Photoelectron-photofragment coincidence spectroscopy of the mixed trihalides.

Abstract

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.