In order to obtain the infrared reflection absorption spectroscopy (IRRAS) on semiconductor or insulator surfaces, IRRAS using a buried metal layer substrate (BML) was proposed. Compared with attenuated total reflection (ATR) arrangement, sensitivity is low, but it is a favorable point that sensitivity exists at wide frequency ranges including the finger print regions. The transmission (TR) arrangement is also applicable for wide frequency ranges, but its sensitivity is slightly lower than that of the BML-IRRAS. The important difference between TR and BML-IRRAS is in the selection rule. TR is sensitive for parallel components, but on the other hand, BML-IRRAS is for perpendicular components. Some recent results about the reaction of hydrogen on the Si(100) surfaces and the IR spectra of self assembled alkyl monolayers (SAMs) investigated by BML-IRRAS are introduced.
Theory of inelastic electron tunneling spectroscopy (IETS) of single adsorbates on metal surfaces with scanning tunneling microscope (STM-IETS) is presented. Adsorbate induced resonance model for studying tunneling processes via vibrational excitation of adsorbates enables us to derive general formulae for the elastic and inelastic tunneling currents. The condition under which the IETS spectrum (d2I/dV2) directly reflects the vibrational density of states of adsorbates was determined. It is found that two different time scales associated with the lifetime of tunneling electrons in the adsorbate and the residence time of electrons due to shuttling between the substrate/tip and adsorbate playimportant roles in the various features of STM-IETS spectra, including peak, dip, derivative-like or asymmetric structures.
Construction of molecular recognition systems at solid/liquid and liquid/liquid interfaces is one of a current topic in host/guest chemistry, because the interfacial regions possess unique physical and structural properties. In this article, we describe an application of second harmonic generation (SHG) spectroscopy to observe the process of molecular recognition at solid/liquid and liquid/liquid interfacial regions. A surface specific complexation reaction of a thiourea-based receptor monolayer with inorganic anions was confirmed at the gold/acetonitrile interface by analyzing second harmonic (SH) intensity as a function of bulk anion concentrations. A molecular association of rhodamine dyes at the heptane/water and hydrophobized-silica/water interfaces was observed by SHG spectra. An analysis of polarization dependent SH signal revealed that a complexation of azoprobe with alkali metal cations caused a change in the molecular orientation at the heptane/water interface.
Coherent lattice oscillation can be excited in semimetals and semiconductors with femtosecond optical pulses. The dephasing of the coherent phonons, typically in the picosecond time scale, is dominated by the interaction with photo-excited carriers, incoherent (thermal) phonons, and defects such as impurities and vacancies. Here we present our femtosecond pump-probe study on the relaxation dynamics of the coherent phonons and the photo-excited carriers in ion-irradiated semimetals and semiconductors. The dephasing rate of the coherent optical phonon in bismuth is increased linearly with increasing ion dose due to the scattering by irradiation-induced defects. The dose dependence of the relaxation of the coherent acoustic phonon of graphite is quantitatively explained by a simple model in which a propagating wave is scattered by single vacancies, whereas that of the optical phonon by a modified mass-defect scattering model. In GaAs, the relaxation dynamics of LO phonon-plasmon coupled modes and photo-excited carriers provide quantitative information of carrier trapping due to vacancies.
Vibrational states of H adsorbed on Ni(111) have been investigated by means of electron energy loss spectroscopy (EELS). In addition to the symmetric (νs) and asymmetric (νas) fundamental modes, the corresponding overtone structures are clearly detected. The overtone of νas, the predominantly parallel-polarized mode, exhibits a doublet structure at 0.5 ML, which is interpreted to result from the delocalization of the excited vibrational states. On the other hand, at high coverage (1.0 ML), the repulsive interactions between the adsorbed hydrogen promote the localization of the vibrational states, and the vibrational excitations are found to be collective (adsorbate phonons). The phonon dispersions are measured along the Γ−M− direction of the surface Brillouin zone, where we find the evidence of the two-phonon bound states. Applying a one-dimensional model of coupled anharmonic oscillators, we derive the anharmonicity and discuss the localized property of the vibrations.
We have investigated the C 1s photoelectron spectra of ethylene on the Si(100) (2×1) surface, using high resolution photoelectron spectroscopy. The vibrational structure has been observed in the C 1s spectra and the C-H stretching mode is found to be dominant. The vibrational splittings of C 1s state are very similar to those in the ground state. It is found that linear coupling model is applicable to this system and this excitation process is a Frank-Condon process. Curvature of the potential energy surface of C-H and C-D bond is almost unchanged upon core ionization.
We performed ab initio plane-wave calculations for NO/Pt(111) using a slab model. The results show that at a low coverage of 0.25 ML, the fcc-hollow site is the most stable adsorption site. Our calculations at a higher coverage of 0.50 ML indicate that NO molecules prefer to be adsorbed at both fcc-hollow and on-top sites rather than only at hollow sites. This adsorption arrangement is consistent with a recent scanning tunneling microscopy experiment. We then calculated the peak intensities of the N-O stretching modes in vibrational spectra. It turned out that the peak corresponding to the fcc-hollow species becomes very small in the presence of the on-top species. This effect is not explained only in terms of the well-known intensity-transfer effect derived from vibrational couplings, but a short-range dipole-dipole interaction involving charge transfer between the substrate and adsorbates turns out to play a major role.
Inelastic tunneling spectroscopy (IETS) combined with scanning tunneling microscopy (STM) has attracted attentions since Wilson Ho group published clear experimental results by use of specially designed STM instruments. Although the results of STM-IETS have clearly shown the vibration features of molecules, their behavior is case sensitive and not straight forward unlike the data from conventional IETS measurements. We believe that accumulation of STM-IETS data on various molecules and surfaces are critical for the further development of this technique. In this context it is quite important to evaluate that commercially available STM machines are capable of obtaining such vibrational information. In this paper, our recent STM-IETS system using a commercially available STM is introduced, and the key issues for the successful measurement of STM-IETS are described. In addition of a single point STM-IETS measurement, we show mapping of vibrational feature using conventional software, which can contribute to the understanding of the mechanism and future chemical analysis.