| Abstract
| - DNA has been at the center of an imaging effort since the invention of the scanning tunneling microscope(STM). In some of the STM imaging reports the molecules appeared with negative contrast, i.e., “submerged”under the metal background and darker. We demonstrate the phenomenon of contrast inversion in DNASTM imaging by controlled and spontaneous contrast inversions and by the dependence of the DNA apparentheight with respect to the surface on the imaging bias voltage. Using these characterizations, we formulatea model explaining the above phenomenon by resonant tunneling through virtual states in the vacuum betweenthe STM tip and the DNA molecule.
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