Lateral Hopping of CO on Ag(110) by Multiple Overtone Excitation

Origin of photoinduced DC current and two-level population dynamics in a single molecule

Studying the photoinduced changes of materials with atomic-scale spatial resolution can provide a fundamental understanding of light-matter interaction. A long-standing impediment has been the detrimental thermal effects on the stability of the tunneling gap from intensity-modulated laser irradiation of the scanning tunneling microscope junction. Photoinduced DC current transduces photons to an electric current and is widely applied in optoelectronics as switches and signal transmission. Our results revealed the origin of the light-induced DC current and related it to the two-level population dynamics and related nonlinearity in the conductance of a single molecule. Here, we compensated for the near-visible laser-induced thermal effects to demonstrate photoinduced DC current spectroscopy and microscopy and to observe the persistent photoconductivity of a two-level pyrrolidine molecule. The methodology can be generally applied to the coupling of light to scan probes to investigate light-matter interactions at the atomic scale.

Dissociation of Single O2 Molecules on Ag(110) by Electrons, Holes, and Localized Surface Plasmons

A detailed understanding of the dissociation of O₂ molecules on metal surfaces induced by various excitation sources, electrons/holes, light, and localized surface plasmons, is crucial not only for controlling the reactivity of oxidation reactions but also for developing various oxidation catalysts. The necessity of mechanistic studies at the single-molecule level is increasingly important for understanding interfacial interactions between O₂ molecules and metal surfaces and to improve the reaction efficiency. We review single-molecule studies of O₂ dissociation on Ag(110) induced by various excitation sources using a scanning tunneling microscope (STM). The comprehensive studies based on the STM and density functional theory calculations provide fundamental insights into the excitation pathway for the dissociation reaction.

Dissociation Mechanism of a Single O2 Molecule Chemisorbed on Ag(110)

The dissociation of O₂ molecules chemisorbed on silver surfaces is an essential reaction in industry, and the dissociation mechanism has long attracted attention. The detailed dissociation mechanism was studied at the single-molecule level on Ag(110) by using a scanning tunneling microscope (STM). The dissociation reaction was found to be predominantly triggered by inelastically tunneled holes from the STM tip due to the significantly distributed density of states below the Fermi level of the substrate. A combination of action spectroscopy with the STM and density functional theory calculations revealed that the O₂ dissociation reaction is caused by direct ladder-climbing excitation of the high-order overtones of the O-O stretching mode arising from anharmonicity enhanced by molecule-surface interactions.

Binding Behavior of Carbonmonoxide to Gold Atoms on Ag(001)

The adsorption behavior of single CO molecules at 4 K bound to Au adatoms on a Ag(001) metal surface is studied with scanning tunneling microscopy (STM) and inelastic electron tunneling spectroscopy (IETS). In contrast to earlier observations two different binding configurations are observed—one on top of a Au adatom and the other one adsorbed laterally to Au on Ag(001). Moreover, IETS reveals different low-energy vibrational energies for the two binding sites as compared to the one for a single CO molecule bound to Ag(001). Density functional theory (DFT) calculations of the adsorption energies, the diffusion barriers, and the vibrational frequencies of the CO molecule on the different binding sites rationalize the experimental findings.

Dynamics of Single-Molecule Dissociation by Selective Excitation of Molecular Phonons

Breaking bonds selectively in molecules is vital in many chemistry reactions and custom nanoscale device fabrications. The scanning tunneling microscope (STM) has proved to be an ideal tool to initiate and view bond-selective chemistry at the single-molecule level, offering opportunities for the further study of the dynamics in single molecules on metal surfaces. We demonstrate H─HS and H─S bond breaking on Au(111) induced by tunneling electrons using low-temperature STM. An experimental study combined with theoretical calculations shows that the dissociation pathway is facilitated by vibrational excitations. Furthermore, the dissociation probabilities of the two different dissociation processes are bias dependent due to different inelastic-tunneling probabilities, and they are also closely linked to the lifetime of inelastic-tunneling electrons. Combined with time-dependent ab initio nonadiabatic molecular dynamics simulations, the dynamics of the injected electron and the phonon-excitation-induced molecule dissociation can be understood at the atomic scale, demonstrating the potential application of STM for the investigation of excited-state dynamics of single molecules on surfaces.

