Multiphoton ionization study of intra- and intermolecular effects on the photodissociation of methyl iodide

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Abstract
A time-of-flight mass spectrometer (TOFMS) was used to obtain multiphoton ionization (MPI) mass and wavelength spectra of methyl iodide. The sample was studied in a molecular jet environment which allowed preparation of isolated molecules and/or clusters of different size. Excitation was performed to access the dissociative valence A state and the predissociative Rydberg C state. Valence state excitation produced no TOF ion signal for (CH3 I)n, n=1, but produced I+2 for n=2 and CH3 I+ for large n. The presence of CH3 I+ is an indication of cluster ‘‘caging’’ of the chromophore and consequential stabilization of the valence state by intermolecular interactions. REMPI spectra of the Rydberg C state origin and C–I stretch produced results that reflect the longer lifetime of this intermediate state. Spectral line shapes and intensities, and mass fragmentation patterns are discussed in light of the photodissociation dynamics of the intermediate Rydberg state and the effect of intermolecular interactions on this process.