The ground-breaking work of the Zewail group (one of the five discoveries cited in the announcement of Ahmed Zewail's receiving the 1999 Nobel Prize in Chemistry) demonstrated experimentally that the reaction of photodissociation of cyclobutane to form two molecules of ethylene proceeds through two steps: first, forms the tetramethylene diradical, and then, proceeds to the products. Our model nicely reproduces this reaction, as shown in an animation file and figure below, and provides much detailed dynamics information for the understanding of the reaction mechanism.

The animation file from our simulation of the photodissociation of cyclobutane to form two ethylene molecules is shown here. A 60 fs FWHM laser pulse, centered at 6.5 eV with a fluence of 42.20 kJm-2, is applied. The formation of the tetramethylene diradical reaction intermediate and later on two ethylene molecules occurs at about t = 127 fs and 160 fs respectively.


Distance variation with the time of C-C making up one ethylene molecule, as shown in the right, indicates that the C-C single bond begins to shorten at about 130 fs and becomes a double bond after about 200 fs. This time period corresponds to a lifetime for the radical reaction intermediate.