Coadsorbate effects on adsorbate vibrational properties

A quantitative analysis of infrared absorption spectra to determine coadsorbate induced relative changes of the vibrational polarizability α_v of an adsorbate mode and of the dielectric screening ? due to this extra species is presented. Four (ternary) coadsorption systems consisting of the Ru(0 0 0 1)-(2 × 2)-(X + CO + O) layer with additional coadsorbates X = H, NO, CO, or O (all of them occupying the remaining empty fcc site) have been studied with FT-IRAS, TDS, LEED and work function change measurements. On-top CO is thereby used as a probe molecule to monitor coadsorbate effects on the dielectric properties of the layer. The vibrational polarizability α_v associated with the internal C-O stretch mode (ν_C-O) of on-top CO is lowered by all coadsorbates. The dielectric screening ? within the adsorbate layer is reduced in the presence of the atomic coadsorbates O and H whereas an increase of ? is found for the molecular coadsorbates, threefold coordinated CO and NO. The derived changes of α_v and dielectric screening ?, as well as the involved line shifts of ν_C-O and ν_Ru-CO can be understood in terms of the standard Blyholder backbonding model, i.e. CO 5σ charge donation to the metal combined with a backdonation to electronic states with 2π∗ character.     

Shift and broadening of the vibrational mode for a diatomic molecule adsorbed on a metal surface

The shift and broadening of the vibrational frequency of a diatomic molecule adsorbed on a metal surface are calculated on the basis of electromagnetic interactions. Finite molecular size and nonlocal metal response are included in a calculation of the response function, whose pole is the complex normal mode frequency. Finite molecular size is described by a model of a point-like vibrational mode within a polarizable sphere. Nonlocal metal response is described by a semiclassical infinite barrier model with a Lindhard dielectric constant generalized to include core polarization and finite electron lifetime. When applied to the case of CO adsorbed on Cu and if the molecule is positioned so as to fit the observed width, it is found the predicted shift is a factor of three or more greater than that observed.     

First-principle calculations of the experimental vibrational spectrum of a surface adsorbate: anharmonic resonance coupling between fundamental and binary modes

By including relevant physical properties in our modeling of a surface adsorbate system we can, from first principles, correctly calculate all experimentally observed features in the vibrational spectrum of CH3CH2O- and CD3CH2O- adsorbed on Cu(100). That is, we reproduce the number of observed modes, the vibrational frequencies, and intensities of the modes including the presence/absence of binary overtone and combination modes. No scaling was performed. Our calculations show that the anharmonic terms of the potential energy surface of free ethanol are transferrable to the corresponding surface adsorbate, ethoxy, while the harmonic terms are not.     

Low energy adsorbate vibrational modes observed with inelastic helium atom scattering: CO on Pt(111)

We report on new low energy loss and gain features which appear in the time-of-flight distributions of He atoms scattered from CO adsorbed on Pt(111). On the basis of calculated normal mode energies we assign the 6 meV features to the excitation of the CO vibrational mode corresponding to a hindered translation of the upright molecule parallel to the surface. The energy and intensity of the 6 meV mode as a function of coverage, sample temperature and scattering conditions are investigated. At high coverages, concurrent with the appearance of a well ordered (4 × 2) overlayer structure, the 6 meV mode is observed to broaden and shift in energy to about 7.4 meV. The dispersion curve measured at high coverages is flat suggesting that there is no direct coupling between hindered translations of adjacent CO molecules.     

Theory of adsorbate vibrational excitation by inelastic electron scattering

For electrons specularly reflected at a metal surface the differential cross-section for excitation of a single adsorbate vibration is calculated, with special application to the adsorbed CO group.     

Low-frequency vibrational modes of glutamine

High-resolution terahertz absorption and Raman spectra of glutamine in the frequency region 0.2 THz-2.8 THz are obtained by using THz time domain spectroscopy and low-frequency Raman spectroscopy. Based on the experimental and the computational results, the vibration modes corresponding to the terahertz absorption and Raman scatting peaks are assigned and further verified by the theoretical calculations. Spectral investigation of the periodic structure of glutamine based on the sophisticated hybrid density functional B3LYP indicates that the vibrational modes come mainly from the inter-molecular hydrogen bond in this frequency region.     

Shift and broadening of hadronic resonances formed in nuclear interactions

The mass shift and broadening of the isobar Δ(1232) formed in the reaction ¹²C(p, n)Δ ^{++ 11}B*, are explained by taking into account the elastic and inelastic rescattering of the isobar decay products by the residual nuclear system. The computational results for the mass decrease and broadening of the peak are in good agreement with the experimental data. Predictions are given for a process of the same type on the deuteron, pd→nnΔ⁺⁺. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 171–176 (10 August 1998)     

Bi-induced vibrational modes in GaAsBi

We have studied GaAs_1−xBi_x (up to x3%) using Raman scattering with two different polarization configurations. Two Bi-induced phonon modes are observed at 186 cm⁻¹ and 214 cm⁻¹ with increasing Raman intensity as the Bi concentration increases. By comparing Raman selection rules for the observed Bi-induced phonon modes with those for the substitutional N vibrational mode (GaN mode) in GaAsN, the phonon mode at 214 cm⁻¹ is identified as originating from substitutional Bi at the As site in GaAsBi.     

