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.

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