Transition state probing and fragment rotational dynamics following impulsive bond breakage of HgI2

The optical anisotropic response following 30 fs, 266 nm photolysis of mercuric diiodide in ethanol solution was measured to explore transition state dynamics leading to bond fission as well as fragment rotational dynamics in the asymptotic limit for two-body dissociation. The reactive motion in the vicinity of the transition state is accompanied by modifications of the nature of the electronic transition utilized for optical detection. The initial motion away from the originally prepared Franck-Condon region through the transition state toward the fragments effectively rotates the transition moment of the electronic probe resonance in the molecular frame of the dynamically evolving system. After the reactive portion including bond breakage is complete, the anisotropy reveals an inertial component on ultrafast time scales well below 1 ps. The time constant associated with this inertial decay is much faster than the free-rotor time constant of HgI implying that impulsive bond breakage of the parent molecule results in rotational excitation of the diatomic fragment. It is argued that excitation of fragment rotational degrees of freedom presumably arises from pronounced anisotropies of the excited state potential which is known to be bent for isolated HgI2. Furthermore, coherent vibrational motion of the diatomic product gives rise to periodic modulations of the anisotropy due to stretch-bend coupling in the dynamically evolving system.