Pablo J. Bruna

Strongly bound multiply excited states of B+2 and B2

The potential curves, transition energies (Te), and spectroscopic constants (Re, ωe) of several low‐lying electronic states of B+2 and selected doubly excited states of B2 are given. The data have been obtained by using a multireference single‐ and double‐excitation (MRD) configuration interaction (CI) approach and a triple‐zeta plus polarization AO basis set. The B+2 ground state, which is found here to be X 2Σ+g, shows a rather shallow potential curve (Re =4.015 bohr, ωe =423 cm−1) when compared with that of X 3Σ−g of B2(Re =3.00 bohr, ωe =1051 cm−1, exptl.). The first excited state of B+2, namely 1 2Πu, lies at Te =0.30 eV. Moreover, double excitations relative to X 2Σ+g are essential for the description of a large number of excited states, such as σuσg→π2u(1 4Σ−u, 1 2Σ−u, 1 2Δu, 1 2Σ+u) and σ2u→π2u(1 4Σ−g,1 2Σ−g, 1 2Δg, 1 2Σ+g). Similarly, 2 2Πu arises from the triple excitation σ2uσg→π3u. In the same order as given above, such multiple excitations lead to a significant gain in bond strength (i.e., sh...

Strongly bound metastable states of B2+2

The stabilities of about 25 electronic states of B2+2 have been investigated using a multireference CI (MRD‐CI) method and an AO basis set composed of 6s4p2d contracted Gaussian species per atom, including semidiffuse functions relevant for an adequate description of charge transfer interactions. The ground state X1∑+g (σ2gσ2u) is repulsive, as expected by its electronic configuration with a zero bond order. In spite of this and the doubly‐positive charge, many excited states are found to be metastable, four of them (11∏g, 11∏u, 13∑−g, and 11Δg ) having potential wells from 1.52 eV (11∏u) to 2.83 eV (13∑−g). Relative to the ground state configuration, the metastable states arise from the excitations σu→σg(3∑+u), σu→πu(3,1∏g), σ2u→σgπu(3,1∏u), and σ2u→π2u(3∑−g, 1Δg, 1∑+g); they are analogous to those states showing deep local minima in B2 and B+2.Differences in stabilities among quasibound states can be explained on the basis of the asymptotic ΔE’s between repulsive channels B++B+ and appropriate higher‐ly...

The electronic spectrum of ScH: An MRD-CI study

The vertical electronic spectrum of ScH has been studied up to transition energies of 4·00 eV by using highly correlated MRD-CI wavefunctions. The potential curves for the triplet and singlet states related to the first two dissociation channels are investigated. The separation X 1∑+ -1 3Δ is found to be 0·33 ± 0·01 eV. Relative to 1 3Δ, the triplet states of the second dissociation channel are clustered in the 1·70 eV energy region. Several electronic states occupying the 4π (4p) MO are located from 2·0 to 3·0 eV above 1 3Δ. The first members of the Rydberg 5s series, i.e. 3, 1Δ(1δ7σ25s) and 3∑+ (6σ7σ25s) are found near 4·0 eV. The calculated 5s term values compare well with the experimental data of Sc atom. An experimental absorption band at 18 000 cm-1 can be related to the transitions X 1∑+ -2 1∑+ and X 1∑+ -2 1Π. Two emission regions (12 000 and 18 000 cm-1) are assigned to several possible combinations between higher-lying upper states and all triplet and singlet states from the first dissociation c...

Electronic structure of the B2- radical

The stability and structure of 10 states of B2- have been investigated with highly correlated MRD-CI wavefunctions. This radical possesses the largest number of stable states (positive electron affinity) so far reported for a diatomic anion: no less than eight states lie below B2.

Doubly excited states of B2, B2+, B22+ and B2−

Abstract Extensive multi-reference (single and) double configuration-interaction (MRD-CI) calculations carried out on B2, B2+ and B22+ predict the existence of several doubly excited states generated by the excitation σu2→πu2 or σ22→σgπu. The abundance of these strongly bound molecular states having short Re and high ωe, results from the fact that the first excited atomic states are of the type sp2 (B) and sp (B+). In the dication B22+, double excitation is essential for the existence of several metastable states, some of them with deep potential wells (e.g. 2.83 eV for l3Σg−). The anion B2−shows no less than eight stable states below neutral B2; here, the doubly excited states are resonant. Photodetachment from X4Σg−[(σg2σu2)σgπu2] of B2−; should be instrumental for the experimental characterization of the relative stabilities of X3Σg−(σg→∞), 15Σu−(σg→∞)and I3Πu(πu→∞) in neutral B2. Several optical transitions are proposed for future experimental studies of the doubly excited states presented in this work. In the case of B2−, photodetachment from stable states of type (σg2σu)σgmπun(with m + n = 4) may play an important role in the experimental detection of several strongly bound states of B2.

