Web edition: July 4, 2008
For the record
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 measurements, 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 Na8, Na20, Na40 and Na58. By the time Na40 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 Na92 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.
Having read of amnesia victims’ inability to imagine a beach scene in “Thanks for the future memories” (SN: 6/21/08, p. 26), I wondered whether they have normal dreams involving scenes and story lines. It would seem that dreaming would be impaired in these people. If not, it would raise questions about why the imagining tasks are different when awake and asleep. If dreaming is impaired, it would be worth investigating how loss of such an apparently important function affects their waking hours.
WALLACE MAGATHAN, MIAMI, FLA.
Eleanor Maguire says that her amnesiac patients report that they don’t dream anymore. However, since their memories are impaired, Maguire says, they may have dreams but simply not remember having them. --SUSAN GAIDOS