of KH

The vibrational and rotational molecular constants have been determined for the A¹Σ⁺  X¹Σ⁺ system of the isotopic potassium hydrides: KH and KD. From these experimental term values, isotopically combined PMO-RKR-van der Waals potentials are constructed, which are checked by direct integration of radial Schrödinger equation. As an additional check on the accuracy of the potentials, the wavefunctions were used to compute B_ν, D_ν, H_ν and L_ν values. The agreement between the calculated and experimental results is quite satisfactory. Franck-Condon overlap integrals and probability density distributions have been calculated. The anomalous behaviour of the A¹Σ⁺ state may be observed in the probability density functions of the lowest vibrational levels.     

Vibronic coupling in the A2Π and B2Σ+ electronic states of the NCS radical

The spin-rovibronic energy levels of the A(2)Π and B(2)Σ(+) electronic states of thiocyanate radical have been calculated variationally, using high-level ab initio coupled diabatic potential energy surfaces. Computations up to J = 7∕2 have been performed, obtaining all levels with K ≤ 3 (Σ(1/2),Π(1/2,3/2),Δ(3/2,5/2),Φ(5/2,7/2)), for energies up to 2000 cm(-1) above the A(000)(2)Π(3∕2) level. The available experimental data have been critically reviewed in the light of the theoretical findings.     

The potential energy curves and probability density distributions of the X1Σ+ and A1Σ+ states of RBH

The potential energy curves PMO—RKR—van der Waals of the electronic A¹Σ⁺ and X₁Σ⁺ states of RbH have been determined. The potentials obtained are self-consistent with the experimental data because they have been tested by direct numerical solution of the radial Schrödinger equation. From exact vibrational eigenfunctions probability density distributions and Franck—Condon factors have been calculated over the range of vibrational levels observed. It is observed that the anomalous behaviour of the A¹Σ⁺ state arises in the υ′ = 1, 2 and 3 levels with probability density functions similar to those of a harmonic oscillator.     

Characterization of C4H in the A2Π and X2Σ+ states by double resonance four-wave mixing

The B(2)Π-X(2)Σ(+) electronic spectrum of C(4)H has been studied by degenerate and double resonance four-wave mixing. The technique identifies vibrational levels in the X(2)Σ(+) ground state. Its sensitivity and unique characteristics permit detection of new levels. The A(2)Π state lying 222 cm(-1) above the X(2)Σ ground state is also observed, confirming the analysis from anion photoelectron spectroscopy but with improved accuracy. Vibrational level determination in the A(2)Π electronic manifold up to 700 cm(-1) above v = 0 is made. A Renner-Teller analysis is carried out for the two lowest bending modes v(6) and v(7) in the A(2)Π state by diagonalization of the effective Hamiltonian matrix. The Renner-Teller parameters ∈(6), ∈(7), and ∈(67), the vibrations ω(6) and ω(7) and the spin-orbit coupling constant A(so) are determined.     

Rotational analysis and deperturbation of the A2Π → X2Σ+ and B'2Σ+ → X2Σ+ emission spectra of MgH

Deperturbation analysis of the A(2)Π → X(2)Σ(+) and B(')(2)Σ(+) → X(2)Σ(+) emission spectra of (24)MgH is reported. Spectroscopic data for the v = 0 to 3 levels of the A (2)Π state and the v = 0 to 4 levels of the B'(2)Σ(+) state were fitted together using a single Hamiltonian matrix that includes (2)Π and (2)Σ(+) matrix elements, as well as off-diagonal elements coupling several vibrational levels of the two states. A Dunham-type fit was performed and the resulting Y(l,0) and Y(l,1) coefficients were used to generate Rydberg-Klein-Rees (RKR) potential curves for the A (2)Π and the B'(2)Σ(+) states. Vibrational overlap integrals were computed from the RKR potentials, and the off-diagonal matrix elements coupling the electronic wavefunctions (a(+) and b) were determined. Zero point dissociation energies (D(0)) of the A(2)Π and B'(2)Σ(+) states of (24)MgH were determined to be 12,957.5 ± 0.5 and 10,133.6 ± 0.5 cm(-1), respectively. Using the Y(0,1) coefficients, the equilibrium internuclear distances (r(e)) of the A(2)Π and B'(2)Σ(+) states were determined to be 1.67827(1) ? and 2.59404(4) A?, respectively.     

