BO: Low-lying electronic states obtained by configuration-interaction studies

Configuration-interaction studies were performed on low-lying ²Σ⁺, ²Σ⁻, ²Π, ²Δ, ⁴Σ⁺, ⁴Σ⁻, ⁴Π, and ⁴Δ states of BO. Sixteen stable states have been found, among them four 3s_B Rydberg states. Spectroscopic constants are compared with those of the four observed states and with theoretical results obtained by others. Particular attention is given to the observed perturbations in the C²Π state, which can be explained by interaction with 1²Δ, 1²Σ⁻, 3²Σ⁺, and other states. Comparison with isoelectronic molecules CN and CO⁺ reveals many similarities.     

Electronic states of SiF and SiF+: Configuration-interaction studies

For SiF, low-lying ²Π, ⁴Π, ²Σ⁺, ⁴Σ⁺, ²Σ⁻, ⁴Σ⁻, ²Δ, and ⁴Δ states were studied by configuration-interaction methods, using a double-zeta plus polarization basis set with 4s, 4pσ, 4pπ, and 5pπ Rydberg orbitals. Potential energy curves and spectroscopic constants for 17 stable valence and Rydberg states are given. The lowest ²Σ⁻ state is repulsive. There is good agreement with known spectroscopic constants. Besides A²Σ⁺, another semidiffuse state, 1⁴Π, is predicted. For the isoelectronic PO molecule, 2π → 3π valence excitations lie below Rydberg excitations, such that 2²Π of PO derives from 2π → 3π, whereas 2²Π of SiF derives from 3π → 4pπ. Dipole moments of X²Π and A²Σ⁺ at their respective R_e, and the radiative lifetime of A²Σ⁺ were calculated. For SiF⁺, many electronic states were investigated, but only two, the X¹Σ⁺ ground state and 1³Π are found to be stable. The ionization energy of SiF is calculated to be 6.87 eV (adiabatic) and 7.05 eV (vertical).     

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.     

A CASSCF/MRCI study of the low-lying Rydberg states of CIO

An ab initio calculation has been performed on the lowest seven doublet and six quartet Rydberg states of CIO at the CASSCF/MRCI level and with basis sets suitable for the extended molecular orbitals of such states (aug-cc-pVTZ with up to eleven extra Gaussian functions). Calculations on the quartet states reveal the energy ordering of Rydberg orbitals to be 4sσ, 4pπ, 4pσ;, 3dδ, 3dσ and 3dπ. The calculated doublet ab initio potential curves confirm experimental assignments of the C²Σ- and F²Σ- states but require reassignments for the symmetries of the D (²Δ), E (²Π) and H (²Δ) Rydberg states. These revisions are supported by spin-orbit coupling calculations that suggest the separation between the Ω components is small. In addition, a ²Σ⁺ state has been identified as the likely upper state for two previously unassigned vibronic bands recorded in absorption studies.     

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.     

Singlet-triplet anticrossings between ungerade states of H2

Singlet-triplet anticrossings in the H₂ molecule have been observed between two ungerade states, B′(3p)¹Σ_u⁺ and f(4p)³Σ_u⁺. This is the first time that an observed H₂ anticrossing has involved a state which can radiate directly to the ground state. Analysis yields accurate values for the zero field separations between two pairs of rotational and vibrational levels. It also yields a value for the Fermi contact interaction in the triplet state as well as the difference in the orbital angular momentum g factors for the two states. From linewidth measurements, we deduce a rigorous lower limit to the radiative lifetime of the B′¹Σ_u⁺ state and a (nearly equal) most reasonable value for it. It is shown that the perturbation between the two states is quite weak leading to little singlet-triplet mixing in zero field. The experimental data establish that the oscillator strength for the forbidden transition from the ground state to the f(4p)³Σ_u⁺ state is at least seven orders of magnitude smaller than that of the allowed transition to the B′¹Σ_u⁺ state.     

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.     

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.     

Analysis of autoionizing Rydberg states in the vacuum ultraviolet absorption spectrum of HCl and DCl

The photographic absorption spectrum of HCl and DCl above the first ionization limit (750 – 950 Å) has been analyzed. Two Rydberg series converging to the second ionization limit (²Σ⁺) have been identified. No analyzable rotational structure was observed, indicating that the autoionization rate is faster than the rotational period. The symmetries of the Rydberg states cannot be determined conclusively, but several possibilities are discussed.     

Resonance-enhanced multiphoton ionization detection of a new 3Σ+-a3Π band system in CS

Resonance-enhanced multiphoton ionization has been used to detect absorption transitions from the nascent states of CS produced in the two-photon laser flash photolysis of CS₂. Transitions are seen from the a³Π state of CS to a previously unobserved ³Σ⁺ state. Rotational assignments of the (1, 0), (0, 0), (0, 1), and (0, 2) bands have been made; the derived constants for those bands are similar to those of the CS⁺ ion, in good agreement with the assumption that the upper electronic state is the 3sσ³Σ⁺ Rydberg state.     

Rydberg states of carbon dioxide and carbon disulfide

The electronic absorption spectra of carbon dioxide and carbon disulfide have been reexamined. Model potential calculations have been used to calculate the energies of excited states in Rydberg approximation, and (npσ) and (npπ) Rydberg series have been assigned. For both molecules, the lowest excited ¹Π_g and ¹Π_u states are identified as Rydberg states. The lowest ${}^1\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ state is mainly Rydberg for CO₂, but contains some valence character for CS₂, There is no evidence for transitions to additional valence states of these symmetries.It is shown that $\text{LCAO}\text{MO}$ predictions about excited states can be misleading because of near-linear dependencies which arise in multicenter expansions. A consideration of the united atom orbitals for CO₂ and CS₂ predicts that there should be only the number of low-energy excited states which are found from the spectral analysis.     

