Single-molecule reaction and characterization by vibrational excitation

Controlled chemical reaction of single trans-2-butene molecules on the Pd(110) surface was realized by dosing tunneling electrons from the tip of a scanning tunneling microscope at 4.7 K. The reaction product was identified as a 1,3-butadiene molecule by inelastic electron tunneling spectroscopy. Threshold voltage for the reaction is approximately 365 mV, which coincides with the vibrational excitation of the C-H stretching mode. The reaction was ascertained to be caused by C-H bond dissociation by multiple vibrational excitations of the C-H stretching mode via inelastic electron tunneling process.     

Adsorbate motions induced by vibrational mode coupling

Adsorbate motions are discussed with a primary attention focused on the coupling between a vibrational mode excited by ultrafast laser heated hot-electrons or by inelastic tunneling electrons with scanning tunneling microscope and the reaction coordinate (RC) mode. Recent experimental results have demonstrated an efficient reaction pathways involving an indirect excitation of a frustrated translational mode, rather than its direct excitation for adsorbate hopping on surfaces. Elementary processes are briefly described for hopping of CO molecules on a laser heated stepped Pt surface, where excitation of the frustrated rotation mode has been found to plays an indispensable. Calculation of the inelastic tunneling current (ITC) for excitation of the C-O stretch mode of a CO molecule is combined with a theory of anharmonic mode coupling to activate the frustrated translation mode above the barrier. The hopping rate as a function of the bias voltage agrees with the experimental result. An unified theory of single-, and two-electron processes for ITC-induced motions induced by an indirect excitation of the RC-mode via mode coupling is also applied to reproduce a crossover from hopping to desorption of a single NH₃ molecule on Cu(1 0 0) with an increase in the tunneling current.     

Electronic excitation of ice monomers on Au(111) by scanning tunneling microscopy

Scanning tunneling microscopy, inelastic tunneling spectroscopy, and electron induced manipulation are used to investigate electronic excitation of D₂O monomers and small clusters adsorbed at the elbows of the Au(111) reconstruction. Diffusion of molecules, dissociation of clusters, and rearrangement of the reconstruction is induced by electronic excitation. Threshold energies of between 200 and 250 meV and of 446 meV are explained by combined vibrational modes of D₂O molecules. External vibrational modes of D₂O molecules on Au(111) are identified by inelastic tunneling spectroscopy at ≈18, 30, and 41 meV.     

Spatial variations in submolecular vibronic spectroscopy on a thin insulating film

Scanning tunneling spectroscopy on single naphthalocyanine molecules adsorbed on an ultrathin aluminum oxide film exhibits electron-vibronic coupling that varies with the position of tunneling over the molecule. The spectra at different positions are composed of several series of equally spaced peaks, which are interpreted as progression of progressions of molecular vibrational modes. The spatial variations correlate with the molecular orbital structure, revealing spatially dependent electron-vibronic coupling and selective vibrational excitation.     

Symmetry selection rules for vibrationally inelastic tunneling

A combined experimental and theoretical study is presented for the C-D stretch mode excitation of acetylene isotopes, C2HD and C2D2, on Cu(100) via inelastic electron tunneling (IET) in a scanning tunneling microscope junction. The calculated IET images using density functional theory show that the measured signal from C2D2 derives from the antisymmetric stretch mode. Selection rules are derived and involve the constraint imposed by the IET image on the symmetry characters of the vibrational mode and the adsorbate-induced electron states at the Fermi level.     

Electronic excitation and thermal effects in alkali-halide cluster anions

We have observed electronically excited states in alkali-halide cluster anions with one excess electron. Using photoelectron spectroscopy, we have found two narrow states in (KI)-2, (NaI)-2, and (NaCl)-2, consistent with a dipole-bound electron, while larger cluster anions exhibit a single broad excited state. In the larger systems, electronic excitation is often accompanied by vibrational excitation and thus a change in cluster temperature. Such temperature changes affect cluster structure and in some cases lead to rapid thermal isomerization.     

Adsorbate-substrate vibrational modes of benzene on Ag(110) resolved with scanning tunneling spectroscopy

Using a low temperature scanning tunneling microscope, we have detected low energy adsorbate-substrate (external or frustrated) vibrational modes of benzene molecules adsorbed on a Ag(110) surface. We demonstrate that such vibrations represent a fingerprint of the molecules' chemical state and environment; two different vibrational spectra are measured on molecules populating two different adsorption states. We also find that the distortion of the adsorption geometry of the molecules may give rise to the excitation of additional (initially hidden) modes. Important differences in the spatial distribution of the inelastic signal are also observed for these external modes.     

