Understanding Ozone

Chemists have puzzled for the past 30 years over the strange behavior of ozone molecules in the Earth’s atmosphere.

Ozone (O₃), a gas known for its ability to absorb the sun’s potentially harmful ultraviolet rays in the stratosphere, performs that function when it splits into a single oxygen atom and an O₂ oxygen molecule and subsequently recombines.

Scientists 30 years ago discovered that when ozone breaks up, its O₂ photofragment showed a strong preference to its rotational states with even quantum numbers, over odd quantum numbers.

Why?

Distinguished Professor Hua Guo in UNM’s Department of Chemistry and Chemical Biology, with colleagues from Texas A&M University, has the answer.

In a paper published in 2020 in the Proceedings of the National Academy of Sciences, Guo explains how he solved the problem and why it matters.

“The rotational quantum number dictates how fast the O₂ molecule rotates and it contains important information about how the O₃ molecules break up in an excited electronic state,” Guo said. “The O atom is largely formed in its excited electronic state and it can have a large impact on the formation of the hydroxyl radical, which serves as the detergent in cleaning various pollutions in Earth’s atmosphere.”

Using a supercomputer due to the complexity of the calculations required, Guo’s team was able to solve the differential equation and definitively explain why the 02 photofragment disproportionately prefers rotational states with even quantum numbers.

According to Guo, “the even-odd propensity in the O₂ rotational states stems from preference of O₃ to break up within the molecular plane.” While plans for follow-up research have not been made at this time, Guo explained the importance of better understanding the processes of how ozone forms.