Abiotic molecular oxygen production-Ionic pathway from sulfur dioxide

Abstract

Molecular oxygen, O₂, is vital to life on Earth and possibly also on exoplanets. Although the biogenic processes leading to its accumulation in Earth's atmosphere are well understood, its abiotic origin is still not fully established. Here, we report combined experimental and theoretical evidence for electronic state-selective production of O₂ from SO₂, a chemical constituent of many planetary atmospheres and one that played an important part on Earth in the Great Oxidation Event. The O₂ production involves dissociative double ionization of SO₂ leading to efficient formation of the [Formula: see text] ion, which can be converted to abiotic O₂ by electron neutralization or by charge exchange. This formation process may contribute substantially to the abundance of O₂ and related ions in planetary atmospheres, such as the Jovian moons Io, Europa, and Ganymede. We suggest that this sort of ionic pathway for the formation of abiotic O₂ involving multiply charged molecular ion decomposition may also exist for other atmospheric and planetary molecules.

Fig. 1.. Electron coincidence spectra and breakdown diagrams of doubly ionized SO₂.

(A) Electron pairs measured in coincidence with two ions in red (from fourfold events) and one ion in purple (from threefold events) upon photoionization of SO₂ at 40.81-eV photon energy. For comparison, the black curve is a higher-resolution electron pair–only spectrum of the total double ionization at the same photon energy previously discussed in (27). The bar combs mark the vertical ionization energies computed at the MRCI/aug-cc-pV(Q+d)Z level of theory at the neutral SO₂ (X ¹A₁) ground-state equilibrium geometry, i.e., at an O─S─O angle of 120^∘ and SO distance of 2.7 bohr [see (28) for more details]. (B) Breakdown diagram of the major detectable decay channels of doubly ionized SO₂. (C) Metastable isomers of ${\text{SO}}_2^{2+}$, which are accessible in the energy region where ${\mathrm{O}}_2^{+}$ + S⁺ is detectable. We give their relative energies with respect to the SO₂ (X¹A₁) vibrationless level.

Fig. 2.. Potential energy surface cuts and minimum energy paths of the lowest electronic states of SO22+ for varying bond distance of S.

(A) One-dimensional PES cuts of the lowest states of ${\text{SO}}_2^{2+}$ for the in-plane angle θ = 89° along the bond distance of S to the center of mass of O₂, where the O₂ distance is kept fixed at its value in the neutral SO₂ (X ¹A₁) ground-state equilibrium geometry (i.e., 4.6 bohr). (B) Corresponding MEP for bent ${\text{SO}}_2^{2+}$ and O─O─S²⁺. (C) Thermodynamic thresholds of ${\mathrm{O}}_2^{+}$ + S⁺. The reference energy is that of SO₂ (X ¹A₁) in its vibrationless ground state. Other cuts are given in figs. S2 to S5.

Fig. 3.. Potential energy surface cuts of the lowest electronic states of SO22+ for varying angle φ.

(A) PES cuts of the lowest electronic states of ${\text{SO}}_2^{2+}$ by varying angle φ from 0° to 120°. (B) Thermodynamic thresholds of ${\mathrm{O}}_2^{+}$ + S⁺. The reference energy is that of SO₂ (X ¹A₁) in its vibrationless ground state. Other cuts are given in figs. S2 to S5.

Fig. 4.. Potential energy surface cuts of the lowest electronic states of SO22+ for varying angle τ.

(A) PES cuts of the lowest electronic states of ${\text{SO}}_2^{2+}$ by varying angle τ from 0° to 180°. In (B), the thermodynamic thresholds of ${\mathrm{O}}_2^{+}$ + S⁺ are indicated. The reference energy is that of SO₂ (X ¹A₁) in its vibrationless ground state. Other cuts are given in figs. S6 to S9.

Fig. 5.. Potential energy surface cuts and minimum energy paths of OOS²⁺.

(A) One-dimensional PES cuts of OOS²⁺ with an O─O distance at the neutral O₂ (${\mathrm{X}}^3{\mathrm{\Sigma }}_g^{-}$) ground-state equilibrium geometry. (B) MEPs of OOS²⁺ with a stable ¹A^′ state in quasi-linear configuration. In (C), the thermodynamic thresholds of ${\mathrm{O}}_2^{+}$ + S⁺ are indicated. The reference energy is that of SO₂ (X ¹A₁) in its vibrationless ground state. Other cuts are given in figs. S6 to S9.