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The electrostatic potential mapped on the electron density contour of gas phase molecules is used to identify several molecules physisorbed on transition metals imaged by scanning tunnelling microscopy within the molecular HOMO–LUMO gap. It is rationalized that the inverted of this potential, representing the potential energy felt by a negative test charge, is a good concept to interpret changes in the dielectricity constant through the influence of physisorbed molecules and thus to understand their submolecular resolution images. The validity of the concept is demonstrated on dichlorobenzene, chloronitrobenzene, and two azobenzene derivates on the surfaces of Au(111), Ag(111), and Cu(111).

Initially invented to image surfaces down to atomic scale, the scanning tunneling microscope (STM) has been further developed in the last few years to an operative tool, with which atoms and molecules can be manipulated at low substrate temperatures at will with atomic precision in different manners by using solely the tip-adparticle forces. In this way various artificial structures on naoscale have been created and in situ characterized with the STM. Such structures as well as single molecules can be investigated by scanning tunnelling spectroscopy (STS) both with respect to their local electronic and even vibrational properties. Modifications of single molecules can be induced by using the tunnelling electron current: Rotations, diffusional jumps, vibrational excitations, desorption, dissociation and even association can be induced in individual molecules, often in a rather precise way by tuning the voltage into the energy levels of specific vibrations or electronic levels. These possibilities give rise to startling new opportunities for physical and chemical experiments on the single atom and single molecule level. Here a brief overview on results obtained with these new techniques is given.

Electrons tunnelling inelastically in a scanning tunnelling microscope have been used to induce the diffusion of water monomers adsorbed on the (111) faces of Ag, Cu, and Au via electrons coupling to vibrational modes of the molecules. Electrons of higher energies transform amorphous water clusters into crystalline clusters. At energies above 1.6 eV, the water molecules within the clusters are dissociated only after crystallization of the amorphous ice clusters. When the energy is sufficient to populate an anti-bonding orbital at 3 V the surface is oxidized.

Local vibrational spectra of meta-dichlorobenzene molecules adsorbed on different parts of the Au(111) reconstruction are investigated using a low-temperature scanning tunneling microscope. The spectra show substantial variations on subnanometer length scale. While for the molecule physisorbed on either the hcp or the fcc domain of the reconstruction only low-energy modes are beyond the detection limit, higher-energy modes are observed for the molecule chemisorbed at the elbow site. The different adsorption strengths of the molecules manifest themselves in an energy shift of the modes. These shifts are used to identify through which part the molecule is bonded to the surface.

We investigate two fundamental steps of a nonadiabatic surface process, the photo-induced movement and approach of CO molecules on the Cu(111) surface, at a hitherto unachieved single-molecule level through scanning tunneling microscope imaging. For the close approach of two CO molecules, we not only determine the nonadiabatic diffusion barrier (87 meV), but also discover a femto-second-laser-induced transient attraction (30 meV) of the usually repelling CO molecules.

We study the interaction of low-energy electrons with crystalline ice (${\mathrm{D}}_{2}\mathrm{O}$) on Cu(111) by low-temperature scanning tunneling microscopy and spectroscopy. Electrons induce dissociation of the molecules with a threshold of $\ensuremath{\approx}3\text{ }\text{ }\mathrm{eV}$. The large dissociation yield of the order of ${10}^{\ensuremath{-}8}/\mathrm{\text{electron}}$ and the extended area of dissociation are attributed to a shift in conduction band during the dissociation. Voltage dependent differences in imaging of ice and dissociated ice are reflected in the spectroscopic signature.

Fig. 1. (colour online) meta-Dichlorobenzene: (a) LUMO, HOMO, and ESP as calculated with ArgusLab 4.0.1, Mark A. Thompson, Planaria Software LLC, Seattle, WA, http://www.arguslab.com; in ESP the purple end of the spectrum shows regions corresponding to decreased electron density and green shows regions of increased electron density; (b) STM image on Au(1 1 1), 17 pA, 85 mV, the line depicts the distance between the two chlorine atoms; it has same length as the line in the ball-and-stick model; (c) STM image on Au(1 1 1) with modified tip, 60 mV, 410 pA, 9 K, lines as in (b).