Femtosecond velocity map imaging of dissociative ionization dynamics in CF3I

Abstract

The multi-photon dissociation dynamics of CF₃I has been studied with femtosecond pump–probe spectroscopy and velocity map ion imaging. The CF₃⁺ and I⁺ fragments produced in a time-delayed pump–probe excitation are detected with a two-dimensional ion imaging setup in a velocity map imaging configuration. The ion images for the selected ionic photofragments provide the velocity and angular distribution of the recoiling fragments. The experiments were performed with both parallel and perpendicular polarization geometry of the pump laser, at 264 nm, versus the probe laser at 396 nm. The velocity and angular distributions provide information on the multi-photon pathways and the potential energy surfaces involved. The CF₃⁺ fragments are mainly formed by a two-photon pump excitation at 264 nm, via the one-photon resonant A band, to the 5pπ7sσ(²Π_1/2) Rydberg state followed by a one-photon probe excitation to the CF₃I⁺ parent states with subsequent dissociation. Analysis of the I⁺ data indicates that at most delay times the fragments are formed via a two-photon absorption at 264 nm to the Rydberg state, followed by a two-photon transition at 396 nm to the state of the parent ion. However, at delay times around 200–400 fs the kinetic energy distribution of the I⁺ fragments changes dramatically relative to 0 fs and 1000 fs. The origin of the very slow I⁺ fragments is probably a bound–free–bound excitation via the repulsive A band to a higher lying ion-pair state. It is shown that the ion imaging technique combined with femtosecond time-resolved spectroscopy provides a direct view of the complex dynamics and multi-photon pathways involved in the dissociative photodynamics of CF₃I.