Recent Topics

Last update on May 25, 2011

Fig.1 Fig.2

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Low-temperature observation of the softened C-H stretching vibrations of cyclohexane on Rh(111) [Takanori Koitaya, Atsushi Beniya, Kozo Mukai, Shinya Yoshimoto, and Jun Yoshinobu, Phys. Rev. B 80, 193409 (2009) DOI: 10.1103/PhysRevB.80.193409 ].

The C-H stretching vibrations of cyclohexane on Rh(111) were investigated using infrared reflection absorption spectroscopy between 20 and 89 K. At 20 K, the softened C-H stretching band consists of several sharp peaks, ranging from 2500 to 2700ÝcmÄ|1. The wide-range distribution of the softened C-H stretching peaks results from inhomogeneity of adsorption environments. With increasing the substrate temperature, each softened C-H stretching peak becomes significantly broadened, but the normal C-H stretching peaks are little changed. These results indicate that the local interaction between the softened C-H species and the Rh(111) surface is sensitive to a thermally excited low-energy mode. The temperature-dependent broadening of a soft mode at low temperature is analyzed using a vibrational dephasing model, where the softened C-H stretching mode is anharmonically coupled with a thermally excited frustrated translation mode.

The adsorption states and growth process of the first layer and multilayer of water (D2O) on Rh(111) above 135ÝK were investigated using infrared reflection absorption spectroscopy (IRAS), temperature programed desorption, spot-profile-analysis low-energy electron diffraction, and scanning tunneling microscopy (STM). At the initial stage, water molecules form commensurate (Ã3ÝÃ3)R30° islands, whose size is limited for several hexagonal units; the average diameter is Ý 2.5Ýnm. This two-dimensional (2D) island includes D-down species, and free OD species exist at the island edge. With increasing coverage, the D-up species starts to appear in IRAS. At higher coverages, the 2D islands are connected in STM images (Fig.1). By the titration of Xe adsorption we estimated that the D-down domain occupies about 55% on Rh(111) at the saturation coverage. Further adsorption of water molecules forms three-dimensional ice crystallites on the first water layer; thus, the growth mode of crystalline water layers on Rh(111) is a Stranski&endash;Krastanov type. We have found that an ice crystallite starts to grow on D-down domains and the D-down species do not reorient upon the formation of a crystalline ice.
Fig. 1

The microscopic diffusion processes of NO molecules on Pt(997) at low coverage were investigated using time-resolved infrared reflection absorption spectroscopy (TR-IRAS). When NO molecules adsorb on Pt(997) at low temperature, each molecule transiently migrates on the surface from the first impact point to a possible adsorption site. At 11  K, the molecules are trapped at four adsorption sites on Pt(997): the on-top sites on the (111) terrace (OT), the hollow sites on the (111) terrace (HT), the bridge sites at the step (BS) and the hollow sites at the step downstream (HS). Based on the initial population ratio for these sites, the mean lateral displacement by transient migration is estimated to be 4.1  Å. By heating the surface to 45  K, the HS species migrate up to the BS sites; the migration barrier is roughly estimated to be 120  meV. In the temperature range from 70  to  77  K, TR-IRAS measurements were carried out to observe the site change of OT species to the adjacent HT sites at isothermal conditions; the activation barrier and the preexponential factor are estimated to be 200  meV and 2.0x10¹¹  s^-11, respectively. In the temperature range from 100  to  110  K, the HT species migrate across the terrace and finally reach the BS sites. The activation barrier between the HT sites and the preexponential factor are estimated to be 290  meV and 6.5x10¹¹  s^-11, respectively, from the TR-IRAS data together with kinetic Monte Carlo simulations. On the whole, the quantitative microscopic picture of NO migration on Pt(997) has been established.

