Molecular oxygen (O₂) is surprisingly rare in the universe, despite oxygen's abundance. Its presence on Earth, and to a lesser extent other celestial bodies, poses a significant question regarding its prebiotic formation. The early Earth's atmosphere is thought to have been primarily composed of H₂O, CO₂, and N₂, with minimal O₂. The prevailing theory for prebiotic O₂ production involved VUV photodissociation of CO₂ followed by O atom recombination. This paper challenges that assumption by investigating an alternative pathway: the three-body dissociation (TBD) of water molecules. This is particularly relevant given the abundance of water in the early Earth's atmosphere and in other oxygen-poor environments like comet comae. Understanding prebiotic O₂ formation mechanisms is critical to understanding the development of life-sustaining atmospheres on Earth and potentially other planets. The investigation seeks to quantify the contribution of H₂O photodissociation to prebiotic O₂ production, potentially revealing a previously underestimated oxygen source.

Previous research has focused primarily on CO₂ photodissociation as the main source of prebiotic oxygen. Studies have shown that VUV photodissociation of CO₂ can lead to the formation of CO and O atoms, with subsequent three-body recombination producing O₂. Recent work has identified direct O₂ production pathways via VUV photodissociation of CO₂ and dissociative electron attachment to CO₂. However, the photodissociation of H₂O, a dominant oxygen carrier, has been largely neglected as a major contributor to O₂ production, with the prevailing belief that it primarily produces OH and H atoms. Recent observations of abundant molecular oxygen in the coma of comet 67P/Churyumov-Gerasimenko, however, indicated a correlation between O₂ and H₂O, challenging this assumption and highlighting the need for further investigation into the role of H₂O photodissociation in oxygen production. Existing photochemical models for comets have failed to account for this abundance of O₂, further emphasizing the gap in our understanding.

The study used the Dalian Coherent Light Source (DCLS) to investigate the photochemistry of H₂O in the vacuum ultraviolet (VUV) region (90-110 nm). A tunable VUV free electron laser was employed, providing an intense source of VUV radiation. The H-atom Rydberg tagging time-of-flight (HRTOF) technique was used to detect dissociated hydrogen atoms. This technique provides high sensitivity and allows for the determination of the kinetic energy release (TKER) distribution of the fragments. The H₂O sample was generated in a supersonic beam at a rotational temperature of approximately 10 K, ensuring well-defined initial conditions for the photodissociation experiments. TOF spectra were recorded with the detection axis aligned both parallel and perpendicular to the polarization vector of the VUV-FEL radiation. From these spectra, 3D flux diagrams of the H atom fragments were constructed and subsequently analyzed. The TKER distributions for parallel and perpendicular directions were used to identify the different dissociation channels. The observed TKER distributions were then compared to those expected from binary and three-body dissociation channels. Simulation techniques were used to fit the experimental data to assess the relative contributions of different dissociation channels (i.e., O(³P) + 2H and O(¹D) + 2H), determining their branching ratios. Additional calculations were performed to assess the overall rates of O atom production from H₂O photodissociation and to compare these rates to those obtained from CO₂ photodissociation.

The experiments revealed that three-body dissociation (TBD) is the dominant pathway in the VUV photochemistry of water at wavelengths shorter than 107.4 nm. The two major TBD channels identified were O(³P) + 2H and O(¹D) + 2H. Analysis of the kinetic energy release (TKER) distributions, coupled with simulations, showed that the TBD channels accounted for 67% of the dissociation at 107.4 nm, increasing to above 80% at shorter wavelengths. The branching ratios for the different dissociation channels (both TBD and binary) were determined for multiple wavelengths (92-109 nm). Notably, the O(¹D) + 2H channel, previously unreported in H₂O photodissociation, was found to have a significant contribution, especially at lower kinetic energies. The study also determined that the TBD processes do not proceed via a simultaneous concerted mechanism, but rather via a sequential dissociation mechanism involving several internal conversions and non-adiabatic couplings. Calculations incorporating solar VUV flux, absorption cross-sections, and experimentally determined quantum yields for O atom production indicate that approximately 21% of H₂O photoexcitation events result in O atom formation. Comparing O production rates from H₂O and CO₂, and considering the relative abundances of these molecules in the early Earth's atmosphere, suggests that H₂O photodissociation could have been a substantial source of prebiotic O₂, potentially even exceeding that of CO₂.

The findings demonstrate that H₂O TBD is a significant pathway for O atom production under conditions relevant to the early Earth and other oxygen-poor environments. This previously underestimated process should be incorporated into photochemical models, significantly impacting our understanding of prebiotic O₂ formation. The production of a significant amount of metastable O(¹D) atoms is also crucial because of its high reactivity, potentially influencing other atmospheric chemistry, such as the formation of formaldehyde. The significant contribution of H₂O photodissociation to O₂ production challenges the long-held view of CO₂ as the primary source of prebiotic oxygen, suggesting a more complex interplay of atmospheric processes in the evolution of oxygen-rich atmospheres. The implications extend beyond the early Earth, impacting our understanding of oxygen evolution in water-rich planetary atmospheres in general. Further studies could refine the models by including other potential reactions of these oxygen atoms, leading to a more complete picture of atmospheric evolution.

This study conclusively shows that three-body dissociation of H₂O is a significant pathway for producing oxygen atoms, particularly at VUV wavelengths. This challenges the previous assumption that CO₂ was the primary source of prebiotic oxygen. The abundance of water in various extraterrestrial environments suggests a widespread role for this mechanism in prebiotic O₂ formation. Further research could investigate the detailed dynamics of the TBD channels and the subsequent reactions of the highly reactive O(¹D) atoms in various atmospheric conditions, contributing to a more accurate model of early planetary atmospheres.

The study focused on specific VUV wavelengths and may not fully represent the entire range of solar radiation. The experimental conditions, though carefully controlled, might not perfectly replicate the complexity of early planetary atmospheres. The theoretical calculations rely on assumptions about atmospheric composition and conditions, and uncertainties in these parameters could influence the quantitative estimates of oxygen production rates.