Probing the shape of atoms in real space

The structure of single atoms in real space is investigated by scanning tunneling microscopy. Very high resolution can be obtained by a dramatic reduction of the tip-sample distance. The instabilities which are normally encountered while using small tip-sample distances are avoided by oscillating the tip of the scanning tunneling microscope vertically with respect to the sample. The surface atoms of Si(111)-(7×7) with their well-known electronic configuration are used to image individual samarium, cobalt, iron, and silicon atoms. The resulting images resemble the charge density corresponding to $4f,$ $3d,$ and $3p$ atomic orbitals.