Electron photodetachment from transient negative ions in the multiphoton ionization of CH₃I

Photoelectron spectra produced by the laser multiphoton ionization of CH₃I show a strong band due to absorption of an additional photon above the ionization threshold and resembling either inverse bremsstrahlung or "above threshold ionization." The spectra are ascribed to a process of capture of Coulomb-retarded photoelectrons to form transient CH ₃I^- ions which are then photodetached before, during and after dissociation to CH₃+I^- products. Both CH ₃^- and I^- ions are observed. Several aspects of the process are discussed in terms of a potential energy diagram.