Detection and Characterization of Anharmonic Overtone Vibrations of Single Molecules on a Metal Surface

Inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope (STM) is a powerful technique used to characterize the vibration and spin states at the single-molecule level. While IETS lacks hard selection rules, historically it has been assumed that vibrational overtones are rarely seen or even absent. Here we provide definitive experimental evidence that the hindered rotation overtone excitation of carbon monoxide molecules adsorbed on Ag(110) can be detected with STM-IETS via isotope substitution. We also demonstrate that the anharmonicity of the overtone excitation can be characterized and compared between adsorption sites and find evidence of anisotropy in the vibrational anharmonicity for CO adsorbed on the [11[over ¯]0] step edge.

Strong Substrate Mediation of Attractive Lateral Interactions of CO on Cu(110)

The mechanism of chemical reactions between adsorbed species is defined by the combined effects of the adsorbate-substrate potential landscape and lateral interactions. Such lateral interactions are therefore integral to catalytic processes, but their study is often complicated by "substrate mediation", the regulation of a two-body potential between adsorbed particles by the surface itself. Substrate mediation can influence the sign and magnitude of lateral interactions. There are notable exceptions of ordered structures forming at low coverage, indicative of short-range attractive forces where repulsive forces are expected to dominate, suggesting a strong substrate-mediated contribution. To explore further the origins of such interactions, we have investigated the adsorption of CO on Cu(110) using a combination of low-temperature microscopy and first-principles calculations. Our studies reveal that lateral adsorbate interactions, which are constrained by the metal surface, regulate the bonding between the adsorbate and substrate. Anisotropic CO-CO coupling is seen to arise from a perfect balance between the intermolecular accumulation of charge that acts as a glue (chemical coupling) at sufficiently large distances to avoid repulsive effects (dipole-dipole coupling and Pauli's repulsion between electron clouds).

Anharmonicity in a double hydrogen transfer reaction studied in a single porphycene molecule on a Cu(110) surface

Anharmonicity plays a crucial role in hydrogen transfer reactions in hydrogen-bonding systems, which leads to a peculiar spectral line shape of the hydrogen stretching mode as well as highly complex intra/intermolecular vibrational energy relaxation. Single-molecule study with a well-defined model is necessary to elucidate a fundamental mechanism. Recent low-temperature scanning tunnelling microscopy (STM) experiments revealed that the cis↔cis tautomerization in a single porphycene molecule on Cu(110) at 5 K can be induced by vibrational excitation via an inelastic electron tunnelling process and the N-H(D) stretching mode couples with the tautomerization coordinate [Kumagai et al. Phys. Rev. Lett. 2013, 111, 246101]. Here we discuss a pronounced anharmonicity of the N-H stretching mode observed in the STM action spectra and the conductance spectra. Density functional theory calculations find a strong intermode coupling of the N-H stretching with an in-plane bending mode within porphycene on Cu(110).

Probing Intermolecular Coupled Vibrations between Two Molecules

Intermolecular interactions can induce energy shifts and coupling of molecular vibrations. However, the detection of intermolecular coupled vibrations has not been reported at the single molecule level. Here we detected an intermolecular coupled vibration between two CO molecules, one on the surface and another on the tip within the gap of a subkelvin scanning tunneling microscope, and analyzed the results by density functional calculations. We attribute the evolution of the energy and intensity of this coupled vibration as a function of tip-sample distance to the tilting and orbital alignment of the two CO molecules.

Single-Molecule Dynamics in the Presence of Strong Intermolecular Interactions

In contrast to conventional spectroscopic studies of adsorbates at high coverage that provide only spatially averaged information, we have characterized the laterally confined shuttling dynamics of a single molecule under the influence of intermolecular interactions by vibrational spectroscopy using a scanning tunneling microscope. The bridge sites on Pt(111) are only occupied by a CO molecule that is surrounded by four other CO molecules at on-top sites. The bridge-site CO undergoes laterally confined shuttling toward an adjacent on-top site to transiently occupy a metastable site, which is slightly displaced from the center of an on-top site through repulsive interaction with adjacent on-top CO molecules. Analysis of action spectra for the shuttling events reveals the C-O stretch frequency of the metastable CO. We also constructed a modified potential energy surface incorporating the intermolecular interaction, which reveals the underlying mechanism and provides a new way to experimentally determine detailed information on the energetics of the metastable state.