For the record

Science News, July 19, 2008 by Walt De Heer, Keith Clemenger, Winston A. Saunders

We read with interest your article on superatoms ("Small, but super," SN: 6/21/08, p. 14) and would like to add a note on the experimental discovery. In the mid-1970s the late Walter Knight decided to investigate small metallic particles with a molecular beam apparatus. When Walt de Heer joined the group in 1979, Knight gave him the freedom to redesign the apparatus. De Heer built a new cluster source and a quadrupole mass analyzer with a range from 1-10,000 amu; clusters were ionized using a broadband UV arc lamp - all features that later proved to be essential for the discovery of electronic shell structure.

In fall 1983 Knight left for a sabbatical at Oxford. Keith Clemenger was performing polarizability measurement Winston Saunders was optimizing the quadrupole mass analyzer for larger sizes and de Heer was measuring magnetic properties of small alkali clusters. Robust maxima progressively appeared in the abundance spectra NaB, [Na.sub.20], [Na.sub.40] and [Na.sub.58]. By the time [Na.sub.40] was observed, Clemenger speculated the features had an electronic origin.

In February 1984, we were confident of the reality of the electronic shell structure and the observation of Na~~ finally left no doubt. We convinced Knight to visit in March. He solicited theoretical help from Marvin Cohen and graduate student Mei-Yin Chou, whose calculations verified the shell structure interpretation. The paper was written in April 1984 and appeared in the June 11, 1984 Physical Review Letters, p. 2141.

WALT DE HEER, ATLANTA, GA.

KEITH CLEMENGER, BEIJING, CHINA

WINSTON A. SAUNDERS, HILLSBORO, ORE.