Oxygen in our atmosphere comes mainly from photosynthesis and not many multi-step abiotic processes producing oxygen from carbon dioxide are known.
But now work with a vacuum ultra violet laser (simulating uv light in the upper atmosphere) shows that the resulting excitation of carbon dioxide molecules can lead to the production of oxygen by disassociation. Vacuum ultra violet (200 – 10 nm; 6.20 – 124 eV) is strongly absorbed by atmospheric oxygen, but 150–200 nm wavelengths can propagate through nitrogen. This is particularly intriguing since it would be controlled by the oxygen concentration in the upper atmosphere. A lack of oxygen would lead to an increase of available vacuum uv available to trigger the disassociation of any carbon dioxide present. This could be a continuous and natural process where carbon dioxide, excited by solar ultra violet light in the upper atmosphere, is broken down to produce oxygen.
Perhaps this happens often enough and in sufficient volume to dampen CO2 concentration increase in the atmosphere.
UC Davis chemists have shown how ultraviolet light can split carbon dioxide to form oxygen in one step. Credit: Zhou Lu
Z. Lu, Y. C. Chang, Q.-Z. Yin, C. Y. Ng, W. M. Jackson. Evidence for direct molecular oxygen production in CO2 photodissociation. Science, 2014; 346 (6205): 61
Abstract: Photodissociation of carbon dioxide (CO2) has long been assumed to proceed exclusively to carbon monoxide (CO) and oxygen atom (O) primary products. However, recent theoretical calculations suggested that an exit channel to produce C + O2 should also be energetically accessible. Here we report the direct experimental evidence for the C + O2 channel in CO2 photodissociation near the energetic threshold of the C(3P) + O2(X3Σg–) channel with a yield of 5 ± 2% using vacuum ultraviolet laser pump-probe spectroscopy and velocity-map imaging detection of the C(3PJ) product between 101.5 and 107.2 nanometers. Our results may have implications for nonbiological oxygen production in CO2-heavy atmospheres.
UC Davis graduate student Zhou Lu, working with professors in the Departments of Chemistry and of Earth and Planetary Sciences, has shown that oxygen can be formed in one step by using a high energy vacuum ultraviolet laser to excite carbon dioxide. (The work is published Oct. 3 in the journal Science).
“Previously, people believed that the abiotic (no green plants involved) source of molecular oxygen is by CO2 + solar light — > CO + O, then O + O + M — > O2 + M (where M represents a third body carrying off the energy released in forming the oxygen bond),” Zhou said in an email. “Our results indicate that O2 can be formed by carbon dioxide dissociation in a one step process. The same process can be applied in other carbon dioxide dominated atmospheres such as Mars and Venus.”
Zhou used a vacuum ultraviolet laser to irradiate CO2 in the laboratory. Vacuum ultraviolet light is so-called because it has a wavelength below 200 nanometers and is typically absorbed by air. The experiments were performed by using a unique ion imaging apparatus developed at UC Davis.
Such one-step oxygen formation could be happening now as carbon dioxide increases in the region of the upper atmosphere, where high energy vacuum ultraviolet light from the Sun hits Earth or other planets. It is the first time that such a reaction has been shown in the laboratory. According to one of the scientists who reviewed the paper for Science, Zhou’s work means that models of the evolution of planetary atmospheres will now have to be adjusted to take this into account.
Tags: carbon dioxide, disassociation, oxygen, uv light
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