Quantum mechanics' exclusion principle allows a new microscope to see the individual atoms in a molecule.
Jülich researchers began with a scanning tunneling microscope with a thin metal tip which scans the specimen surface like the needle of a record player and registers the atomic irregularies as tiny electric currents.
To increase the sensitivity for organic molecules, they put a molecule of two deuterium atoms, also called heavy hydrogen, on the tip.
Short-range Pauli repulsion, a quantum-physical force between the deuterium and the sample, modulates the conductivity and allows the measurement of the fine structures.
According to the quantum mechanical Pauli exclusion principle, no two electrons in close proximity can occupy the same state. As they approach, the increasing overlap of the wavefunctions causes some electrons to be forced into higher energy states. The absorbed energy can be measured by the microscope tip.