ISOTOPIC ANOMALY IN DIMER EMISSION FROM ALLOY LIQUID-METAL-ION-SOURCEMASS-SPECTROSCOPY

In a pure copper liquid-metal-ion source (LMIS) the three Cu-2(+) isot opes are emitted with intensities close to the natural abundances whil e for a Au0.5Cu0.5 LMIS we only observe the (CuCu+)-Cu-63-Cu-63 and (C uCu+)-Cu-65-Cu-65 homoisotopic species. A similar phenomenon appears f or the emission of Ge: from pure germanium compared to a Au0.73Ge0.27 alloy. We observe that the emission of the heteroisotopes is strongly reduced in the alloy case. We propose the following interpretation. In the electric-field zone close to the surface the two atoms of the het eroisotopic ion have different trajectories. As a consequence the mole cular M(2)(+) ion (M=Cu or Ge) is deformed and an electronic excitatio n appears which makes easier the tunneling of the outer electron from M(2)(+) to the bulk available levels of the liquid metal or alloy. Mor eover, the electronic structure of the alloy is such that the tunnelin g effect is easier than for the metal (for example, the work function is larger for Au0.5Cu0.5 than for Cu). Then, the absence or reduction of the Cu-2(+) or Ge-2(+) heteroisotope intensities in alloy LMIS woul d be due to the conjunction of two effects: presence of an electronic excitation specific to heteroisotopes and easier tunneling effect for alloys.