FEMTOSECOND OBSERVATION OF A CONCERTED CHEMICAL-REACTION

A time-resolved study is presented of the photodissociation of methyle ne 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 lambda(equivalent) = 155 nm. Discrimination over competing pathway s is achieved by wavelength-selective monitoring of the nascent I-2 dy namics. The results show clear evidence that breakage of the two carbo n-iodine bonds and formation of the iodine-iodine bond is concerted. T he reaction takes place in less than 100 fs and is characterized by co herent vibrational motion in the iodine molecule.