Self-consistent potential energy curves for the A1Σ+ and X1Σ+ states of isotopic cesium hydrides

From the spectroscopic experimental data available in the literature we have determined the mass-reduced Dunham coefficients for the A¹Σ⁺ ↔ X¹Σ⁺ system of the isotopic species CsH and CsD. Based upon these results, for both ground and excited states of cesium hydride and deuteride we report new hybrid rotationless potential energy curves (PMO-RKR-van der Waals) up to the dissociation. As a consistency check on the accuracy of the potentials the eigenvalues were calculated by direct numerical integration of the radial Schrödinger equation and found to agree within the rms error 0.39 cm⁻¹ (X¹Σ⁺) and 0.41 cm⁻¹ (A¹Σ⁺) with the experimental vibrational energies. From the wavefunctions, the rotational constants B_υ, centrifugal distortion terms D_υ, H_υ and L_υ, Franck—Condon factors, and probability density distributions were obtained. The probability density distributions for the lowest vibrational levels of the A¹Σ⁺ show an anharmonicity associated with the anomalous behavior of that state.     

Fourier spectrometry investigation of the 1 3Σg+ →a 3Σu+ transition of the 7Li2 molecule

The 1 ³Σ_g⁺ → a ³Σ_u⁺ transition in the ⁷Li₂ molecule has been observed in the 8200–10 000 cm⁻¹ region with a high resolution Fourier spectrometer. Rotational analysis of 1 ⩽ υ′ ⩽ 7 of 1 ³Σ_g⁺ and 0 ⩽ υ″ ⩽ 7 of a ³Σ_u⁺ has been carried out. We found D_e(a ³Σ_u⁺) = 332.5 ± 1.0 cm⁻¹ that gives T_e(a ³Σ_u⁺) = 8184.3 ± 1.5 cm⁻¹ and D_e(1 ³Σ_g⁺) = 7090.4 ± 1.5 cm⁻¹ with T_e = 16330 ± 2 cm⁻¹.     

Comment on “ab initio investigation of low-lying 2Σ+ and 2Π states of NO2+”

Energies of the singlet stales of the doubly charged nitric oxide cation have been determined experimentally by the technique of double charge transfer, and are compared with previous results of full-valence MC SCF plus first-order CI techniques. The good agreement obtained permits an assignment of some of the peaks in the experimental spectrum, and confirms the accuracy of these calculations.     

Adiabatic versus diabatic descriptions of the lowest Rydberg and valence 1Σ+ states of HCl

In this contribution we first report new ab initio self-consistent field configuration interaction calculations of the first excited adiabatic potential of (1)Σ(+) symmetry, the 2(1)Σ(+) or B(1)Σ(+) state, which presents two minima and can thus be seen as made up of the Rydberg E(1)Σ(+) and the valence V(1)Σ(+) states. Based on the computed 2(1)Σ(+) potential, we devised a theoretical procedure to compute the vibronic structure in order to try to explain the energy levels observed in the region above 76 254.4 cm(-1) which display an irregular vibrational structure, indicative of spectral perturbations. We try to find out which representation of the electronic states, the diabatic or the adiabatic one, is best suited to replicate the lowest observed vibronic levels of the E and V states. To this end, we deduce, from the 2(1)Σ(+) potential and its complementary adiabatic potential, two diabatic potentials. We then carry out a coupled equation treatment based on these diabatic potentials. The results of this treatment indicate that, in the present case, the adiabatic representation is better than the diabatic one to describe the observed vibronic levels. This is due, as expected, to the existence of a strong electrostatic interaction between the two diabatic potentials.     

