Photoionization of ammonia clusters: Detection and distribution of unprotonated cluster ions (NH3)+n, n=2–25

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Abstract
Unprotonated ammonia cluster ions (NH3)+n of various sizes (n=2–25) are detected for the first time, in addition to normally observed protonated ions (NH3)n−1H+, via the molecular‐beam VUV photoionization by using Ar (11.83, 11.62 eV), Kr (10.64, 10.03 eV), and Xe (9.57 eV) resonance lamps. The observation of the species indicates that there exist potential minima for the various (NH3)+n cluster ions along the proton‐transfer reaction coordinates. Considerations based on intracluster ion–molecule reactions reveal that the unprotonated cluster ions have the proton‐transferred structures of (NH3)n−2⋅NH+4 ⋅⋅⋅ NH2 rather than (NH3)+n which usually lead to the dissociated protonated products (NH3)n−1H+. Furthermore, the results also provide evidence that the so‐called ‘‘magic numbers’’ found in the intensity distributions of both the protonated (NH3)4⋅NH+4 and unprotonated (NH3)+5, i.e., (NH3)3⋅NH+4⋅NH2 cluster ions suggest the special stability of a certain ion cluster rather than that of the corresponding neutral cluster.