DELAYED IONIZATION IN TRANSITION METAL-CARBON CLUSTERS - FURTHER EVIDENCE FOR THE ROLE OF THERMIONIC EMISSION

The delayed ionization behavior of several single and binary transitio n metal-carbon cluster systems, in which metallocarbohedrenes (Met-Car s) are produced, is reported. The single metal Met-Cars, M8C12, where M=Ti, Nb, or Zr, and the binary metal Met-Cars, TixMyC12, where M=Zr, Nb, or Ta and x+y = 8, are investigated at various selected photoioniz ation wavelengths (266, 355, 532, 590, and 620 nm) and over a range of fluences (20 to 120 mJ/cm(2)). In addition, a variable temperature ex periment is performed in order to estimate the initial internal energy of the clusters after expansion from a laser vaporization sourer. Del ayed ionization, which is observed at all wavelengths studied, is well described by a statistical model based on an adaptation of the Richar dson-Dushman relation. The new findings reported herein provide eviden ce that all of phase space is sampled, supporting the proposal that th e observed delayed ionization is thermionic in character. The temporal dependence of the ionization is found to correlate with the ionizatio n potentials of two systems whose values are available. The ionization potential trends of the mixed metal Met-Cars can by predicted using t he thermionic emission model. (C) 1998 American Institute of Physics. [S0021-9606(98)01221-5]