Investigating the ultraviolet photodissociation of bromocyclopropane with ultrafast electron diffraction

We have studied the photodissociation of gas-phase bromocyclopropane in the gas-phase by 200-nm wavelength ultraviolet radiation using ultrafast electron diffraction. Bromocyclopropane is a prototypical molecule in the study of organobromides, a class of molecules which have a significant impact on atmospheric ozone depletion through their photochemistry. Previous studies have revealed two possible reaction pathways for the photodissociation of bromine from bromocyclopropane; either the C-Br bromine bond dissociates leaving behind a cyclopropyl ring, or there is a concerted opening of the cyclopropyl ring along with the bromine C-Br bond dissociation. In this work, both our experimental and simulation results indicate that the majority of the UV-photoexcited BCP molecules (84 ± 10 %) follow the first reaction pathway in which the cyclopropyl ring remains closed after the bromine dissociation by homolytic C-Br bond cleavage. This direct bond dissociation occurs within the experimental time resolution of 300 fs. In order to differentiate between the possible reaction end-products, both of which have diffraction signals dominated by the bromine atom dissociation, a new analysis method has been employed which is more sensitive to the structure of the end-products.