Monday, December 28, 2009

Imaging Superatoms

A superatom is a composite of atoms, either homogeneous or heterogenous, that exhibits a similar electronic profile to a given single atom in the periodic table. Using photoelectron imaging, the figure below shows that a superatom of titanium oxide (bottom row) mimics the electron energetics of a single nickel atom (top row).

Previous experiments have shown that a cluster of 13 aluminum atoms behaves like a single iodine atom. Now, there appears to be a kind of arithmetic for superatoms. Here's how it works.

Using the periodic table of elements, start at Ti (2nd of the 1st-row transition metals) with atomic number 22. It has 4 electrons in its outermost shell ([Ar] 3d2 4s2). Now, move 6 elements to its right (in the same row), because O possesses 6 outer atomic-shell electrons ([He] 2s2 2p4). The element you land on is Ni with atomic number 28 = 22 + 6. Since Ni has 10 outer-shell electrons ([Ar] 4s1 3d9), it is isoelectronic with the 4 + 6 outer-shell electrons of molecular titanium oxide. See "Clusters: A bridge between disciplines," PNAS, Dec 2009.