Isotopic anomaly in dimer emission from alloy liquid-metal-ion-source mass spectroscopy.

Abstract

In a pure copper liquid-metal-ion source (LMIS) the three ${\mathrm{Cu}}_{2}^{+}$ isotopes are emitted with intensities close to the natural abundances while for a ${\mathrm{Au}}_{0.5}$${\mathrm{Cu}}_{0.5}$ LMIS we only observe the $^{63}\mathrm{Cu}^{63}$${\mathrm{Cu}}^{+}$ and $^{65}\mathrm{Cu}^{65}$${\mathrm{Cu}}^{+}$ homoisotopic species. A similar phenomenon appears for the emission of ${\mathrm{Ge}}_{2}^{+}$ from pure germanium compared to a ${\mathrm{Au}}_{0.73}$${\mathrm{Ge}}_{0.27}$ 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 heteroisotopic ion have different trajectories. As a consequence the molecular ${\mathit{M}}_{2}^{+}$ ion (M=Cu or Ge) is deformed and an electronic excitation appears which makes easier the tunneling of the outer electron from ${\mathit{M}}_{2}^{+}$ to the bulk available levels of the liquid metal or alloy. Moreover, the electronic structure of the alloy is such that the tunneling effect is easier than for the metal (for example, the work function is larger for ${\mathrm{Au}}_{0.5}$${\mathrm{Cu}}_{0.5}$ than for Cu). Then, the absence or reduction of the ${\mathrm{Cu}}_{2}^{+}$ or ${\mathrm{Ge}}_{2}^{+}$ heteroisotope intensities in alloy LMIS would be due to the conjunction of two effects: presence of an electronic excitation specific to heteroisotopes and easier tunneling effect for alloys. \textcopyright{} 1996 The American Physical Society.