Se. Kooi et Aw. Castleman, DELAYED IONIZATION IN TRANSITION METAL-CARBON CLUSTERS - FURTHER EVIDENCE FOR THE ROLE OF THERMIONIC EMISSION, The Journal of chemical physics, 108(21), 1998, pp. 8864-8869
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]