Measurements of the delayed ionization of the zirconium Metallocarbohedrene (Met-Car, Zr8C12), obtained employing a recently developed reverse field technique (RFT), are presented. Two methods have been used in the past to study delayed ionization: the "passive" method, where the shape of the mass peak in a mass spectrum is studied, and the "active" method, where a blocking field technique is used to sample the delayed ions created during specific time intervals. The RFT is a newly modified version of the blocking field technique, which allows the relative amount of delayed ionization during 50 ns time slices to be measured starting at the time the excitation laser interacts with the species under study. The fitting of the thermionic emission model to the delayed ionization data of the clusters investigated in the present study is described in detail. Previous use of the thermionic emission model, as applied to the blocking field technique, did not mathematically account for the longevity of the extraction pulse. A modified thermionic emission model, which accurately describes the reverse field technique, is presented. Comparisons between the data and model fit of the Met-Car are made, showing excellent agreement. (C) 2003 American Institute of Physics.