Laser induced fluorescence spectroscopy of the C̃¹B₂X̃¹A₁ band of jet-cooled SO₂: rotational and vibrational analyses in the 235-210 nm region

Laser induced fluorescence spectra of the C̃¹B₂X̃¹A₁ band of SO₂ in the 235-210 nm region were measured under jetcooled conditions. By high-resolution (≈ 0.08 cm⁻¹) measurements of the vibronic transitions, which were well separated from each other due to rotational and vibrational cooling, the rotational constants and term values for the 33 vibrational levels were determined. For the other four levels, only the term values were determined. A total of 573 ro-vibronic transition wavenumbers are presented. It was found that the C rotational constants exhibit considerable fluctuation over the whole observed energy range due to the C-axis Coriolis interaction. The observed vibrational term values were utilized for the extension of the secure vibrational assignments to the higher energy region. However, due to the strong 1:2 Fermi interaction between v₁ (symmetric stretch) and v₃ (anti-symmetric stretch), definitive vibrational assignments for the transitions to the vibrational levels above 2000 cm⁻¹ were found to be intrinsically impossible except for the C̃(v₁, v₂, v₃) = (1, n, 0), (2, n, 0), (3, n, 0) and (1, n, 2) vibrational levels. By constructing the three-dimensional Hamiltonian with a vibrational potential expanded to the fourth-power of the normal coordinates, the expansion coefficients were determined by the least-square fitting to the observed term values and those of several known lowlying vibrational levels. The Franck-Condon intensity pattern calculated using the vibrational wavefunctions derived as eigenfunctions was consistent with the observed pattern below the C̃(1, 4, 2) band, above which the predissociation occurs. The derived vibrational eigenfunctions showed that the v₁ and v₃ modes mix with each other significantly in the higher energy region above 2000 cm⁻¹, which is consistent with the difficulty encountered in assigning the definitive v₁ and v₃ vibrational quantum numbers. The vibrational wavefunctions were further utilized to analyze the C-axis Coriolis interaction, and the counterpart levels of the perturbation having an odd v₃ quantum number were identified for the lowlying vibrational levels. By measurements of the hot-band transitions from X̃(0, 1, 0), the v₂ (bend) fundamental wavenumber of the electronic ground X̃¹A₁ state was determined to be 517.90(3) cm⁻¹.