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The initial formation of the samarium/silicon interface is studied by combining scanning tunneling microscopy ͑STM͒, low-energy electron diffraction, and ab initio pseudopotential calculations. A Si(111)3ϫ2-Sm reconstruction is formed at a Sm coverage of 1/6 monolayer. High-resolution STM images reveal a strong bias-voltage dependence for the 3ϫ2 reconstruction. In the empty-state STM images, the rows with a ϫ2 periodicity are shown at high bias voltages, and attributed to Sm atoms. At low bias voltage, an additional double row feature with a ϫ1 periodicity appears and dominates the empty-state image as the bias voltage decreases. The paired protrusions with the ϫ1 periodicity in the double rows in the empty-state image are attributed to Si atoms. In the filled-state STM images, double rows of protrusions forming zigzag chains with a 3ϫ1 structure are observed and assigned to Si atoms. We propose a honeycomb chain-channel model for the 3ϫ2 phase, common to alkali metals and alkaline-earth metals on Si͑111͒. Simulated STM images based on this model are in excellent agreement with experiment.

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The effect of radiations on supercoiled plasmid DNA has been investigated by using atomic force microscopy (AFM). The DNA molecules were deposited on a substrate and observed by AFM. Alternatively, DNA at different scavenger concentrations was initially exposed to different types of radiations (alpha and X rays) at various doses. After irradiation, fragments (open circular and linearised strands) were observed corresponding to single strand breaks and double strand breaks in DNA. This result indicates the capabilities of AFM for the qualitative detection of strand modifications due to irradiation. The amount of each class of topology enables a quantitative response to be determined for both types of radiation (alpha, X). A value of the radiosensitivity of DNA was obtained as a function of the scavenger concentration. Strong accordance was found between AFM results and those obtained by use of gel electrophoresis. The advantage of AFM in comparison with traditional techniques is the possibility of analysing the radiation effects on one molecule. Indeed, taking the example of alpha particles, it is shown that it is easy to measure the sizes of linear strands by AFM. Such additional or even precise results are difficult to obtain with gel electrophoresis since, in such a case, data are lost through smearing.