Vibrational branching ratios for the A3Πi → X3Σ− system of OH+ and OD+

Near-thermal charge exchange between He⁺ and H₂(D₂O) is used as a source of OH⁺(OD⁺) A³H_i→ X³Σ^? emission. A comparison between experimental emission branching ratios and those calculated in the r-centroid approximation suggests that the electronic transition moment varies as a function of the r-centroid.     

The predissociation of Li2 b3Πu by a3Σ+u

⁶Li₂ 1³Δ_g(F₁) → b³Π_u(F₁v = 0–11) rotationally resolved fluorescence spectra are recorded following perturbation-facilitated optical—optical double resonance excitation of 1³Δ_g via spin—orbit mixed A¹Σ⁺_u ∼ b³Π_u(F₁e) intermediate levels. The f-symmetry Λ-components of b³Π_u(F₁) are broadened above the 0.05 cm⁻¹ detection threshold owing to predissociation by the vibrational continuum of the a³Σ⁺_u state. The observed v = 0–11, N = 31f level widths were used to determine the potential energy curve for the Li₂ a³Σ⁺_u state in the region 2.35 < R < 2.60 Å and 11200 < E < 14900 cm⁻¹ (relative to E = 0 at the minimum of X¹Σ⁺_g). The a³Σ⁺_u ∼ b³Π_u curve crossing is at R = 2.57 Å and E = 11246 cm⁻¹ and the electronic part of the − BN·LL-uncoupling matrix element is 〈b Π¦L⁺ ¦aΣ〉 = 1.216H at an R-centroid Rv_bϵ_a = 2.61Å.     

Estimation of a priori errors in ab initio calculations of thermochemical values for the example of the dissociation energies of the ZnO and ZnS molecules

The total energies of ZnO(¹Σ), ZnO(³Π), ZnO^?(²Σ), ZnO⁺(²Σ), ZnS(¹Σ), ZnS(³Π), ZnS^?(²Σ), and ZnS⁺(²Σ) were calculated ab initio by the CCSD(T) method with the use of atomic basis sets including 80, 84, and 93 functions for O, S, and Zn, respectively. Similar calculations were performed for the Zn atom [Zn(¹ S), Zn(³ P), Zn⁺(² S), Zn²⁺(¹ S)] and several oxygen and sulfur states [O(³ P), O^?(² S), O(¹ D), O₂(³Σ), O ₂ ^? (²Π), O₂(¹Δ), S(³ P), S^?(² S), S(¹ D), S₂(³Σ), S ₂ ^? (²Π), and S₂(¹Δ)]. The ideology of engagement groups suggested by us is considered. According to this approach, data treatment can be performed on the assumption that the errors in all the 24 results obey the normal distribution law. As a result, we obtained D _e(ZnO) = 1.70 ± 0.21 and D _e(ZnS) = 1.57 ± 0.25 eV (at a 95% confidence level).     

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.     

Ab initio interaction and spectral properties of CO+–He

In this contribution, ab initio methods have been used to study the open-shell CO⁺–He van der Waals (vdW) complex in both the ground and the first Π excited electronic state. Calculations were performed at the UCCSD(T) level of theory in the framework of the supermolecule approach using the cc-pVTZ basis set complemented with a set of standard bond functions in the middle of the vdW bond. Calculations predict a most-stable equilibrium conformation with β e=45°, R _e =2.85 Å and D _e =275 cm^?1 for the ground CO⁺(X²Σ)–He(¹S) state and β e=90°, R _e =2.70 Å and D _e =218 cm^?1 for the excited CO ⁺(A²Π)–He(¹S) state. The dipole moment μ and independent components of the field polarizability α of the CO ⁺–He vdW complex have been studied at the calculated equilibrium geometry of these states. The vertical excitation energies from the ground CO⁺(X ²Σ)–He(¹S) to the excited CO⁺(A²Π)–He (¹S) electronic state and corresponding shifts in the fluorescent spectrum with respect to the isolated CO⁺ molecule are also presented     

On the origin of energy barriers in the excited states of He2

The A ¹Σ⁺_u and C ¹Σ⁺_g states of He₂ have been examined using self-consistent (spatially projected) generalized valence bond (GVB) wavefunctions. We find both states to have humps (0.06 and 0.22 eV, respectively) at large R (3.1 Å and 2.1 Å, respectively). The repulsive nature of these states at large R results from non-bonding interactions between the singlet pairs of orbitals located on different centers. For R smaller than the size of the excited He orbital (2s or 2p), the state becomes attractive if the symmetry is such that the wavefunction can build in attractive He⁺₂(²Σ⁺_u) character.     

