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Some aspects of the electronic spectra of small triatomic molecules Hallin, Karl-Eliv Johann

Abstract

Several electronic transitions of NO₂, SO₂, and CS₂ have been photographed, and the rotational structures of some of the bands have been analysed. In the spectrum of CS₂, large numbers of 'hot' bands in the first electronic absorption systems of ¹²CS₂ and ¹³CS₂ (3400-4100 A) have been analysed from high-dispersion plates, and accurate rotational constants have been obtained for the overtones of the ground state bending vibration up to v₂ = 6 and ℓ = 3 for ¹²CS₂ and v₂= 4, ℓ = 2 for ¹³CS₂. The energy differences between the various levels with the same ℓ value have been determined to an accuracy of about ±0.006 cm⁻¹ but (because of the parallel polarization of the electronic transition) the absolute energies of levels with ℓ > 0 could not be obtained from these spectra. A detailed rotational analysis of the (0,0) band of the 2²B₂- X²A₁ electronic transition of N0₂, at 2491 A, has been carried out. Although the lines are broadened as a result of predissociation, it has been possible to determine the five quartic centrifugal distortion constants and the spin-rotation coupling constant εaa for the upper ν₃' state. The centrifugal distortion constants allow the position of the unseen vibrational level ν₃' to be estimated : the results offer no support to the suggestion of Coon, Cesani and Huberman that there is a double minimum potential function in the antisymmetric stretching coordinate of the ²B₂ state. The geometrical structure of the zero-poi level of the ²B₂ state is r(N - 0) = 1.314₂ A, ∠0N0 = 120.8₇°, and its lifetime (as calculated from the linewidths) is 42 ± 5 picoseconds. About 160 rotational lines in the region 7370 - 7410 A in the electronic spectrum of NO₂ have been assigned. The lines form the K = 0, 1, and 2 sub-bands of a perturbed parallel band where the upper state A constant is about 17 cm⁻¹. In a diabatic representation, the band can be considered to be a transition within the ground state manifold, which obtains its intensity by vibrational momentum coupling from a nearby band of the A²B₂-X²A₁ electronic transition; its d assignment is 2 13 1-000. Comparison with the spectrum of ¹⁵N0₂ shows that the nearby A₂B₂ level has quite a small amount of vibrational energy, which is not inconsistent with the assignment by Brand, Chan, and Hardwick that the (0,0) band of the A - X transition is at 8350 A. The implications of the electron spin-rotation parameters and the intensity of the 7390 A band are discussed. Rotational analyses have been carried out for the (0,0) bands of the ã³B₁-X¹A₁ absorption systems of S¹⁶0₂ and S¹⁸0₂, from high dispersion plates taken with the gases at dry ice temperature. The rotational analysis of the (0,0) band of S¹⁶0₂ given by Brand, Jones and di Lauro is confirmed in-general, but their values for the anisotropic spin fine structure constants are found to be in error. Our new values remove the discrepancy in the sign of the spin-spin interaction parameter β=E between the gas phase work and the solid state value given by Tinti. This discrepancy had been rationalized by Brand, Jones and Di Lauro in terms of a different choice of phases for the angular momentum operators, but this argument is shown to be incorrect. The spectrum of S¹⁸0₂ confirms our new values for the spin constants in detail. The C¹B₂-X¹A₁ absorption spectra of S¹⁶0₂ and S¹⁸0₂ between 2350 and 2270 A have been analysed in detail from high dispersion plates taken with the gases at dry ice temperature. The centrifugal distortion constants for the (0,0) band are found to disagree with those reported by Brand, Chiu, Hoy and Bist; rotational constants for the other members of the ν₂' progression are given to high precision. Effective constants for the C¹B₂ state of S¹⁸0₂ are reported. Irregularities due to Coriolis coupling observed in the 002 levels of both isotopic species have' been deperturbed to giventhe rotational constants of the unseen 011 levels. A similar deperturbation of the levels 012 and 100 of S¹⁶0₂ has been used to give the rotational constants of the unseen 021 level of the C¹B₂ state. The spectroscopically determined Coriolis and anharmonic coupling constants are reported. An estimate of the energy of the unseen 001 level from the band origins observed gives 236 and 234 cm⁻¹ for S¹⁶0₂ and S¹⁸0₂, respectively. The large anharmonicity observed in the ν₃' manifold confirms the double minimum potential in Q₃ suggested by Brand, Chiu, Hoy and Bist, but indicates that the barrier height is smaller than the value of 100 cm⁻¹ that they report. A detailed theoretical analysis of the direct spin-orbit interaction between electronic states of the same spin multiplicity has been carried out.

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