Laser-induced phosphorescence of SO₂ in solid neon: Direct observation of the b̃³A₂ state in the¹⁶OS¹⁸O molecule

Laser excitation of isotopically labeled SO₂ molecules in a neon matrix has allowed the measurement of dispersed phosphorescence spectra (ã³B₁-X̃¹A₁) in the wavelength region 387.5-526.3 nm (19 000-25 800 cm^-1). The origin of the ã-X̃ transition lies at 25 747 cm^-1 in ¹⁶OS¹⁶O, shifted 18 cm^-1 to the red from the gas phase; the three vibrational frequencies of the X state are nearly unperturbed in the neon matrix (1149, 520, and 1362 cm^-1). As the zero-phonon lines are sharp and well resolved, it has been possible to monitor emission from a specific isotopomer to obtain its excitation spectrum. The excitation spectra of matrixisolated ¹⁶OS¹⁶O and ¹⁸OS¹⁸O in the region 350.9-388.5 nm (25 740-28 500 cm^-1) are similar to those previously observed in the gas phase, with strong perturbations affecting all vibrational levels above ãB₁(110) at 27 005 cm^-1 (370 nm). Two additional groups of bands are observed for the unsymmetrical isotopomer ¹⁸OS¹⁶O; they are identified as transitions to levels of the ã³B₁ state with v₃ = 1 and as direct transitions to the b̃₃A₂ state, which both become allowed in the lower symmetry. The v₃ frequency of the ã³B₁ state of ¹⁸OS¹⁶O is 922 cm^-1; for the b̃³A₂ state, v₁ - 791 cm^-1 and v₂ ≃ 315 cm^-1, while the b̃ (001) level z lies 26 653 cm^-1 above the X̃ (000) level.