Number 132 (Story #1), June 10, 1993 by Phillip F. Schewe and Ben Stein THE STM CAN NOW DISTINGUISH AMONG SIMILAR MOLECULES adsorbed onto a metal surface. Identifying molecules with scanning tunneling microscopy (STM) is actually more challenging than identifying individual atoms because the electrons that the STM "sees" when scanning molecules are often arranged in delocalized orbitals that are not centered on specific atoms. Researchers at IBM-Almaden, with the help of chemists at the Technical Hochschule Darmstadt in Germany, imaged a series of isomers--molecules with the same chemical composition but different structures--adsorbed onto a platinum substrate. The IBM scientists are the first to demonstrate the power of combining STM images with improved calculations to recognize such similar molecules as three different isomers of the methylazulene molecule, a double-ring aromatic molecule with a CH3 group attached at different locations. This ability to distinguish such similar molecules with the STM is a first step toward allowing researchers to identify directly the individual reactants, products and, perhaps, intermediate configurations of chemical reactions on surfaces. (V.M. Hallmark et al, Phys. Rev. Lett., 14 June 1993.)