Probing vibrationally mediated ultrafast excited-state reaction dynamics with multireference (CASPT2) trajectories

Excited-state trajectories computed at the complete active space second-order perturbation theory (CASPT2) reveal how vibrational excitation controls the molecular approach to the intersection space that drives the photodissociation of a prototypical halogenated methyl radical, namely CF ₂I. Translating the Franck-Condon structure along the ground-state CASPT2 vibrational modes in this system followed by propagating the displaced structures in the first excited doublet state simulates specific vibrational excitations and vibrationally mediated dynamics, respectively. Three distinct situations are encountered: the trajectories (i) converge to an energetically flat segment of the intersection space, (ii) locate a segment of the intersection space, and (iii) access a region where the intersection space degeneracy is lifted to form a ridge of avoided crossings. The computational protocol documented herein can be used as a tool to design control strategies based on selective excitation of vibrational modes, including adaptive feedback schemes using coherent light sources.