Figure 1: Mass spectra of reactant clusters and their entire range of alloys.

Electrospray ionization mass spectra (ESI MS) of Ag₂₅(DMBT)₁₈ (a), Au₂₅(PET)₁₈ (b) and a mixture of the two at a Ag₂₅:Au₂₅ molar ratio of 0.3:1.0 measured within 2 min after mixing (c). The peak labels in (c) shown as numbers in red (m) and blue (n) in parentheses give the numbers of Ag and Au atoms, respectively, in the alloy clusters of formula Ag_mAu_n(SR)₁₈. Schematic structures of I and II (with the R group as –CH₃, not the real ligands) are also shown. Colour codes for the atoms in the inset pictures: red (Au), green (Ag), yellow (sulfur), black (carbon) and white (hydrogen). The inset in (c) is the expanded region of the same mass spectrum between m/z 5,300 to 7,150. c and its inset shows that entire range (n=1–24) of alloy clusters, Ag_mAu_n(SR)₁₈ (m+n=25), are formed. The peak labelled with ‘*’ in (c) is due to an unassigned dianionic species. DMBT is 2,4-dimethylbenzenethiol and PET is 2-phenylethanethiol.