Shift and broadening of Na D lines at low pressure

The collision broadenings of the Na D lines were calculated from the transmission measurements of the vapor in sub-μTorr and μTorr regions by modeling the line shape with the Voigt profile. From the line shift, the average time between collisions is calculated. The ratio of the width to the shift is of the order of 10⁻⁴ implying that the impact approximation is valid. Expanding the line widths and shifts in powers of the product of number density and mean velocity of the atoms, the radiative transition probabilities, and the fundamental line shape constants: collision cross-sections and asymmetry of the lines are obtained. Moreover, from the line shifts and using the estimated value of the optical collision diameter obtained from the line widths, the average C₃ value, over the temperature ranges the experiment covers, is calculated.     

Lattice vibrational modes in potassium thiocyanate

Calculations on long-wavelength lattice vibrations have been made for cystallline KNCS within the framework of the rigid ion model. The short range force constants and effective charges of the ions have been determined from the observed frequencies. Unobserved frequencies are predicted, and normal coordinates have been obtained and schematized.     

Surface vibrational modes in disk-shaped resonators

The natural frequencies and distributions of displacement components for the surface vibrational modes in thin isotropic elastic disks are calculated. In particular, the research is focused on even solutions for low-lying resonant vibrations with large angular wave numbers. Several families of modes are found which are interpreted as modified surface modes of an infinitely long cylinder and Lamb modes of a plate. The results of calculation are compared with the results of the experimental measurements of vibrational modes generated by means of resonant excitation in duraluminum disk with radius of ≈90 mm and thickness of 16 mm in the frequency range of 130–200 kHz. An excellent agreement between the calculated and measured frequencies is found. Measurements of the structure of the resonant peaks show splitting of some modes. About a half of the measured modes has splitting Δ_fsplit/_fmode$\mathrm{\Delta }{f}_{\mathrm{split}}/f_{\mathrm{mode}}$ at the level of the order of 10⁻⁵. The Q-factors of all modes measured in vacuum lie in the interval (2…3) × 10⁵. This value is typical for duraluminum mechanical resonators in the ultrasonic frequency range.     

Normal vibrational modes in benzil crystals

A group-theoretical analysis of the vibrational spectrum of benzil crystals was carried out. The selection rules and normal coordinates were found. Raman scattering spectra of benzil single crystals were studied in polarized light within a broad temperature range from 100 K to the melting point (T _m=96°C). The experimental data were found to correlate with theory. The temperature dependences of the spectral-line half-widths were used to separate the observed vibrations into translational and librational modes, and the correspondence between the experimentally observed vibrations and their normal coordinates was established in some cases.     

Instability of vibrational modes in hexagonal lattice

The phenomenon of modulational instability is investigated for all four delocalized short-wave vibrational modes recently found for the two-dimensional hexagonal lattice with the help of a group-theoretic approach. The polynomial pair potential with hard-type quartic nonlinearity (β-FPU potential with β > 0) is used to describe interactions between atoms. As expected for the hard-type anharmonic interactions, for all four modes the frequency is found to increase with the amplitude. Frequency of the modes I and III bifurcates from the upper edge of the phonon spectrum, while that of the modes II and IV increases from inside the spectrum. It is also shown that the considered model supports spatially localized vibrational mode called discrete breather (DB) or intrinsic localized mode. DB frequency increases with the amplitude above the phonon spectrum. Two different scenarios of the mode decay were revealed. In the first scenario (for modes I and III), development of the modulational instability leads to a formation of long-lived DBs that radiate their energy slowly until thermal equilibrium is reached. In the second scenario (for modes II and IV) a transition to thermal oscillations of atoms is observed with no formation of DBs.     

Infrared-active vibrational modes of single-walled carbon nanotubes

The IR-active vibrational modes of single-walled carbon nanotubes have been observed by optical transmission through thin films of bundled nanotubes. Because IR-active chemical functional groups, e.g., -COOH, -OH, might be attached to the tube walls and contribute additional spectral features, we have also studied the effects of chemical purification and long-term high-temperature vacuum annealing on the IR spectrum. Through comparison with theory, we are able to assign much of the sharp structure observed in our IR spectra.     

Direct laser pumping of adatom-surface vibrational modes

A classical model for direct-laser-induced vibrational excitation of an adatom-lattice bond is investigated in order to resolve discrepancies between previous treatments. We also give a critical discussion on the applicability of the model to real systems.