A vacuum-ultraviolet laser pulsed field ionization-photoelectron study of sulfur monoxide (SO) and its cation (SO⁺)

Vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) spectroscopy has been applied to the study of the sulfur monoxide radical (SO) prepared by using a supersonically cooled radical beam source based on the 193 nm excimer laser photodissociation of SO₂. The vibronic VUV-PFI-PE bands for the photoionization transitions SO⁺(X²Π_1/2; v⁺ = 0) ← SO(X³Σ^-; v = 0); and SO⁺(²Π_3/2; v⁺ = 0) ← SO(X³Σ^-; v = 0) have been recorded. On the basis of the semiempirical simulation of rotational branch contours observed in these PFI-PE bands, we have obtained highly precise ionization energies (IEs) of 83 034.2 ± 1.7 cm^-1 (10.2949 ± 0.0002 eV) and 83 400.4 ± 1.7 cm^-1 (10.3403 ± 0.0002 eV) for the formation of SO⁺(X²Π_1/2; v⁺ = 0) and SO⁺(²Π_3/2; v⁺ = 0), respectively. The present VUV-PFI-PE measurement has enabled the direct determination of the spin-orbit coupling constant (A₀) for SO⁺(X²Π_1/2,3/2) to be 365.36 ± 0.12 cm^-1. We have also performed high-level ab initio quantum chemical calculations at the coupled-cluster level up to full quadruple excitations and complete basis set (CBS) extrapolation. The zero-point vibrational energy correction, the core-valence electronic correction, the spin-orbit coupling, and the high-level correction are included in the calculation. The IE[SO⁺(X ²Π_1/2,3/2)] and A₀ predictions thus obtained are found to be in remarkable agreement with the experimental determinations. © 2011 American Institute of Physics.