In theory, the creation of ozone - caused by the breakdown of oxygen in the atmosphere by ultraviolet light - is a mass-independent process. However, experiments by Klaus Mauersberger and colleagues at the Max-Planck institute for nuclear physics in Heidelberg indicate that the collisions between oxygen atoms and molecules that form ozone are not mass-independent (Science 283 370).

The German researchers studied the six different atom-molecule reactions that lead to the formation of ozone. By creating ozone in a series of different gas mixtures, each containing different amounts of the three oxygen isotopes, the German group was able to study the reaction rates for each isotope. It found that the reaction rate for creating heavy molecules of ozone was up to 50% higher than that for creating lighter versions.