Lasers wrest oxygen from carbon dioxide

Process may alter view of how Earth’s atmosphere formed

With a zap, a beam of ultraviolet light can blast carbon dioxide into oxygen gas, O2. Because the sun hurls the same radiation, the reaction hints that breathable air could have formed on Earth before the dawn of photosynthetic organisms or even in other CO2-rich atmospheres like those on Mars and Venus, researchers argue in the Oct. 3 Science.

The finding is extremely exciting, says chemist Simon North of Texas A&M University in College Station. Though scientists had theorized that the detritus of a carbon dioxide breakup could include O2, proving it has been extremely difficult. To do it, North says, “the authors have made a beautiful set of challenging measurements.”

Coauthor Cheuk-Yiu Ng a physical chemist at the University of California, Davis, says the researchers first thought they would need the high energies a synchrotron can produce. But the circular particle accelerators are very expensive and can be difficult to access. Instead, Ng and his colleagues assembled a set of lasers that shoot adjustable wavelengths of ultraviolet light in a vacuum. One laser shatters the carbon dioxide and a second beam identifies the molecular debris.

Normally a CO2 molecule will break apart to form a molecule of carbon monoxide and an oxygen atom. This break requires the least amount of energy because in a molecule of CO2, the carbon sits between the two oxygens, so only one bond has to break to form CO and O. But chemists theorized that if CO2 is shot with enough energy, its atoms could become superexcited and jumble to form unexpected structures and fragments.

Chemists predicted that energizing CO2 could cause the two oxygens to link, forming a three-atom ringed structure. Next, one of the two oxygens could break away from the carbon, creating a linear molecule with the oxygens side by side. Finally, O2 could break free from the deformed CO2.

Though the laser-based experiment couldn’t detect each of the stages of this atomic scrambling, the researchers did clearly detect a lone carbon when the second laser ripped an electron off the atom. This measurement suggested that O2 did indeed form, though in tiny quantities – only 5 percent of the CO2 broke up this way.

Because the wavelengths that wrecked CO2 in the experiment are also produced by the sun, the finding suggests that these beams of radiation could create O2 and other unexpected molecular wreckage elsewhere in the universe, Ng says. “This experiment opens up many possibilities,” he adds, hinting at the alien molecules yet to be identified from the newfound chemical zapping. And just as a recent study proposed a way to make O2 from water using UV light (SN: 4/19/14, p. 11), the new study also suggests lifeless planets may carry small amounts of oxygen.

Most interesting is that the creation of O2 could influence how a planet’s atmosphere develops, says physical theoretical chemist Alexander Mebel of Florida International University in Miami. This includes the early atmosphere of Earth. About 2.4 billion years ago, the planet underwent the “Great Oxidation Event” around when photosynthetic microbes originated.

If O2 was already present in the atmosphere prior to that time, however, it could have helped form a livable planet for oxygen breathers, such as animals. “This may affect the entire picture of the prebiotic atmosphere of Earth,” Mebel says. But, he adds, how important those pioneering oxygen molecules were for the formation of Earth’s atmosphere is unclear. Next, he says, researchers will need to include this newfound chemical reaction in computer simulations of how atmospheres form.

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