Time-resolved Coulomb explosion imaging of photodissociation mechanisms in C₂X₄I₂.

Imaging the nuclear motion during photoinduced reactions is useful to elucidate the ultrafast non-Born-Oppenheimer dynamics that drive chemical processes. We present work that utilizes molecular coulomb explosion imaging in conjunction with velocity map imaging (VMI) to track the structural changes during the photodissociation of atomic iodine from diiodoethane (C₂H₄I₂) and tetrafluorodiiodoethane (C₂F₄I₂). The photochemistry of C₂F₄I₂ has been previously shown to undergo elimination in a two step mechanism.  The first photodissociation of the C–I bond to produce C2F4I• + I• happens on the femtosecond timescale, then a secondary dissociation to produce C₂F₄• + 2I• proceeds on the picosecond timescale. To image gas phase structural dynamics, we employ a pump-probe scheme with a 267 nm pump and a 100 eV femtosecond pulse produced by a free electron laser (FEL) as the probe. The soft x-ray light is tuned such that the atomic iodine is selectively ionized allowing us to track the reaction’s progression from the point of view of the halogen. The VMI technique allows us to present the time-resolved momentum distributions of molecular fragments during the dissociation. In addition to mapping the momentum vectors to a coulomb curve, the covariant angles between the momentum vectors provides further insight into structural dynamics.