Molecular ion spectroscopy: Mass selected, resonant two-photon dissociation spectra of CH3I+ and CD3I+

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Abstract
We present a new ion spectroscopic technique by which it is possible to investigate larger molecular ions by scanning an entire electronic state and coincidentally measuring and, hence, fixing the mass of the fragment ion observed. The technique involves primary resonance excitation together with secondary absorption followed by dissociation as detection process, employing the absorption of two photons in the ion. This method accesses the nondissociating vibronic levels of an electronic state rather than just the narrow range of predissociating levels, as in one‐photon dissociation spectroscopy. Spectra of CH3I+ and CD3I+ over a range of 7000 cm−1, beginning at the origin of the à state, were recorded with progression bands up to n=21. These spectra allow for the first time an unambiguous assignment of the Ã←X̃ transition of CH3I+ and CD3I+. The exact positions of the electronic origin as well as of the origin of several progressions and new values for several vibrational constants of methyl iodide cations are reported. Some 300 bands in both ions were measured and assigned with this technique.