Ab initio study of the molecular vibrations and electronic structure of the X,B, D and F2Σ+ states of AlO

Molecular vibrations and electronic structure of the X²Σ⁺, B²Σ⁺, D²Σ⁺, and F²Σ⁺ states of AlO are studied by carrying out ab initio configuration interaction calculations and molecular vibration calculations using accurate potential energy functions. An avoided crossing between the D²Σ⁺ and F²Σ⁺ potential energy curves occurs in the neighborhood of 4.0 a₀ and results in irregular vibrational levels of the D and F²Σ⁺ states. The vibrational constants for the F²Σ⁺ state are predicted from the vibrational levels not involved in the irregularity. Configuration mixing is important in describing the B, D, and F²Σ⁺ states. The F²Σ⁺ state at and around its well minimum and the D and F²Σ⁺ states in the avoided crossing region are characterized in terms of their main configurations and dipole moment functions.     

A comparison between the potential energy curves for the X1Σg+ state of H2 and D2

The electronic potential for the ground state of H₂ and D₂, molecules has been calculated from spectroscopic molecular constants. Numerical integration of the radial wave equation gives accurate self-consistent values (an eigenvalue mean deviation of about 1 cm⁻¹). A comparison between different potentials is reported.     

Temperature dependence of the relaxation rate constants of the a 1Δg and b 1Σ +g states of oxygen isotopes

The relaxation rate constants of the low-lying electronic singlet states, a ¹Δ_g and b ¹Σ⁺_g , of gaseous natural O₂ and of the isotope ¹⁸O₂ were investigated as a function of temperature from 100 to 295 K. The measured increase of the rate constants with temperature is in good agreement with a theory of electronic-to-vibrational-translational energy transfer. The significant effects of the different electronic states and of the isotope masses on the absolute values of the relaxation rate constants, which range from 1.0× 10⁻²⁰ to 3.9× 10⁻¹⁷ s⁻¹ molecule⁻¹ cm³ at 295 K, are discussed.     

Charge exchange of O2+ with Cs: spectroscopy and predissociation pathways for the Πg Rydberg states of O2

Electron capture by keV O₂⁺ (X₂Π_g) ions from Cs atoms yields predominantly the ¹Π_g and ³Π_g Rydberg states of O₂ which subsequently predissociate. Through the use oftranslational spectroscopy on the neutral product atoms, we have located a number of vibrational levels of these states and determined the dissociation channels. Furthermore we have observed competition between diabatic and adiabatic behaviour in the dissociating channels.     

Bond functions in molecular excited states: MRD CI calculations for the A3Σ+u,B3Πg and W3Δu states of N2

Using double-zeta plus polarization (DZP) basis sets systematically augmented with a variety of bond functions, the term dissociation energies are calculated for the A³Σ⁺_u, B³Π_g and W³Δ_u states of N₂. It is found that the best agreement with literature values is generally with a basis set composition of DZP augmented by a set of s, p and d orbitals at the bond midpoint. The excited state potential energy curves and spectroscopic constants for the B³Π_g state are calculated from this basis and compared with experimental values. Good agreement was obtained, considering the small basis set size, with the spectroscopic constants ω_e, ω_eχ_e, ω_ey_e, B_e and α_e and the dissociation energy D_e (e.g., D_e = 3.38 (3.681, exp.), 4.75 (4.897) and 4.77(4.873) eV for the A, B and W stages, respectively). Poorer agreement was obtained for the term energy T′₀ (7.92 versus 7.35 eV, exp., for the B state). The error in term energy arises largely from an error in the calculated ⁴S → ²D splitting (2.705 versus 2.383 eV, exp.), and shifting the potential curve for the B state by a constant amount leads to much improved agreement relative to the ground state. The counterpoise correction applied to the potential curve of the B state causes a drastic deterioration of the results and shows qualitatively incorrect behaviour, and is therefore not recommended for calculations of this type.