Photodissociation dynamics of CH₃I and CD₃I probed by zero kinetic energy photoelectron spectroscopy

One-color two-photon ionization of methyl iodide, resulting in the CH₃I⁺ and CD₃I⁺ ions in the X̃ ²E_3/2 and ²E_1/2 electronic states, is investigated using the zero kinetic energy (ZEKE) photoelectron technique. Unlike the conventional photoelectron spectra, the two-photon spectra are resonantly enhanced via the dissociative Ã-state continuum and exhibit an extensive vibrational structure. A long progression in the C-I stretching vibration ν₃⁺ is observed, and reliable values for all the vibrational frequencies of the ions and for the ionization potentials are obtained. The appearance of the spectra is discussed in terms of consecutive two-photon absorption via a dissociative intermediate state and a time-dependent picture of spectroscopy. Experimental evidence for the influence of the asymmetric ν₆ vibration on the photodissociation process is presented. It is shown that two-photon ZEKE spectroscopy via a dissociative intermediate state can serve as a powerful probe of the photodissociation dynamics. It also allows one to explore regions of the ground-state potential energy surface of the ion that are far from equilibrium.