Search

Dissociative adsorption of doubly substituted benzene molecules leads to formation of benzyne radicals. In this study, co-adsorbed hydrogen molecules are used in scanning tunneling hydrogen microscopy to enhance the contrast of the meta- and the para-isomers of these radicals on Cu(111) and Au(111). Up to three hydrogen molecules are attached to one radical. One hydrogen molecule reveals the orientation of the carbon ring and its adsorption site, allowing discrimination between the two radicals. Two hydrogen molecules reflect the bond picture of the carbon skeleton and reveals that adsorption on Cu(111) distorts the meta- isomer differently from its gas-phase distortion. Three hydrogen molecules allow us to determine the bond picture of a minor species.

The complete bilayer is commonly considered as the termination of the (0001) surface of hexagonal ice. Experiments on thin crystalline ice structures grown on Cu(111) demonstrated a termination by admolecule structures on top of the bilayer. Modeling of complex admolecule terminations including admolecule clusters and decorated hexagon adrows within density functional theory and high-resolution STM imaging are combined for the structural analysis and to reveal possible causes for the apparent distinction. A dominant admolecule structure that appears during a short anneal at 130 K is identified as an arrangement of water dimer and trimers. By the combined approach, detailed models for decorated hexagon adrows are derived. Such structures possess low energy; however, the proton-ordered bilayer is more favorable at a small margin. Yet, energetically unfavorable bonding of water, for example, in thin ice films may drive the formation of admolecule terminations, for which kinetic effects still are an important...

The orientation of water molecules on water bilayers is investigated on Cu(111) by a combination of scanning tunneling microscopy and density functional theory. Theory predicts that the application of a field reorients the adsorbed water molecules at a distance of close to a nanometer from the surface. Experimental evidence is presented for this prediction. Furthermore, the process differs strongly between adsorption on two and on three ordered layers. We propose that these results give insight into the behavior of the diffusive layer close to electrodes.

Arrhenius plots are often used to determine energy barriers and prefactors of thermally activated processes. The precision of thus determined values depends crucially on the precision of the temperature measurement at the sample surface. We line out a procedure to determine the absolute temperature of a metal sample in a cryogenic scanning tunneling microscope between 5 K and 50 K with sub-Kelvin precision. We demonstrate the dependence of prefactor and diffusion energy on this calibration for diffusion of CO on Cu(111) and on Ag(100) measured in the temperature range from 30 K to 38 K and 19 K to 23 K, respectively.

We investigate the distance distribution of two anorganic molecules (CO, H 2 O), one organic radical ( para -benzyne), and one strongly dipolar molecule ( ortho -dinitrobenzene) on the (111) faces of copper and silver. Above the onset of diffusion, their distribution is influenced by the surface state and oscillates. While CO, H 2 O, and para -benzyne show the expected oscillation period of ≈ λ F / 2 , ortho -dinitrobenzene oscillates with λ F / 4 . The position of the first maximum in these oscillations is consistent with a perfect scatterer for the anorganic molecules, but inconsistent for the radical and the dipolar molecule. This observation is utilized to explain the double periodicity observed in the distance distribution of ortho -dinitrobenzene.

Inelastic electron tunneling spectroscopy at low temperatures was used to investigate vibrations of Au(111) and Cu(111). The low-energy peaks at 9 millielectron volts (meV) on Au(111) and 21 meV on Cu(111) are attributed to phonons at surfaces. On Au(111), the phonon energy is not influenced by the different stacking of the surface atoms, but it is considerably influenced by different atomic distances within the surface layer. The spatial variation of the phonon excitation is measured in inelastic electron tunneling maps on Au(111), which display atomic resolution. This atomic resolution is explained in terms of site-specific phonon excitation probabilities.

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.

A combination of femtosecond laser excitation with a low-temperature scanning tunneling microscope is used to study long-range interaction during diffusion of CO on Cu(111). Both thermal and laser-driven diffusion show an oscillatory energy dependence on the distance to neighboring molecules. Surprisingly, the phase is inverted; i.e., at distances at which thermal diffusion is most difficult, it is easiest for laser-driven diffusion and vice versa. We explain this unexpected behavior by a transient stabilization of the negative ion during diffusion as corroborated by ab initio calculations.

