Electron-ionization-dissociation dynamics of laser-excited SO2B̃B11/ÃA21 studied using velocity map imaging

It is well known that electron-driven chemistry for electronically excited molecules is markedly different from that for their ground-state counterparts, yet in most cases, reaction mechanisms and electron scattering cross sections are determined by theory alone due to experimental challenges. We report the development of a pump-probe velocity map imaging (VMI) apparatus, where laser-excited molecules in a molecular beam subsequently interact with a dissociating electron beam. The product ions are probed using a calibrated VMI mass spectrometer to report absolute cross sections and information on the dynamics of the dissociation reactions. We demonstrate the apparatus on the electron-ionization-dissociation (EID) process of SO2 molecules excited to the electronically mixed Clements' manifold B̃B11/ÃA21. Compared to ground-state SO2 molecules, we observed that for the state-selective EID formation of SO+ cations: (a) the thermodynamic threshold is lowered by the energy of the electronic excitation and (b) the partial cross sections with equivalent total energy are enhanced by factors of 2-4 for two distinct vibrational bands of the Clements' manifold. Finally, the VMI images reveal that the SO+ fragments from the electronic excited states of SO2 are formed with kinetic energies up to 0.5 eV, whereas those from the ground state are predominantly below 0.2 eV. The change in the kinetic energy distribution is attributed to intramolecular vibrational relaxation and molecular geometry changes on the lower adiabatic surface of the Clements' manifold, which shift the Franck-Condon region of the dissociative cationic state.