We investigated the adsorption states of 2-methylpropene and propene on Si(100)c(4x2) using low-temperature scanning tunneling microscopy. We have found that regioselective cycloaddition reactions (di-sigma bond formation) occur between the asymmetric alkene molecules and the asymmetric dimers on Si(100)c(4x2). First-principles calculations have elucidated that the regioselectivity is closely related to the structures of precursor species and these precursor species have carbocation-like features. Thus, we conclude that Markovnikov's rule is applicable for the cycloaddition of asymmetric alkene with the asymmetric dimer on Si(100)c(4x2).

We investigated the initial stage of water adsorption on Rh(111) at 20 K using infrared reflection absorption spectroscopy (IRAS). In this low coverage region, isolated water molecules and small water clusters are observed. Since thermal diffusion is suppressed at 20 K, the formation of water clusters at low coverage is controlled by both coverage and transient diffusion on the surface. Within a simple random walk model as transient diffusion and clustering process, we estimate the mean lateral displacement from the first impact point to the final adsorption site to be 0.76 nm.

We report the direct observation of a precursor state for the cycloaddition reaction (the di-s bond formation) of ethylene on Si(100)c(4x2) using high-resolution electron energy loss spectroscopy at low temperature, and the meta-stable precursor state is identified as a weakly bonded p -complex type. The activation energy from the p-complex precursor to the di- s bonded species is experimentally estimated to be 0.2 eV. First-principles calculations support the p-complex precursor mediated cycloaddition reaction of ethylene on Si(100)c(4x2).

We investigated low-energy electron-stimulated chemical reactions between CO and water molecules in low-temperature ice using infrared reflection absorption spectroscopy. Carbon dioxide, the formyl radical, formaldehyde, and methanol were produced by electron irradiation of the water/CO/water layered ice.The electron energy threshold and temperature dependence for the chemical reactions were investigated to elucidate the reaction mechanisms.

CO in water ice + e^- -> CO₂, H₂CO, CH₃ OH

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Purely site-specific chemisorption and conformation of trimethylamine on Si(100)c(4x2)

We investigated adsorbed states of CO on Pt(997) at 11 K using infrared reflection absorption spectroscopy. At 11 K, thermal migration is suppressed and thus the initial chemisorption at terrace sites and step sites is controlled by the transient mobility of the adsorbing molecule. The initial occupation ratio between atop CO on the terrace and atop CO at the step is directly determined to be 3.6:1.With a simple isotropic migration model, we estimated the mean lateral displacement from the first impact point to the initial chemisorption site to be 0.68nm .We also discuss the origin of transient mobility of CO on metal surfaces.

The origin and atomic structure of C-defect on Si(100) have been unambiguously identified. Two pairs of enantiomorphic protrusions of C-defect (U,U',D,D') have been observed by low temperature scanning tunnelling microscopy (STM). These are attributed to the dissociative adsorption of single water molecule on two adjacent dimers. Two unreacted dangling bonds on these dimers have different electronic states which are visualized in unoccupied state STM images.

This spectrum was measured with hn=129eV using a Phoibos100 analyzer at KEK-PF BL16B. The overall instrumental resolution was ~40meV, so that the width of each component can be estimated as a function of temperature.

Surface atoms on Si(100) are reconstructed to form a (2x1) dimer row structure. Many experimental and theoretical studies have supported the buckled (asymmetric) dimer with c(4x2) phase as a ground state, although some quantum chemical studies including electron correlation have supported a symmetric dimer as the most stable state. Recently, several low temperature STM studies have reported symmetric dimer images. With decreasing the temperature from 80 K to 20 K, the area of apparent symmetric dimer increases. The origin of symmetric images is a controversial issue (dynamic or static). In this study, using high resolution Si 2p photoelectron spectroscopy, we have investigated the electronic states of Si(100) at 140 K, 100 K and 55 K (Fig.1). By careful analysis of surface core level shift and area intensity of each component in the spectra, the number of asymmetric dimer atoms does not change in this temperature range. Thus, we conclude that the dimer on Si(100) is asymmetric down to 55 K, and the origin of symmetric images in STM may be due to dynamic effects.