Fluorescence lifetime studies of no a 2Σ+(ν = 5, N = 9), B2Π32(ν = 8, J = 8.5), CwΠ32(ν = 1, J = 8.5), D2Σ+ (ν = 0, N = 5) and D2Σ+(ν = 1, N = 9)

Fluorescence decay profiles of NO excited levels slightly above the dissociation limit have been measured by a single-photon counting technique with nanosecond pulse excitation using an iodine flash lamp. Three iodine atomic lines in the vicinity of 180 nm are found to bring NO molecules into the levels A²Σ⁺(ν = 5, N = 9), B²²Π_{$\text{3}\text{2}$}(ν = 8, J = 8.5), C²Π_{$\text{3}\text{2}$}(ν = 1, J = 8.5), D²Σ⁺(ν = 0, N = 5) and D²Σ⁺(ν = 1, N = 9). Extrapolated zero-pressure lifetimes for single rotational levels are obtained, except for the C state where only a lifetime of ⩽0.4 ns was obtained. Self-quenching rate constants are also determined under higher-pressure conditions. Helium was found to quench the NO A²Σ⁺(ν′ = 5) fluorescence very efficiently.     

Laser spectroscopic investigation of the X 2Σ1/2 state of the van der Waals molecule NaAr

The laser-excitation spectrum of the transition X ²Σ⁺ → A ²Π of NaAr has been investigated using a supersonic expansion of a mixture of sodium vapor and argon gas for production of the molecules. In comparison to preyous investigations the rotational constants of the vibrational levels ν″ = 2, 3 and 4 of the X ²Σ⁺ state could in addition be determined. From our results we deduce a value of R_e = 5.008(5) × 10^?10 m for the equilibrium internuclear distance and of D_e = 41.7(δ) cm^?1 for the well depth of the X ²Σ⁺ state.     

The X1Σ+ ground state of kh near the dissociation limit

Fluorescence excited in the A¹Σ⁺ -X¹Σ⁺ system of ³⁹KH by the 4880 Å argon-ion laser line gives information about the ground state as far as the last bound rovibrational level. This is identified as J = 6 in v = 23, and, assuming a limit midway between J = 6 and J = 7, D_e(KH) = 14776 ± 4 cm⁻¹.     

CAS SCF Cl calculations for the 3Σ−g, 1Σ+g, 3Σ+u, and 5Δu states of Sc2

CAS SCF CI (SD) calculations have been carried out for the ³Σ^?_g, ¹Σ⁺_g, ³Σ⁺_u, and ⁵Δ_u states of Sc₂ using large gaussian basis sets. The ³Σ^?_g, ¹Σ⁺_g, and ³Σ⁺_u states arise from the ²D(4s² 3d¹) + ²D(4s² 3d¹) limit of Sc₂ and are found to be only weakly bound (D_c ≈ 0.06 eV and R_c ≈ 8.0a₀). The ⁵Δ_u state arises from the ²D(4s² 3d¹) + ⁴F(4s¹ 3d¹ 4p¹) atomic limit. This state is found to be strongly bound relative to its limits (D_c ≈ 0.8 eV and R_c ≈ 7.0a₀).     

Laser-induced fluorescence in supersonic nozzle beams predissociation in the Rb2 C1πu and D 1πu states

By exciting Rb₂ in a supersonic nozzle beam with a pulsed dye laser in the C ¹Π_u-X ¹Σ⁺_g and the D ¹Π_u-X ¹Σ⁺_g band system, we find evidence tor different predissociation processes The products appear as follows from the C state, Rb^* (5 ²P_{$\text{3}\text{2}$}) exclusively, and from the D state Rb*(4²D_{$\text{3}\text{2}$}) predominantly, followcd by Rb^*(5 ²P_i-5²S) cascade radiation In addition, a lower bound of D_e(Rb₂X¹Σ⁺_g)? 3939± 10 cm^?1 is obtained.     

Theoretical evidence for multiple 3d bondig in the V2 and Cr2 molecules

Calculated CAS SCF potential curves are reported for the ³Σ_g^? state of V₂ and the ¹Σ_g⁺ state of Cr₂. At the CAS SCF level the ³Σ_g^? state of V₂ is calculated to be bound (R_c = 1.77 Å ω_c = 593.6 cm^?1, D_e 0.33 eV) and to involve a triple 3d bond; while the Cr₂ potential curve is not bound but shows a shoulder near the experimental R_e and the wave function shows multiple 3d bonding in this region.     

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.     

