Ab initio potential curves, dipole moments, and transition probabilities for the low-lying states of arsenic oxide

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Abstract
Relativistic effective core potentials (RECPs) are employed in the framework of spin‐orbit configuration interaction method to compute potentials curves and one‐electron properties for a large number of electronic states of the arsenic oxide molecule. Good agreement is noted between calculated and experimental data for the spectroscopic constants of states with T e values at or below 40 000 cm−1. The calculations predict that the lowest excited Λ–S state is the π→π* a 4Π and it is argued that some experimental results of Kushawaha et al. originally thought to correspond to the A″ 2Σ+–X 2Π transition should be reassigned as a 4Π–X 2Π. There is general agreement that the corresponding π→π* 2Π is the upper state in the G 1,G 2→X 2Π band systems, with computed T e values only 600 cm−1 smaller than observed, and discrepancies in r e and ω e values of 0.01 Å and 16–20 cm−1, respectively. The b 4Σ− and I 2Φ Ω components are found to be the next lowest‐energy states, but it is pointed out that the experimental L–F splitting is too large to be attributed to the b 1 4Σ− 1/2–b 2 4Σ− 3/2 energy difference. Strong perpendicular transitions are computed for the A 2Σ+–X 2Π band system, and the upper state is found to undergo homogeneous perturbations by a number of neighboring states which should have important effects on the A–X vibrational intensity distribution. The B 2Σ+ state has a large amount of Rydberg character and is the only low‐lying AsO state with As−O+ polarity. The minimum in its potential curve appears to be almost coincident with a maximum in the A 2Σ+ potential, leading to an onset of a break‐up in the otherwise strong B–X emission intensity pattern at v′=0 and N′=21. On the basis of the present calculations an estimate for the D 0 0 value of the AsO ground state of 4.22 eV can be made, which is ∼0.7 eV smaller than the upper limit for this value given in the literature. Numerous comparisons with analogous calculated results for the heavier Group V oxides, SbO and BiO are made, allowing for a systematic evaluation of the changing role of relativistic effects with increasing atomic number of the heavy atom in this class of molecules.