Assignments of autoionization states of O2

Autoionization states of O₂ leading to the b ⁴Σ_g^-, B ²Σ_g^- and c ⁴Σ_u^- states of O⁺₂ have been well established, while those leading to the a ⁴∏_u and A ²∏_u are not certain. Further, some of the assignments of the autoionization states and the determinations of effective quantum numbers and quantum defects are open to questions. In view of a recent study of isotopic oxygen molecules, the vibrational assignments for the autoionization states leading to the a ⁴∏_u and A ²∏_ucan be unamiguosly assigned. We have systematically examined the known Rydberg series and have suggested new assignments and interpretations for several autoionization states leading to the various states of O₂⁺.     

The RKR potential energy curves for the X1Σ+ and A1Σ+ states of RbH

The A¹Σ⁺-X¹Σ⁺ laser-excited fluorescence spectrum of RbH was observed in the 4760-to 8470-Å region, using the Ar⁺ 4765-Å exciting line, and v″ = 6–12 were observed for the first time. New spectroscopic constants were obtained for the X¹Σ⁺ and A¹Σ⁺ states of RbH. New Rydberg-Klein-Rees (RKR) potential energy curves were calculated up to v″ = 12 of the X¹Σ⁺ state and up to v′ = 14 of the A¹Σ⁺ state.     

High-resolution optogalvanic study of the c4(0)1Πu, c′5(0)1Σu+, and a″(0)1Σg+ Rydberg states of N2

A new optogalvanic technique with an rf discharge was applied to a high-resolution study of the Rydberg states of N₂. The Ledbetter band, c₄(0)¹Π_u ← a″(0)¹Σ_g⁺, and a new visible band, c′₅(0)¹Σ_u⁺ ← a″(0)¹Σ_g⁺, were studied at a Doppler-limited resolution of 0.05 cm^?1. A Doppler-free method was also applied to resolve overlapped lines. Precise wavenumbers were determined for the rotational transitions of the two Rydberg bands. The rotational and the centrifugal constants for the lowest Rydberg state, a″(0)¹Σ_g⁺, were determined to be B₀ = 1.913748(42) cm^?1 and D₀ = 6.088(99) × 10^?6 cm^?1, where the numbers in parentheses are the standard deviation and apply to the last digits.     

Quasi-relativistic treatment of the low-lying KCs states

The potential energy curves, permanent and transition dipole moments as well as spin-orbit and angular coupling matrix elements between the KCs electronic states converging to the lowest three dissociation limits were evaluated in the basis of the spin-averaged wavefunctions corresponding to pure Hund’s coupling case (a). The quasi-relativistic matrix elements have been obtained for a wide range of internuclear distance by using of small (9-electrons) effective core pseudopotentials of both atoms. The core-valence correlation has been accounted for a large scale multi-reference configuration interaction method combined with semi-empirical core polarization potentials. The static dipole polarizabilities of the ground X¹Σ⁺ and a³Σ⁺ states were extracted from the closed-shell coupled-cluster energies by the finite-field method. Among the singlet and triplet Σ⁺ states manifold the pronounced avoided crossing effect between repulsive walls of the (2,3)³Σ⁺ states has been discovered and analyzed by finite-difference calculation of radial coupling matrix elements. The resulting transition dipole moments and potentials were used to predict radiative lifetimes and emission branching ratios of excited vibronic states while the calculated angular coupling matrix elements were transformed to Λ-doubling constants of the (1,2)¹Π states and magnetic g-factor of the ground state. The accuracies of the present results are discussed by comparing with experimental data and preceding calculations.     

Rotational distribution in simultaneously excited electronic states of CH

A²Δ, B²Σ^? and C²Σ⁺ states of CH were excited in discharges through flowing C₂H₂ and C₂H₄ and in flames of C₂H₂. The rotational distributions in these states were determined from the measured integrated intensities of rotational lines for bands of A²Δ → X²Π, B²Σ^? → X²Π and C²Σ⁺ → X²Π. The A²Δ state exhibits a non-Maxwellian distribution while the B²Σ^? and C²Σ⁺ states show Maxwellian distributions. The non-Maxwellian distribution of the A²Δ state and the different rotational distributions in the three states are discussed.     

Potential energy curves for the 48 lowest-lying molecular states of LiH+

The electronic energies of the 20 lowest lying ²Σ⁺ states, the 14 lowest lying ²Π states, and the 14 lowest lying ²Δ states of LiH⁺ have been calculated in the range 2 ≤ R ≤ 20 a.u. from a model potential approach and using truncated diatomic orbitals (TDO) as basis set. Results in very good agreement with the more recent literature were obtained for the spectroscopic constants, R_e and D_e, for the ground state X²Σ⁺.     

Spin-rotation and hfs constants for the X2Σ+ and B2Σ+ states of CaI

The molecular-beam, laser-rf double-resonance technique is used to obtain precise values for the spin-rotation and hyperfine interaction constants in the X²Σ⁺ ground state of ⁴⁰Ca¹²⁷I. The measured X-state splitting information is supplemented by BX optical hfs splitting data to obtain hfs constants for the excited B²Σ⁺ state. The results for both states are compared with previously reported values for CaI and other calcium monohalides.