Analytical photoion spectroscopy applied to dissociative single and double photoionization of diatomic molecules (H2, N2, O2, CO,and NO)

This work reports the principle, advantage, and limitations of analytical photoion spectroscopy which has been applied to dissociative photoionization processes for diatomic molecules such as H₂, N₂, CO, and NO. Characteristic features observed in the differential photoion spectra are summarized with a focus on (pre)dissociation of(i) multielectron excitation states commonly observed in the inner valence regions,(ii) shape resonances, and(iii) doubly charged parent ions. Possible origins for negative peaks in the differential spectra are discussed. This spectroscopy is applied to the reported photoion branching ratios for D₂ (and H₂ at high energies). The main findings are as follows: (1) The direct dissociation of theX ²Σ _g ⁺ (1sσ _g ) state of D ₂ ⁺ , the two-electron excited state¹Σ _u ⁺ (2pσ _u 2sσ _g ) of D₂, and the²Σ _u ⁺ (2pσ _u ) state of D ₂ ⁺ appear clearly in the differential spectrum, as previously observed for H₂. (2) Decay of H ₂ ⁺ (D ₂ ⁺ ) to H⁺ (D⁺) above 38 eV is due to the direct dissociation of highly excited states of H ₂ ⁺ (D ₂ ⁺ ) such as the²Σ _g ⁺ (2sσ _g ) and high-lying Rydberg states converging on H ₂ ²⁺ (D ₂ ²⁺ ). (3) In the ionization continuum of H ₂ ²⁺ (D ₂ ²⁺ ) peculiar dissociation pathways are observed. The differential photoion spectra for O₂ derived from the reported photoion branching ratios are also presented. The (pre)dissociation of theb ⁴Σ _g ^? ,B ²Σ _g ^? , III²Π _u ,²Σ _u ^? , and^2,4Σ _g ^? states of O ₂ ⁺ appears as the corresponding positive values in the spectra in accord with previous observations. Some other dissociation pathways possibly contributing to the spectra are discussed including dissociative double ionization.     

Laser-induced fluorescence of CsH: The X1Σ+ state dissociation energy

Laser-induced fluorescence of CsH from the A¹Σ⁺ electronic state to the X¹Σ⁺ state was recorded using high-resolution Fourier transform spectrometry. Ground-state vibrational levels were observed from ν″ = 1 to the dissociation limit. These measurements showed anomalies in the X¹Σ⁺ potential energy curve due to the avoided crossing of ionic and covalent potential curves. Accurate molecular constants were derived for the lower X¹Σ⁺ vibrational levels. The observation of a quasibound level gave the first experimental determination of the dissociation energy (in cm^?1): 14802 ? D_c ? 14813.     

Emission spectra produced from thermal energy charge transfer reactions of Ar+ ions with CS and PN radicals

CS⁺(B²Σ⁺-A²Π_i) and PN⁺(B²Σ⁺-X²Σ⁺) emissions were observed for υ′ = 0 and 1 from argon afterglow reactions of CS and PN radicals. The rotational constant (B₀) of the CS⁺(B) state was estimated to be 0.696±0.002 cm^?1 from the difference between band head and band origin. The dependence of each emission intensity on the voltage applied to the ion-collector grids and on the argon pressure indicated that Ar⁺ was a plausible candidate. Vibrational populations of the CS⁺(B) and PN⁺(B) states obtained from the emission spectra shifted to lower vibrational levels in comparison with those expected from the energy resonance and Franck-Condon factors for ionization. This is explainable as the distortion of target radicals by approach of reactant ions.     

Theoretical investigation of the B′1Σu+ state of the hydrogen molecule

Potential energy curve for the excited B′¹Σ_U⁺ state of the hydrogen molecule was computed for internuclear distances 1.2 <R < 5.0 au. The dissociation energies evaluated for H₂, HD and D₂ are above the experimental results by 0.9, 3.1 and 27.2 cm^?1, respectively. The wavefunctions obtained are employed to compute at several internuclear distances the electronic dipole transition moment for the transition to the ground state.     

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.     

Potential energy surfaces for the photodissociation H2O→O(1Dg + H2(X1Σ+g)

The minimum energy pathways for symmetrical dissociation of water into O(¹D_g + H₂(X¹Σ⁺_g) are calculated by the MRD Cl technique for various excited states of H₂O and possible mechanism for the photodissociation are discussed.     

Electronic transition moment of the AΠr-XΣ system of TiN

The A²Π_r-X²Σ⁺ transition of TiN was observed by the dispersed laser induced fluorescence (DLIF) spectroscopy. The relative intensities of the DLIF spectra were analyzed to determine the dependence of the electronic transition moment, R_e(r), on the internuclear distance, r, as R_e(r)∝{1−0.281(26)r} (1.380 Å≤r≤1.823 Å). This r-dependence was analyzed simultaneously with the reported values of the spin-orbit constants for A²Π_r and the hyperfine-coupling constants for X²Σ⁺ to evaluate the ionic character of the TiN bond, the 4s atomic character in the 9σ orbital of X²Σ⁺, and the 4p atomic character in the 4π orbital of A²Π_r. These characters were confirmed to be in accordance with the reported theoretical prediction. A strong r-dependence was indicated for the 3d-4p mixing in the A²Π_r state due to the configuration mixing of the Ti(3d⁴) and Ti(3d³4p) states at a large internuclear distance.