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Shape of things to come: molecular STM.

The scanning tunneling microscope's (STM] needle-like probe enables researchers to image the electron clouds surrounding atoms. Making sense of those images, however, requires detailed calculations that predict the shape of electron clouds for individual atoms. The task is even more difficult for molecules.

Now, a team led by Vickie M. Hallmark and Shirley Chiang at IBM's Almaden Research Center in San Jose, Calif., has come up with a simple calculation technique that provides accurate shapes of the electron clouds surrounding molecules deposited on platinum. They used this prediction method along with high-resolution STM to distinguish among closely related chemicals called isomers - molecules containing the same number and kinds of atoms but built in different geometric arrangements.

As part of a report in the June M PHYSICAL REVIEW LETTERS, the group looked at three isomers of monomethylazulene [a-c, top]. They calculated the shape of each isomer's electron cloud using a conventional technique (a-c, center) and their new approach (ac, bottom). In an STM image of a mixture of molecules (d), they marked isomer a with a square and isomer b with a circle; the image of isomer c is shown at right (e). The shapes predicted by the new calculations - which include the influence of the platinum substrate - more accurately reflect those seen in the high-resolution STM images, Chiang and Hallmark assert. This advance brings researchers a step closer to using STM to track the reactants, intermediates, and products of chemical reactions on metal surfaces.
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Title Annotation:calculation technique developed to aid identification of isomers imaged by scanning tunneling microscope
Publication:Science News
Article Type:Brief Article
Date:Jun 26, 1993
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