Ultraviolet and visible fluorescence produced by controlled electron impact on SO₂

Abstract

The emission spectrum in the ultraviolet and visible region of the optical spectrum (2000–8000 Å) produced by electron impact on SO2 has been studied under single-collision conditions. Two major features have been observed: the A3Π → X3Σ− system of the SO fragment (2400–2650 Å) and a broad, unstructured emission extending from 2500 to 4600 Å, which has been identified as arising from the excitation–de-excitation of the SO2 molecule. Absolute emission cross sections for the SO system have been measured for incident electron energies from threshold to 500 eV, and a value of 5.2 × 10−18 cm2 has been found at 100 eV with an uncertainty of approximately ± 18%. The maximum in the emission cross section is at 30 eV (6.5 × 10−18 cm2). The measured threshold of 11.5 eV is about 1 eV above the minimum energy required for partial fragmentation of SO2 into SO (A3Π) and a ground-state oxygen atom, indicating that very little translational kinetic energy is carried away by the dissociation fragments in the near-threshold region. A complex excitation–de-excitation scheme involving three different excited states of the SO2 molecule, e.g., the 8a1 → 3b1, 1.3B1 states and the 5b2 → 3b11A2 state, produces the broad 2500–4600 Å emission feature. At 100 eV, an emission cross section of 12.2 × 10−18 cm2 has been measured for the SO2 system. Several O1 and S1 emission lines have also been identified, yet they are found to be extremely weak with estimated emission cross sections smaller than 10−21 cm2 at 100 eV impact energy.

Galileo Imaging of Atmospheric Emissions from Io

“…Laboratosy spectra of LV-visible emission from SO, bombarded by electrons at various energies (4,25) show ~nolecular bands extending to visible wavelengths that are brighter in violet than in green wavelengths, just as seen in the SSI data. If SO, is the source of the blue glows, then 10's spectrum may allow us to constrain the temperature of the electrons exciting the emissions.…”

The complete UV spectrum of SO2 by electron impact, 2, The middle ultraviolet spectrum

“…the work byMiller and Becker [1987] for MUVI shows good agreement in relative energy dependence. However, the cross sections found in this study are lower by a factor of about 2.25.…”

“…Previous MUV laboratory studies of SO 2 by electron impact induced fluorescence techniques have been conducted by Miller and Becker [1987]. Their studies extended from 200 to 550 nm but did not include the important threshold energy range 5-11 eV for SO 2 direct excitation and dissociative excitation processes.…”

Partial differential cross sections for the ionization of the SO2 molecule by electron impact

“…Numerous experimental studies of electron collisions with the SO 2 molecule have been carried out in the past, most notably the analysis of the optical emission spectra resulting from electron impact on SO 2 and electron energy loss studies. 3,4 The recent measurements and calculations for the total SO 2 ionization cross section have been reported by Cadez et al, 5 Szmytkowski and Maciag, 6 Kim et al 7 and Lindsay et al 8 Absolute partial SO 2 electron impact ionization cross sections have been reported previously by Smith and Stefenson 9 and Orient and Srivastava. 10 Their results disagreed by as much as a factor of 3.5.…”