Femtosecond real-time probing of reactions. V. The reaction of IHgI

Abstract

The dissociation reaction of HgI2 is examined experimentally using femtosecond transition‐state spectroscopy (FTS). The reaction involves symmetric and antisymmetric coordinates and the transition‐state is well‐defined: IHgI*→[IHgI]‡* Q S’ Q a’ q →HgI+I. FTS is developed for this class of ABA‐type reactions and recurrences are observed for the vibrating fragments (symmetric coordinate) along the reaction coordinate (antisymmetric coordinate). The translational motion is also observed as a ‘‘delay time’’ of the free fragments. Analysis of our FTS results indicates that the reaction wave packet proceeds through two pathways, yielding either I(2 P 3/2) or I*(2 P 1/2) as one of the final products. Dissociation into these two pathways leads to HgI fragments with different vibrational energy, resulting in distinct trajectories. Hence, oscillatory behaviors of different periods in the FTS transients are observed depending on the channel probed (∼300 fs to ∼1 ps). These results are analyzed using the standard FTS description, and by classical trajectory calculations performed on model potentials which include the two degrees of freedom of the reaction. Quantum calculations of the expected fluorescence of the fragment are also performed and are in excellent agreement with experiments.