Imaging rotation of a radical co-fragment in UV-induced photodissociation of dihalomethanes

Numerous studies of the A-band photodissociation of dihalomethanes suggest that more than 80% of the excess photon energy can be deposited into internal degrees of freedom, triggering rotation of the molecular reaction product. We directly map such rotation upon the C-I bond cleavage in CH₂BrI and CH₂ClI at 264 nm by time-resolved Coulomb explosion imaging. The dissociating molecule is multiply-ionized by the 790-nm probe pulse and breaks up into CH₂⁺ and two halogen ions, detected in coincidence. The rotation of CH₂Br or CH₂Cl products shows up as an oscillatory structure (period ~270 fs) in the delay-dependent kinetic energies and relative emission angles of the detected ions. Our results agree well with the simulation based on a rigid-rotor model of the radical co-fragment rotation with increasing C-I bond length and suggest that the dissociating molecule periodically approaches a nearly linear geometry, where the fragment located in the middle acquires minimal kinetic energy.