One-color multiphoton threshold photoelectron spectra of methyl bromide, and their comparison with methyl iodide

Threshold photoelectron spectra (ZEKE) of methyl iodide, bromide, and chloride are investigated using a one-color, two-photon ionization. Resonances due to a dissociative intermediate state are responsible for differences between spectra of the three halides, with long progressions in the C-I stretching vibration being observed in CH3I, where the one photon absorption is near the maximum of the dissociative continuum, while the vibrational structure is much less prominent in the lighter halides, where this does not occur. The spectra provide accurate ionization potentials of the three compounds as well as insights into their dissociation dynamics, and extensive information about the vibrational structure of their cations. (C) 2002 American Institute of Physics.