Unimolecular Dissociation of Cluster Ions Following Multiphoton Ionization and Subsequent Proton Transfer Reaction

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Abstract
Neutral clusters of ammonia comprised of up to 65 monomer units are subjected to multiphoton ionization following their production in a supersonic expansion. Resonance enhanced ionization via two‐photon absorption to the B̃(v') and C̃'(v') states in the monomer and the isotope 15NH3 is observed over a wide wavelength range, but no resonance enhancement is found for the production of the cluster ions, (NH3)n · H+ Following ionization, a proton transfer reaction ensues which leads to the production of protonated ammonia clusters. Unimolecular dissociation of the ions is studied via an electrostatic reflectron incorporated in the field‐free region of the time‐of‐flight drift tube. The rate of Unimolecular dissociation corresponding to the loss of one molecule of ammonia from cluster ions is found to increase strongly with cluster size n, while there is a decrease in rate with cluster size for the loss of two ammonia molecules.