NEAR-THRESHOLD PHOTOIONIZATION OF NICKEL CLUSTERS - IONIZATION-POTENTIALS FOR NI-3 TO NI-90

The threshold photoionization efficiency (PIE) curves for nickel clusters in the size range Ni3 to Ni90 have been measured by laser photoionization with detection by time-of-flight mass spectrometry. Both warm (≤298 K) and cold (≤77 K) clusters have been studied. The PIE curves for 298 K clusters display thermal tails, while these tails are smaller for cold clusters. Cluster ionization potentials (I.P.s) have been determined by two methods: the Watanabe procedure and linear extrapolation of the PIE curves. Dramatic dependence of I.P. on cluster size is found for clusters smaller than 11 atoms, while the I.P.s of larger clusters decrease relatively smoothly and nearly monotonically from 5.84 eV for Ni11 to 5.56 eV for Ni 90. The I.P.s for clusters larger than Ni40 show the linear dependence on reciprocal radius (R-1) predicted by the conducting spherical drop model of small particle I.P.s, but do not fit the model quantitatively unless the limiting (R-1 → 0) work function is reduced by 0.46 eV from the bulk polycrystalline value. The differences between the thermal tails of the room temperature and 77 K PIE curves diminish with increasing cluster size, suggesting a reduced difference between neutral and ionic structures for larger clusters. In general, there is poor agreement between our experimental results and theoretically calculated I.P.s for small nickel clusters, with the exception of the recently reported tight-binding theory results of Pastor et al. [Chem. Phys. Lett. 148, 459 (1988)]. © 1990 American Institute of Physics.