Femtosecond observation of a concerted chemical reaction

A time-resolved study is presented of the photodissociation of methylene iodide (CH2I2) to produce a carbene diradical and molecular iodine. This reaction pathway occurs only at high excitation energies (vacuum ultraviolet) and is initiated in this case by a two-photon transition at λequivalent = 155 nm. Discrimination over competing pathways is achieved by wavelength-selective monitoring of the nascent I2 dynamics. The results show clear evidence that breakage of the two carbon-iodine bonds and formation of the iodine-iodine bond is concerted. The reaction takes place in less than 100 fs and is characterized by coherent vibrational motion in the iodine molecule.