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The initial stages of chemical reduction of metallic ions in solution must proceed through the formation of single atoms, followed by the rapid formation of clusters with other atoms until continued coalescence produces observable precipitates or evidence of stable sol formation. Dilute solutions of sodium borohydride (0.0025%) merged with microgram per milliliter concentration solutions of Ag, Pd, and Cu in a custom spectrophotometric flow cell produce transient background-corrected atomic absorption signals during the initial few seconds of the reaction. High-resolution atomic absorption with both a hollow cathode line source and a continuum source reveals that the absorption line profile of the absorbing species is extremely narrow (11 pm for Ag) and not influenced by the aqueous environment. Such selective absorption of radiation suggests that free atoms are being observed in aqueous solution.

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