Ion Imaging of MgI+ Photofragment in Ultraviolet Photodissociation of Mass-Selected Mg+ICH3 Complex.

We have observed images of MgI+ fragment ions produced in ultraviolet laser photodissociation of mass-selected Mg+ICH3 ions at 266 nm. Split distribution almost perpendicular to the polarization direction of the photolysis laser was observed in the photofragment image. Potential energy curves of Mg+ICH3 were obtained by theoretical calculations. Among these curves, the excited complex ion dissociated along almost repulsive potentials with several avoided crossings, which was connected to MgI+ + CH3. In the ground state of Mg+ICH3, the CH3I was bonded with Mg from the iodine side, and the Mg-I-C bond angle was calculated to be 101.1°. The theoretical results also indicated that the dissociation occurred after the 52A' ← 12A' photoexcitation, where the transition dipole moment was almost parallel to the Mg-I bond axis. The MgI+ and CH3 fragments dissociated each other parallel to the direction connecting those center-of-masses, which was 67° with respect to the transition dipole moment of 52A' ← 12A' photoexcitation. Therefore, the fragment recoil direction was assumed to approach perpendicular tendency against the polarization direction under the fast dissociation process. However, calculated potential energy curves showed a complicated reaction pathway for MgI+ production, including nonadiabatic processes, although the experimental results indicated the fast dissociation reaction.