振动和转动激发对N2在Fe(111)表面解离吸附的影响

N₂的解离化学吸附是工业合成氨的速控步骤. 基于最近构建的六维势能面,本文研究了N₂的初始振动激发和转动激发在Fe(111)表面的反应性的作用. 由于该反应具有重要的量子效应,通过六维量子动力学计算研究了入射能量低于1.6 eV 时振动激发的效应. 并采用准经典轨线计算揭示了高入射能量下的振动和转动激发的影响. 通过这些研究发现增加平动能量在一定程度上能提高解离几率,振动激发或转动激发能更有效地促进解离. 这项研究为重原子分子-表面反应的模式特异性动力学提供了有价值的见解.     

Excitation of frustrated translation and nonadiabatic adatom hopping induced by inelastic tunneling

The dynamics of lateral manipulation for cobalt/Cu(111) has been investigated combining the model of vibrational heating and first-principles density functional calculations. The frustrated translational mode responsible for lateral excitation is identified as a vibrational resonance involving a concerted motion between the adatom and surface phonons. The calculated frequency shows good agreement with the onset energy for adatom hopping induced by inelastic tunneling. Simulation of the power law, compared with experiment, suggests that the atom hopping overcomes a nonadiabatic barrier due to the nonequilibrium local heating of the translational mode.     

Local bond breaking via STM-induced excitations: the role of temperature

We discuss the influence of temperature on local bond breaking through multiple vibrational excitations induced by inelastic tunneling in the STM. We focus on hydrogen desorption from the H---Si(111) and H---Si(100) systems, but the results are general. The substrate temperature affects the desorption yield in two important ways: first, lowering the temperature increases the H---Si vibrational energy relaxation time, resulting in a higher effective adsorbate temperature and an increased desorption yield. Second, lowering the substrate temperature decreases the dephasing rate of the H---Si modes (manifested by a decrease of the infrared absorption linewidth), which then reduces the rate of incoherent (Förster) vibrational energy transfer away from the Stark-shifted H---Si mode under the tip. This increases the localization of the vibrational energy and enhances the probability for multiple vibrational excitation and desorption. Finally, we discuss the possible implications of our findings on the mechanism of MOS device degradation by hot electrons.     

Single molecule vibrationally mediated chemistry

Tunnelling electrons may scatter inelastically with an adsorbate, releasing part of their energy through the excitation of molecular vibrations. The resolution of inelastic processes with a low temperature scanning tunnelling microscope (STM) provides a valuable tool to chemically characterize single adsorbates and their adsorption mechanisms. Here, we present a molecular scale picture of single molecule vibrational chemistry, as resolved by STM. To understand the way a reaction proceed it is needed knowledge about both the excitation and damping of a molecular vibration. The excitation is mediated by the specific coupling between electronic molecular resonances present at the Fermi level and vibrational states of the adsorbate. Thus, the two-dimensional mapping of the inelastic signal with an STM provides the spatial distribution of the adsorbate electronic states (near the Fermi level) which are predominantly coupled to the particular vibrational mode observed. The damping of the vibration follows a competition between different mechanisms, mediated via the creation of electron-hole pairs or via anharmonic coupling between vibrational states. This latter case give rise to effective energy transfer mechanisms which eventually may focus vibrational energy in a specific reaction coordinate. In this single-molecule work-bench, STM provides alternative tools to understand reactivity in the limit of low excitation rate, which demonstrate the existence of state-specific excitation strategies which may lead to selectivity in the product of a reaction.     

C+OH(v=0—3,j=0—3)→CO+H反应的准经典轨线研究

运用准经典轨线方法,在碰撞能为1.0eV时,研究了反应物OH分子的振转激发对C+OH反应的立体动力学性质的影响.详细地讨论了该反应在不同反应物振转态下的矢量性质.计算表明,OH分子的振转激发对C+OH反应的矢量性质非常敏感,这种现象在对该反应的标量性质的研究中是不存在的.     

Semiphenomenological analysis of cis-to-trans isomerization of polyacetylene

Semiphenomenological analysis of cis-to-trans isomerization process of polyacetylene is presented. It is shown that excitation of cis-transoid form to trans-cisoid form is necessary in the course of the isomerization. The probability of tunneling of a π electron from trans-cisoid to cis-transoid state is estimated to be about 0.7 per second. The life time of trans-cisoid form can be inferred to be several hours.     

