The provenance of oxygen on Earth and other solar planetary bodies is a fundamental issue. It is widely accepted that the prebiotic pathway of oxygen production in the Earth primitive atmosphere was via vacuum ultraviolet (VUV) photodissociation of CO₂ and subsequent recombination of two O atoms.

In contrast, the photodissociation of H₂O, one of the dominant oxygen carriers, has long been assumed to proceed mainly to produce hydroxyl (OH)- and hydrogen (H)-atom primary products, and its contribution to oxygen production is limited.

Recently, a research group led by Prof. YUAN Kaijun and YANG Xueming from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences revealed oxygen production from the three-body photodissociation of water molecule using the Dalian Coherent Light Source.

Their findings were published in Nature Communications on April 30.

Dalian Coherent Light Source revealing three-body photodissociation of water as an important prebiotic-O₂ source (Image by YUAN Kaijun) 

The VUV free-electron laser facility at the Dalian Coherent Light Source allows the researchers to quantitatively assess the importance of H₂O photochemistry for oxygen production.

"Our experimental results indicated that H₂O under VUV excitation can break into three fragments: one O atom and two H atoms, where the O atoms are in the ¹D and ³P states. The three-body dissociation process is the dominant channel for H₂O photochemistry in the 90-110 nm region," said Prof. YUAN.

The quantitative determination demonstrated that approximately 20% of the H₂O photoexcitation events resulted in O atoms. Considering the water abundance in widely interstellar circumstances such as in interstellar clouds, atmospheres of the solar-family comets, and even in the Earth primitive atmosphere, O production from water photolysis must be an important process. The subsequent recombination of O atoms produced O₂, which represented an important prebiotic O₂-production pathway.

This research was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, Chemical Dynamics Research Center, and the National Natural Science Foundation of China.