Mass spectrometric study of negative, positive, and mixed KI cluster ions by using fast xe atom bombardment

We have recorded the mass spectra of I^-(KI)_n, K⁺(KI)_n, and mixed KI/MI cluster ions (M = Na and Cs) by using the fast Xe atom bombardment method. The mass spectral pattern of I^-(KI)_n cluster ions was similar to that of K⁺(KI)_n cluster ions except for the overall enhanced ion intensity of K⁺(KI)_n cluster ions, which implied that both negative and positive cluster ions were produced through the same mechanism. Magic numbers were observed at n = 13, 22, 31, and 37 which corresponded to structures composed of one, two, three, and four unit cells, respectively, which implied that only multiples of the unit cell structure were extra-stable. The ion intensities in mixed KI/MI (M = Na and Cs) cluster ions depended on the positive ion radius, and as a result, the cluster ions with as many positive ions of ionic radius as close to that of I^- as possible were stronger than other cluster ions, which indicated that cluster ion stability increased as the ionic radius difference between the positive ion and I^- decreased.