Molecular vibration dynamics in molecule-surface interactions

A quantum mechanical coupled channels approach to associative or recombinative desorption and scattering of diatomic molecules is described. The formulation is based on the concept of a reaction path and allows prediction of the vibrational excitation of desorbing molecules. We first consider very light molecules such as H₂ and D₂ desorbing via a Langmuir-Hinshelwood reaction. In a simple model neglecting rotations and substrate vibrations, the dependence of molecular vibrational excitation on incident energy, the curvature of the reaction path and the position and height of the saddle point are discussed. Various experimental results can be described with reasonable parameters. Vibrational excitation in Eley-Rideal reactions and rotational excitations in general are discussed only in a semiquantitative way. For heavier molecules the coupling to substrate vibrations in principle will become more important. Arguments will be presented that for the problem of vibrational excitation in desorption and scattering this coupling may still be neglected approximately. Results for vibrational excitations of CuF desorbing from Cu are in support of this simple point of view.     

Tunneling ionization of atoms with excitation of the core

A general formula is obtained for the probability of tunneling ionization of an atom accompanied by excitation of the core. This formula is a generalization of the Carlson formula for the probability of a single-photon two-electron transition in atoms. The limiting case of this formula, just as that of the Carlson formula, is the well-known random-perturbations approximation. Numerical results are presented for Zn, Sr, and Cd atoms. For these atoms the contribution of the excited states of singly charged ions to the probability of the formation of doubly charged ions is a nonmonotonic function of the laser radiation intensity. Analysis of the tunneling ionization of molecules shows that with overwhelming probability an ion is formed in the ground vibrational state, while for the standard photoionization the distribution over vibrational states is determined by the Franck-Condon factors.     

Electron interaction with vibrational modes in tunneling through a single molecular electronic level

Within the adiabatic approximation, it is shown that the effective electron-phonon interaction Hamiltonian for tunneling through a single molecular electronic level contains two different contributions. The interference of the two interaction channels can lead to either the enhancement or suppression of phonon generation. Conditions determining the intensity of excitation of vibrational modes of the molecule are found.     

Control and characterization of a multistep unimolecular reaction

Electrons from a scanning tunneling microscope were used to break a C-H bond in a single acetylene (HCCH) molecule adsorbed on Cu(001) at 9 K. The product ethynyl (CCH) was characterized by imaging, inelastic electron tunneling spectroscopy, and variable temperature measurements of the rotation rate. Significant changes in the vibrational spectra and bonding geometry accompanied HCCH dissociation. The ethynyl was further dehydrogenated to form dicarbon (CC). Dissociation studies of the isotopes HCCD and DCCD permitted unambiguous identification of the reaction products.     

Theory of inelastic electric current through single molecules

An electronic structure theory has been developed for an inelastic electric current of electron-intramolecular vibration coupling origin in terms of the Keldysh Green function method and the self-consistent Born approximation. Numerical calculations were made for the benzenedithiol molecule linking the two Au(111) electrodes. The calculations successfully reproduce typical features commonly observed in inelastic tunneling spectroscopy. The vibrational excitation energy due to the inelastic current was estimated. The inelastic electric current is quite important for the structural stability and the switching possibility of the molecular device.     

Inducing all steps of a chemical reaction with the scanning tunneling microscope tip: towards single molecule engineering

All elementary steps of a chemical reaction have been successfully induced on individual molecules with a scanning tunneling microscope (STM) in a controlled step-by-step manner utilizing a variety of manipulation techniques. The reaction steps involve the separation of iodine from iodobenzene by using tunneling electrons, bringing together two resultant phenyls mechanically by lateral manipulation and, finally, their chemical association to form a biphenyl molecule mediated by excitation with tunneling electrons. The procedures presented here constitute an important step towards the assembly of individual molecules out of simple building blocks in situ on the atomic scale.     

反应物振动量子数对O(~1D)+HBr→OH+Br立体动力学影响的研究

采用准经典轨线(QCT)方法计算了O(1D)+HBr→OH+Br反应体系的立体动力学反应.基于由Peterson(J.Chem.Phys.113(2000)4598)等人开发的基态势能面上,计算了反应的矢量相关性质,极角p(θ)r及方位角pφ()r以及空间角pθr,φ()r.此外我们还计算了极化微分反应截面(PDDCSs)的分布各矢量性质随各振动量子数变量的变化.结果表明反应受反应物振动量子数影响较大.