Double absorption photofragment spectroscopy: A new tool for probing unimolecular processes

A new technique, double absorption photofragment spectroscopy (DAPS), has been developed for observing the time evolution of unimolecular processes. Molecules isolated from collisions in a molecular beam are prepared in a well defined intermediate state by an initiation laser pulse. As this state evolves in time, it is monitored by a second probe laser pulse which, after a chosen delay, photodissociates the molecules. Information about the state to which the molecules have evolved by the time of the probe pulse can be extracted from the mass, density, and angular and energy distributions of the recoiling fragments. Simple models are explored for the functional form of the angular distributions which can be expected in DAPS experiments. This new technique is demonstrated on the I2 B state (used as a real rather than virtual intermediate) whose lifetime is found to vary between 770 and 1600 nsec, depending upon the set of B state v′J′ levels prepared by the initiation laser pulse. Future DAPS experiments should be able to observe unimolecular electronic and nuclear change over a time span from 10−12 to 10−3 sec, probing such processes as rearrangement, dissociation, internal vibrational transfer, intersystem crossing, and internal conversion.