Ion Imaging of MgI Photofragment in Ultraviolet Photodissociation of Mass-Selected MgICH Complex.

We have observed images of MgI fragment ions produced in ultraviolet laser photodissociation of mass-selected MgICH 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 MgICH 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 + CH. In the ground state of MgICH, the CHI 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 5A' ← 1A' photoexcitation, where the transition dipole moment was almost parallel to the Mg-I bond axis. The MgI and CH fragments dissociated each other parallel to the direction connecting those center-of-masses, which was 67° with respect to the transition dipole moment of 5A' ← 1A' 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.