ABSENCE OF THE DIATOMIC ISOTOPE (CUCU-CU-63-CU-65 IN THE LMIS MASS-SPECTRUM OF THE AU0.5CU0.5())

The Cu-2(+) ions formed in a LMIS (liquid metal ion source) using the pure copper metal exhibit three isotopes with the correct statistical intensities whereas when an Au0.5Cu0.5 alloy is employed we only obser ve the (CuCu+)-Cu-63-Cu-63 and (CuCu+)-Cu-65-Cu-65 homoisotopic specie s. We propose the following explanation. In the electric field zone cl ose to the surface the two atoms of the heteroisotopic ion have differ ent trajectories. Then, the molecular ion is deformed and an electroni c excitation appears which makes easier the tunnelling of the external C-u(2+) electron from the ion to the metallic tip. This new ionizatio n destroys the molecular ion.