Vibrational spectra for a single molecule adsorbed on a solid surface have been obtained with a scanning tunneling microscope (STM). Inelastic electron tunneling spectra for an isolated acetylene (C2H2) molecule adsorbed on the copper (100) surface showed an increase in the tunneling conductance at 358 millivolts, resulting from excitation of the C-H stretch mode. An isotopic shift to 266 millivolts was observed for deuterated acetylene (C2D2). Vibrational microscopy from spatial imaging of the inelastic tunneling channels yielded additional data to further distinguish and characterize the two isotopes. Single-molecule vibrational analysis should lead to better understanding and control of surface chemistry at the atomic level.

The tunneling current from a scanning tunneling microscope was used to image and dissociate single O 2 molecules on the Pt(111) surface in the temperature range of 40 to 150 K. After dissociation, the two oxygen atoms are found one to three lattice constants apart. The dissociation rate as a function of current was found to vary as I 0.860.2 , I 1.860.2 , and I 2.960.3 for sample biases of 0.4, 0.3, and 0.2 V, respectively. These rates are explained using a general model for dissociation induced by intramolecular vibrational excitations via resonant inelastic electron tunneling. [S0031-9007(97)

Tunneling electrons from the tip of a scanning tunneling microscope were used to induce and monitor the reversible rotation of single molecules of molecular oxygen among three equivalent orientations on the platinum(111) surface. Detailed studies of the rotation rates indicate a crossover from a single-electron process to a multielectron process below a threshold tunneling voltage. Values for the energy barrier to rotation and the vibrational relaxation rate of the molecule were obtained by comparing the experimental data with a theoretical model. The ability to induce the controlled motion of single molecules enhances our understanding of basic chemical processes on surfaces and may lead to useful single-molecule devices.

The reversible rotation of a single isolated acetylene molecule between two diagonal sites on the Cu(100) surface at 8 K was induced and monitored with tunneling electrons from a scanning tunneling microscope (STM). Excitation of the C-H (C-D) stretch mode of C 2 H 2 (C 2 D 2 ) at 358 meV (266 meV) led to a 10-fold (60-fold) increase in the rotation rate. This increase is attributed to energy transfer from the C-H (C-D) stretch mode to the hindered rotational motion of the molecule. Inelastic electron tunneling spectroscopy with the STM provides the energies of the stretch modes and allows a quantitative determination of the inelastic tunneling and coupling probabilities.[S0031-9007 (98)06800-8] PACS numbers: 68.35.Ja, 07.79.Cz, 34.50.Ez, 61.16.ChThe concept of the reaction coordinate hinges on the notion that motions of specific bonds dictate the progress of the reaction. By selectively exciting these bonds, the rates and pathways of chemical reactions can be affected. In addition to selective deposition of energy into the reaction coordinates, it is necessary for the reaction to occur prior to the dissipation and randomization of the initially localized energy [1-4]. Fast energy transfer has been the bottleneck in the realization of bond-selected chemistry.The unique capabilities of the scanning tunneling microscope (STM) allow one to excite individual molecules and observe the resulting motions. For example, it was demonstrated that tunneling electrons from the STM can be used to induce and image the dissociation of an isolated O 2 molecule adsorbed on the Pt(111) surface [5]. Inelastic tunneling electrons were used to excite the reaction coordinate, the n͑OO͒ stretch. Excitation of the hindered rotational mode of the nearly parallel bonded O 2 molecule also led to its reversible rotation [6]. By scanning the STM tip at a high bias voltage, reversible rotation of antimony dimers on the Si(100) surface was also observed [7]. Using a tracking technique, quantitative studies were made on the reversible rotation rates of individual Si addimers on the Si(100) surface as a function of temperature and electric field [8].In this paper, tunneling electrons were used to induce and monitor the motions of a single molecule, namely, acetylene adsorbed on Cu(100) at 8 K. We demonstrate a novel application of the STM to directly probe the coupling of a specific vibrational mode of the molecule to its hindered rotational motion. Even though intramolecular energy transfer has been investigated extensively in the gas phase by lasers [1,2], the STM allows a direct and clear visualization of the process. Single molecule vibrational spectroscopy allows the deduction of this coupling and a quantitative determination of the coupling probability. The induced rotation rate was found to decrease as the STM tip was laterally displaced away from the center of the molecule and exhibited an anisotropic spatial dependence of molecular dimensions. It is shown how the analysis of single-molecule vibrational modes and singlemolecule motions ca...

The design and performance of a variable-temperature scanning tunneling microscope (STM) is presented. The microscope operates from 8 to 350 K in ultrahigh vacuum. The thermally compensated STM is suspended by springs from the cold tip of a continuous flow cryostat and is completely surrounded by two radiation shields. The design allows for in situ dosing and irradiation of the sample as well as for the exchange of samples and STM tips. With the STM feedback loop off, the drift of the tip–sample spacing is approximately 0.001 Å/min at 8 K. It is demonstrated that the STM is well-suited for the study of atomic-scale chemistry over a wide temperature range, for atomic-scale manipulation, and for single-molecule inelastic electron tunneling spectroscopy (IETS).