Photoelectron-photofragment coincidence spectroscopy of the mixed trihalides

Photoelectron-photofragment coincidence (PPC) spectroscopy is used to study the photodetachment, photodissociation, and dissociative photodetachment (DPD) of I₂Br^-, IBr₂ ^-, I₂Cl^-, and ICl₂ ^- at 266 nm. The mixed trihalides are asymmetric analogs of the well-studied I₃ ^- anion, with distinguishable dissociation asymptotes and the potential for selective bond breaking. The high beam energy PPC spectrometer used in this study couples an electrospray ionization source, a hexapole accumulation ion trap, and a linear accelerator to produce a 21 keV beam of a particular trihalide. Total, stable, and dissociative photoelectron spectra have been recorded for all the anions, except ICl₂ ^- that does not photodetach at 266 nm. A bound ground state (X) is observed for all the anions, and a dissociative first excited (A) state is also seen for I₂Br^- and I₂Cl^- at low electron kinetic energies (eKE). A 258 nm photoelectron spectrum recorded for I₂Br^- and I₂Cl^- rules out autodetachment of a dipole-bound state as the origin of the low eKE feature. The threshold detachment energy (TDE) of I₂X^- to the X state of the radical is similar to I₃ ^-, whereas the TDE to the radical A state increases with substitution of iodine for a lighter halogen. Two-body DPD is observed for I₂Br^- and I₂Cl^-, resulting in IBr/ICl + I + e^-. For IBr₂ ^- and ICl₂ ^-, the charge symmetric three-body photodissociation of [Br-I-Br]^- and [Cl-I-Cl]^- is seen yielding Br + Br and Br + Br^*, and Cl + Cl and Cl + Cl^* neutral fragments. Evidence for the minimum energy anion structure is observed in all cases, where the iodine atom is located at the center of the trihalide.