Fig.1 Si 2p PES spectra at low temperature. (hn=129eV, normal emission)

Chemical bond between adsorbed unsaturated hydrocarbon molecules and the Si(100)(2x1) surface has been investigated by means of HRPES at PF BL-16B. This undulater beam line has a 24-m High-resolution Spherical Grating Monochromator (H-SGM), which covers the photon energy range from 40 eV to 550 eV. We have carried in our UHV chamber including a LEED optics (Varian) and an old electron analyzer (VSW HA100) and a new SPECS Phoibos100.

In the cases of cyclopentene and cyclohexene, the dangling bond state just below the Fermi level in valence PES spectra has decreased in intensity with increasing the coverage and at the saturation the dangling bond peak disappears almost completely. On the other hand, the HOMO derived states which are clearly observed for the multilayer of molecules at 90K have been significantly attenuated at the original position. Thus, we conclude that the p bond of cyclopentene (and cyclohexene) interacts with the Si dangling bonds. According to the Si 2p spectra, it seems that the SiC bonds are formed between the Si surface and the molecule. This is also supported from the information about the C 1s peak in XPS spectra taken at ISSP.
1,4-cyclohexadiene has two C=C double bonds. There are two possible adsorption structures on Si(100)(2x1); (1) two p bonds interact with Si dangling bonds to form four SiC sigma bonds, and (2) one p bond reacts with one Si dimer to make di-sigma bond and another C=C bond is still intact. Preliminary results from PES in UVSOR-BL5A and XPS in ISSP suggest that the case (2) is plausible. In this case, additional molecular development/modification is possible using organic chemical reactions with 1,4-cyclohexadiene/Si(100). However, further experiments are needed to confirm the adsorption structure and realize this idea.

In order to elucidate electronic properties of adsorbed atoms/molecules and surfaces, a new UHV system including a hemispherical electron analyzer (VG CLAM4), X-ray sources (VG XR3), a UV source (VG UVL-Hi) with a polarizer, LEED (VSI), QMS (Balzers Prisma) and the home-made gas dosing system (a continuous doser with a variable temperature tungsten tube and a MCP doser with a pulse valve) is constructed. The sample is cooled down to <30 K using a closed cycle He refrigerator, and can be rotated around two axis.

(From left: Ms. Hamaguchi, Dr. Yamashita and Mr. Machida)

Evidence for a molecule-substrate hybridized state near the Fermi level (E_F) is presented for Pd(110)c(4x2)-benzene. Observed images of adsorbed benzene near E_F by STM consist of two elongated protrusions separated by a single nodal depression with C₂ symmetry. The existence of a benzene derived state near E_F is also observed by metastable atom electron spectroscopy, and it is assigned to the antibonding states between the 1e_1g molecular orbital of benzene and the Pd 4d orbitals by ab initio molecular orbital calculations.

The microscopic surface diffusion of CO molecules on Ni(100) has been studied with infrared reflection absorption spectroscopy (IRAS). We have observed low temperature kinetics approaching equilibrium between isolated adsorbed CO and c(2x2)-CO domains at a low coverage. The upper limit of the diffusion barrier E_d = 60 ± 7 meV and the preexponential factor A = 10^6.8±1.1 s^-1 were estimated. While the adsorption energy (E_ad) of CO on Ni(100) is approximately 1.3 eV, the diffusion barrier is less than 5% of E_ad. It is shown that the barrier is consistent with the postulated potential energy surface [J. Yoshinobu et al., Phys. Rev.B49,16670-16677 (1994)] derived from the frustrated translation modes and the binding energy difference with a harmonic parabola assumption, and also with the recent microscopic diffusion barrier obtained by quasielastic He atom scattering (QHAS) measurements.

NiCOpotential.JPG

This proposed mechanism can explain most experimental data of CO₂ formation dynamics on Pt(111), and is supported by a recent first principle calculation [A.Alavi et al., Phys. Rev. Lett. 80, 3650 (1998)].