Molecular beam photoionization study of SO2 and (SO2)2

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Abstract
Photoionization efficiency (PIE) data for SO2+ have been obtained with a wavelength resolution of 0.14 Å (FWHM) in the region 625–1005 Å using the molecular beam method. The ionization energy (IE) of SO2 was determined to be 12.348±0.002 eV (1004.08±0.20 Å). Similar to the observation in the PIE curve for O3+, the spacing for steplike structure observed near the threshold was found to be irregular. Weak structures which arise by autoionization from different vibrational states of Rydberg levels were also resolved in this region. The analysis gives average vibrational spacing of 386, 428, 716, 745, 911, 938, and 956 cm−1 for these Rydberg states. The appearance energy (AE) for the photodissociative ionization processes SO2+hν→SO++O+e− and S++O2+e− were measured to be 15.953±0.010 eV (777.2±0.5 Å) and 16.228±0.030 eV (764±1.5 Å), respectively. Using the AE for the formation of SO+ from SO2, and the heats of formation of SO2, SO, and O, the IE of SO is deduced to be 10.28±0.02 eV. This value is in excellent agreement with that reported previously by Dyke et al. From the observed IE (11.72±0.03 eV) of (SO2)2, the IE of SO2, and the estimated binding energy (0.03 eV) of (SO2)2, the bond dissociation energy of SO2+ ⋅ SO2 is found to be 0.66±0.04 eV. Using the measured AE (15.38±0.06 eV) for S2O3+ production from (SO2)2, a lower bound for the binding energy of SO+ ⋅ SO2 was calculated to be 0.60 eV.

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