A search for double-minimum potential functions in excited states of bent triatomic molecules

Abstract

The spectra of the 3700-Å absorption system of SO₂, the 2491-Å absorption system of NC₂ and the 1340-Å system of SO₂ are investigated. The principal aim of this study is to find evidence for double-minimum potential functions in the excited states of these absorption systems. For the 3700-Å system of SO₂, the intensities of bands involving only symmetrical modes may be calculated accurately by application of the Franck-Condon principle. However, four non-temperature-sensitive bands of that system cannot be accounted for by the symmetrical modes. These four bands are given two alternative interpretations in terms of the antisymmetrical mode. For each interpretation, a double-minimum potential function in the Q₃ coordinate explains the positions and intensities of the bands. In the first interpretation, the interval (1⁺ - 0⁺) = 1264.0 cm⁻¹ yields a barrier height of 1700 cm while in the seconu interpretation the interval (1⁺ - 0⁺) = 416.8 cm⁻¹ gives a barrier height of 444 cm⁻¹. In the 2491-Å system of NO₂, the relative intensity of a progression of bands for which the antisymmetrical mode of vibration seems to be well established cannot be explained from a harmonic potential function. The position and relative intensity of the first band of the progression may be explained by using a three-parameter double-minimum potential function. The observed data, (1⁺ - 0⁺) = 713 cm⁻¹ and relative intensity of 1 .2 ± .2, of the first band of the progression yields a barrier height of 800 ± 150 cm. The asymmetric configuration of the molecule in the excited state, determined from the minima of the potential function, shows a departure in bond length of 7% from the average bond length determined from rotational analysis. Eight bands of the 1340-Å system of SO₂ were remeasured and the unusual isotope shifts of two of them were confirmed. Except for those isotope shifts the band system is successfully interpreted in terms of the symmetrical frequencies v₁' = 1068.5 cm⁻¹ and v₂' = 352.1 cm⁻¹. Two geometrical models of the molecule in the excited state are calculated by the FranckCondon principle and band contours are used to determine which model is correct.