Stability of highly excited electronic states of Be+ 2

Multireference configuration interaction calculations on about 40 electronic states of Be+ 2 are reported. The ground state X2Σ+ u (σ2 gσu) is bound, with R e = 4·25 a 0, ωe = 513 cm-1 and D e = 1·94 eV. Excited states showing a stronger bonding arise from the excitations σu → πu (12Πu) and σgσu → π2 u(4,2Σ- g, 2Σ+ g, 2Δg). Doubly excited 14Σ- g has the shortest R e (3·43 bohr), highest ωe (816 cm-1) and largest D e (4·16 eV) so far reported for any diatomic Be species. The existence in Be+ 2 of several strongly bound, doubly excited states is caused by the low energy of Be(sp) + Be+ (p) products. Most of the low-lying Rydberg members 3s, 3p and 3d of Be+ 2 have irregular potential curves (double-minima) because of avoided crossings with valence species. The Rydberg species 22Δg(σu → 3dδg), however, has a regular potential curve. The ionization potentials and Franck-Condon factors for Be2 → Be+ 2 transitions are reported. The adiabatic ionization potentials resulting from σu → ∞ (X1Σ+ g → X2Σ+ u) appears ...

Doubly excited states of C2, C+2, and C2+2

Abstract The stability of strongly bound excited states of C2, C+2 and C2+2 resulting from double excitations of the type σ2u→ MO2 relative to the ground-state configuration (with MO2 standing for 3σ2g,3σg1πu or 1π2u) was investigated using a multi-reference MRD-CI method. For C2, the doubly excited configuration 3σ2g1π4u, corresponds to the experimentally observed E1Σ+g (31Σ+g) state, which exhibits a heavy mixing with the less bonding configurations 2σ2u,1π4u (X1Σ+g) and 2σ2u3σ2g1π2u(21Σ+g). This mixing is reflected in two ways: the E+g state does not show the features expected for a strongly bound state (i.e. short Re and high ωe), but - despite its mainly two-electron 3σg1πu→2σ2u character - it allows the emission band E1Σ+g →A1Πu to be observed with a moderate intensity by one-photon spectroscopy. The states 12Σ+u (2σulπ4u) and 22Σ+g (3σg1π4u) of C+2 as well as 23,1Πu (3σg1π3u) and 31Σg+ (1π+u) of C2+2 exhibit a definite enhanced bond strength relative to the ground state. Radiative decay from all these doubly excited states into lower-lying, singly excited 2σu states should be suitable for experimental study because of favorable Franck-Condon envelopes and the one-electron character of the transition.

Transition probabilities for the B 1Δg-A 1Πu and B′ 1Σg+ -A1Πu infrared bands of C2. An ab initio study

Abstract Several radiative data (e.g., transition moment R e′e″ , oscillator strength f v′v″ , Einstein emission coefficient A v′v″ and lifetime τ v′ ) are derived for the B 1 Δ g -A 1 Π u and B′ 1 Σ g + -A 1 Π u infrared bands of C 2 by using ab initio MRD-CI wavefunctions. The predicted f 00 values are 0.75 × 10 −3 (B←A) and 1.54 × 10 −3 (B′←A), both results being somewhat smaller than the best literature estimate f 00 ≈ 2.21 × 10 −3 for the A←X (Phillips) band. The computed τ 0 0 (B 1 Δ g ) of 141.3 μs represents the longest radiative lifetime so far reported for any dipole-allowed C 2 transition. The lifetime τ 0 (B′ 1 Σ g + ) is only 8 μs, a value which is approximately 50% shorter than the time range from 13 to 19 μs assigned to τ 0 (A 1 Π u ); however, for ν ≥ 2 the lifetimes of both B′ and A states differ by less than 3 μs. Differences between the radiative data for the A, B, B′ and b states are rationalized on the basis of excitation energies and Franck-Condon factors. According to the computed B and B′ lifetimes, the cascading process B′→ A → X might have been responsible for the longer A 1 Π u lifetimes reported experimentally.

Strongly bound doubly excited states of diatomic molecules

Abstract Theoretical calculations are given which show the existence of strongly bound doubly excited states. These states show significant decreases in R e and increases in ω e compared to the ground state. Examples include B 2 and B 2 + , but the discussion is general and may lead to the discovery of many such states.