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COMBO METHOD PROBES SURFACES Fast lasers and STM team up to elucidate reaction mechanism on silicon
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TUDYING DIFFUSION AND
other low-energy surface processes with atomic resolution is relatively easy to do nowadays, thanks to scanning tunneling microscopy (STM) and related methods. But events marked by high-activation-energy barriers generally are not amenable to STM investigations because of competing low-energy events. So while hydrogen diffusion on silicon, for example, has been studied in detail with STM, hydrogen desorption has proven tougher. The limitation has left open questions about the mechanism of H 2 desorption, for example, during silicon film growth from SiH 4 decomposition—an important industrial process. The problem, according to Tony F. Heinz, a professor of physics and electrical engineering at Columbia University is that if experiments are conducted at temperatures low enough to follow the motions of individual hydrogen atoms with STM, then it's too cold for H 2 to desorb readily
And if the temperature is high enough to give molecules sufficient energy to desorb, then surface diffusion occurs so rapidly that STM has little chance of pinning down the positions of atoms and molecules, revealing little about reaction dynamics. Now, by combining nanosecond laser heating with STM, Heinz, Albert Biedermann, and Zonghai Hu of Columbia, and Michael Durr and Ulrich Hôfer of the University of Marburg, in Germany, have flipped the relative rates of hydrogen diffusion and desorption, and probed the H-on-Si system with atomic resolution. Without interferences from diffusion, the group finds that hydrogen molecules are formed through reaction of H atoms on adjacent silicon surface dimers [Science, 296,1838 (2002)]. The interdimer mechanism stands in contrast to a commonly accepted intradimer recombination mechanism. "It's an exciting development in the study of reaction dynamics
LITIGATION
Inverse temperature, 1/T R O U N D A B O U T At low temperatures, H diffusion on Si (red line) outpaces H2 desorption (blue) as shown here in a schematic Arrhenius plot. But with fast laser heating, researchers can cause desorption to outrun diffusion momentarily.
on surfaces," says John J. Boland, a chemistry professor at the University ofNorth Carolina, Chapel Hill. John H. Weaver, a professor of materials science and engineering at the University of Illinois, Urbana-Champaign, comments that the study "offers new insights" into the mechanism of H 2 desorption from silicon. But he notes that the work focuses strictly on thermal causes for desorption. In related studies, researchers have shown that electronic excitations can play a role. "It would be interesting to learn whether there is a coupling between thermal and electronic activation," he says.—MITCH JACOBY
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subject of litigation for some time." It doesn't envision dealing any differently with the issues in this new lawsuit. tices have had a dramatic impact on the ability of people The lawsuit is only the latest and health care systems to ob- to target drug producers. In late tain affordable, effective 2001,30 states together filed an drugs," says Ohio Attorney antitrust lawsuit against BrisGeneral Betty D. Montgomery. tol-Myers over the antianxiety "It's unconscionable that these drug BuSpar. And another practices seem to place profit group of 29 states has sued above patients." Aventis and Andrx for conspirBristol-Myers says the "ac- ing to keep a generic version of tual events at issue are several Aventis' heart drug Cardizem off the market.-ANN THAYER years old and have been the
States Sue Bristol-Myers Over Taxol Generics
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wenty-nine U.S. states have sued Bristol-Myers Squibb, alleging that the drug company illegally blocked generic versions of the anticancer drug paclitaxel. They say the company "knowingly manipulated the patent processes to secure fraudulent patents... that prevented other drug manufacturers from developing generic alternatives."
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The states' attorneys general want unspecified monetary damages. They say Bristol-Myers wrongfully profited from its Taxol brand-name drug monopoly and deprived consumers of less expensive versions. Generic paclitaxel reached the market in 2000, the same year annual Taxol sales peaked at $1.6 billion. "These marketplace prac-
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