Dynamics of the C-O Stretch Overtone Vibration of CO/Ru(001)

Two-exciton bound state quantum self-trapping in an extended star graph

The attractive Bose-Hubbard model is applied for describing the quantum self-trapping in an extended star graph. In the strong coupling limit and when two excitons are created on the core of the star, the dynamics is dominated by pair states whose properties is governed by the branch number $N$. When $N=2$, the star reduces to a linear chain so that the energy does not self-localize. Conversely, when $N\geq3$, a restructuring of the eigenstates arises and a low-energy state occurs describing a pair localized on the core of the star. Preferentially excited, this localized state gives rise to a quantum self-trapping of the energy, a process that intensifies as $N$ increases.

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

A novel type of action spectrum representing multiple overtone excitations of the v(M-C) mode was observed for lateral hopping of a CO molecule on Ag(110) induced by inelastically tunneled electrons from the tip of a scanning tunneling microscope. The yield of CO hopping shows sharp increases at 261±4  mV, corresponding to the C-O internal stretching mode, and at 61±2, 90±2, and 148±7  mV, even in the absence of corresponding fundamental vibrational modes. The mechanism of lateral CO hopping on Ag(110) was explained by the multistep excitation of overtone modes of v(M-C) based on the numerical fitting of the action spectra, the nonlinear dependence of the hopping rate on the tunneling current, and the hopping barrier obtained from thermal diffusion experiments.

Electromagnetic excitation of phonons at C(001) surfaces

The photon-phonon coupling at C(001)-(2 × 1) surfaces and its manifestation in far-infrared reflectance anisotropy spectra (FIR-RAS) are theoretically investigated. We solve the coupled system of equations for the electromagnetic field and lattice vibrations, described within the adiabatic bond charge model (ABCM), with the method of expansion into bulk phonon and photon modes. The calculated FIR-RAS exhibit resonances associated with zone-center surface phonons in good agreement with available HREELS experiments and predictions of vibrational modes for diamond (001)-(2 × 1) surfaces from ABCM and ab initio calculations. Interestingly, the reflectance anisotropy spectra for a C(001)-(2 × 1) surface turn out to be qualitatively different from the spectra for a Si(001)-(2 × 1) surface, reported previously.

Two-dimensional infrared surface spectroscopy for CO on Cu(100): Detection of intermolecular coupling of adsorbates

Surface-specific infrared signals obtained by subjecting the system to two infrared laser pulses are calculated for an admixture of CO and isotopic CO on Cu(100) by using molecular dynamics simulation based on a stability matrix formalism. The two-dimensional profiles of the signals in the frequency domain show both diagonal and cross peaks. The former peaks mainly arise from the overtones of the CO and isotopic CO, while the latter represent the couplings between those. As temperature is increased, the phases of cross peaks in a second-order infrared response function change significantly, while those of diagonal peaks are unchanged. The authors show that the phase shifts are originated from the potential anharmonicities due to the electronic interaction between adsorbates. Using a model with two dipole moments, they find that the frustrated rotational mode activated with temperature has effects on the anharmonicity. These results indicate that two-dimensional infrared surface spectroscopy reveals the anharmonic couplings between adsorbates and surface atoms or between adsorbates which cannot be observed in first-order spectroscopy.

Surface-induced C–O bond anharmonicity of methoxy adsorbed on Cu(100): Experiments and density-functional theory calculations

The anharmonic properties of a surface intermediate, methoxy, adsorbed on Cu(100) are investigated by surface infrared overtone spectroscopy and density-functional-theory electronic structure calculations. The anharmonicity is measured in the zero-coverage limit, and it is observed that the anharmonicity is increased upon adsorption as compared with the free methanol. By combining experiments with calculations we demonstrate that modifications of the anharmonicity of the methoxy species is indeed induced by adsorption onto the copper surface and not by the formation of the methoxy species.

High density gradients in the (3×3)R30°-CO layer on Ru(0001)

The coverage regime just beyond 0.33 ML, representative of a perfectly ordered (square root 3 x square root 3)R30 degrees-CO layer on Ru(0001), has been investigated using infrared-absorption spectroscopy. Different isotopic mixtures of CO have been employed to derive a profound understanding of structural properties of such layers. It is found that extra CO molecules incorporated into the (square root 3 x square root 3)R30 degrees-CO layer affect their nearest neighbor molecules only, and the associated density gradient extends over no more than a few angstroms. Contrary to existing belief, the model system CO on Ru(0001) does not represent a case of an unusually shallow adsorption potential corrugation. Rather, CO experiences an exceptionally strong site preference when adsorbed on Ru(0001). Annealing causes the local distortion of the overlattice to propagate laterally, most probably in a density wave-like manner. Incipient motion on the atomic scale thereby has been detected by means of isotopic labeling of inequivalent molecules within the high density areas. All major conclusions are based on observations of (isotopically labeled) minority CO species which feature negligible dynamical lateral coupling. The majority CO species, on the other hand, provide laterally averaged, unspecific information on the status of the layer.

Relaxation channels of two-vibron bound states in alpha-helix proteins

Relaxation channels for two-vibron bound states in an anharmonic alpha-helix protein are studied. According to a recently established small polaron model [V. Pouthier, Phys. Rev. E 68, 021909 (2003)], it is shown that the relaxation originates in the interaction between the dressed anharmonic vibrons and the remaining phonons. This interaction is responsible for the occurrence of transitions between two-vibron eigenstates mediated by both phonon absorption and phonon emission. At biological temperature, the relaxation rate does not significantly depend on the nature of the two-vibron states involved in the process. The lifetime for both bound and free states is of the same order of magnitude and ranges between 0.1 and 1.0 ps for realistic parameter values. By contrast, the relaxation channels strongly depend on the nature of the two-vibron states which is a consequence of the breatherlike behavior of the two-vibron bound states.

Two-vibron bound states in alpha-helix proteins: the interplay between the intramolecular anharmonicity and the strong vibron-phonon coupling

The influence of the intramolecular anharmonicity and the strong vibron-phonon coupling on the two-vibron dynamics in an alpha-helix protein is studied within a modified Davydov model. The intramolecular anharmonicity of each amide-I vibration is considered, and the vibron dynamics is described according to the small polaron approach. A unitary transformation is performed to remove the intramolecular anharmonicity, and a modified Lang-Firsov transformation is applied to renormalize the vibron-phonon interaction. Then a mean field procedure is realized to obtain the dressed anharmonic vibron Hamiltonian. It is shown that the anharmonicity modifies the vibron-phonon interaction, which results in an enhancement of the dressing effect. In addition, both the anharmonicity and the dressing favor the occurrence of two different bound states whose properties strongly depend on the interplay between the anharmonicity and the dressing. This dependence was summarized in a phase diagram which characterizes the number and the nature of the bound states as a function of the relevant parameters of the problem. For a significant anharmonicity, the low-frequency bound states describe two vibrons trapped onto the same amide-I vibration, whereas the high-frequency bound states refer to the trapping of the two vibrons onto nearest neighbor amide-I vibrations.