Chemical analysis with atom tweezers.

Using the scanning tunneling microscope, researchers can determine the locations of individual atoms on a wide variety of surfaces. However, the technique doesn't allow them to identify the type of atom at a particular site. That limits the technique's usefulness in studying atomic impurities, for example, which may be trapped at a surface kink and could influence such processes as crystal growth.

Now, physicists have developed an instrument that allows them to transfer a small cluster of atoms from a surface to a spectrometer for identification. As the needlelike end of a scanning tunneling microscope rides over a surface, a short voltage pulse pulls atoms from the surface to the tip. Applying a much larger voltage pulse to the collected sample ionizes the atoms and sends them flying. Measuring the time the atoms take to reach a detector gives the ratio of their mass to electric charge, permitting researchers to identify them.

Uwe Weierstall, John C.H. Spence, and their coworkers at Arizona State University in Tempe described their technique last month at an American Vacuum Society meeting in San Jose, Calif.