We compare the adsorption of oxygen molecules on Ag(100) at 60 K and at 100 K. At both temperatures, the molecules form islands. Differences between the species adsorbed at the two temperatures in both low-temperature scanning tunneling microscopy and inelastic electron tunneling spectroscopy are attributed to two different adsorption states, a chemisorbed state after 100 K adsorption and a physisorbed state after 60 K adsorption.

The DNA base thymine is deposited at 100 K on Cu(111) and investigated and manipulated by low-temperature scanning tunneling microscopy at 5 K. At submonolayer coverage paired rows are observed. At monolayer coverage a hexagonal commensurate self-assembled layer with the methyl group pointing away from the surface forms. A reversible local manipulation of molecules within the self-assembled layer is demonstrated. This manipulation is interpreted as an out-of-plane relaxation of molecules within the layer induced by the change of the adsorption geometry of individual molecules between two meta-stable orientations. A positive field of 2–4 V leads to this local change in the molecular arrangement, while a field larger than 4 V restores the original geometry.

Low-temperature scanning tunnelling microscope measurements of an X-shaped dimer, consisting of two amino-nitro-azobenzene molecules, on a Au(111)-surface are presented. Electron-induced manipulation switches either the upper- or the lower-lying molecule. These reversible switching processes are based on the cis-trans isomerization of the azobenzene molecules. The switching of the upper molecule moves the lower-lying molecule in a controlled way. Thus mechanical work is performed. This proof-of-principle shows the ability of azobenzene molecules to act as a molecular-level machine.

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 explore the dynamics of low temperature interfacial water nanoclusters on Cu(111) by femtosecond-laser excitation, scanning tunneling microscopy and density functional theory. Laser illumination can be used to induce single molecules to diffuse within water clusters and across the surface, breaking and reforming hydrogen bonds. A linear diffusion probability with laser fluence is observed up to 0.6 J/m2 and we suggest that diffusion is initiated by hot electron attachment and detachment processes. The density functional calculations shed light on the detailed molecular mechanism for water diffusion that is determined by the local structure of the water clusters.

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.

The structure of D2O clusters on a Cu(111) surface and the femtosecond dynamics of photoexcited excess electrons are investigated by low-temperature scanning tunneling microscopy and two-photon photoemission spectroscopy. Two types of amorphous ice clusters, porous and compact, which exhibit characteristic differences in electron dynamics, are identified. By titration with Xe we show that in both structures solvated electrons preferentially bind on the cluster surface.

A newly established combination of a femtosecond laser with a low temperature scanning tunneling microscope is described, which facilitates one to analyze femtochemistry on metal surfaces in real space. The combined instrument enables focusing the laser to some tens of micrometers and guiding it reproducibly into the tunneling gap with the aid of in situ movable mirrors. Furthermore, a method to determine the focus size on the sample is presented. The focus size is used to calculate the electron and phonon temperatures at the surface. Despite the additional noise introduced by laser operation the vertical resolution of the microscope lies below 1 pm. The potential of the instrument is demonstrated on para-chloronitrobenzene clusters adsorbed on Au(111). Single chloronitrobenzene molecules diffuse upon femtosecond laser irradiation; some smaller clusters rotate by multiples of 30 degrees ; clusters of less compact form rearrange to close-packed clusters.

Scanning tunneling microscopy (STM) is used to study the dissociation of molecular oxygen on Ag(100) induced by inelastic electron tunneling (IET) at 5 K. This dissociation is possible above 3.3 V with a yield of (3.63 ± 0.47) × 10(-9) per electron. Dissociation leads to three different types of hot atom motion: lateral motion, a cannon ball mechanism, and abstractive dissociation. Analysis of the I-t characteristics during dissociation suggests that the dissociation is proceeded by an adsorption site change.