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The adsorption, desorption, and decomposition of formic acid (HCOOH) on Cu(111), Cu(997), Zn-Cu(111), and Zn-Cu(997) were systematically studied by high-resolution x-ray photoelectron spectroscopy, temperature programmed desorption, and infrared reflection absorption spectroscopy. On the clean Cu(111) surface, 13% of formic acid molecules adsorbed at 83 K were dissociated to form bidentate formate species by heating at 300 K; however, on the Zn-Cu(111) surface, only 4% of adsorbed HCOOH molecules were dissociated into the bidentate formate species. On the contrary, 13% of adsorbed HCOOH molecules were already dissociated into monodentate formate species on Cu(997) even at 83 K and 17% of adsorbed formic acid molecules were transformed to bidentate formate species by heating at 300 K, indicating that the stepped Cu surface has higher reactivity for HCOOH dissociation at low temperature. On the Zn-Cu(997) surface, 20% of formic acid became bidentate formate species in contrast to the case with Zn-Cu(111). Thus, the Zn deposited Cu step surface shows special activity for adsorption and dissociation of formic acid. The desorption peak maxima of the formate decomposition products (CO2 and H2) on Zn-Cu(997) were shifted to higher temperatures than those on Cu(997). Zn on Cu surfaces plays an important role in the stabilization of formate species, which probably leads to the decrease in the activation barrier for hydrogenation on the Zn-Cu alloyed surface. [ABSTRACT FROM AUTHOR]

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 2.0×1011 s-1, 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 6.5×1011 s-1, 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. [ABSTRACT FROM AUTHOR]

The authors investigated the initial stage of water adsorption on Rh(111) at 20 K, using infrared reflection absorption spectroscopy. 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 of the transient diffusion and clustering process, the authors estimate the mean lateral displacement from the first impact point to the final adsorption site to be 7.6 Å; an incoming water molecule on Rh(111) is trapped with eight postcollision hops on the average. [ABSTRACT FROM AUTHOR]

The adsorption states and growth process of the first water (D2O) layer on Rh(111) were investigated using infrared reflection absorption spectroscopy, temperature programed desorption, and spot-profile-analysis low energy electron diffraction. Water molecules wet the Rh(111) surface intact. At the early stage of first layer growth, a (&#221A;3&#221A;3)R30° commensurate water layer grows where "up" and "down" species coexist; the up and down species represent water molecules which have free OD, pointing to a vacuum and the substrate, respectively. The up domain was a flatter structure than an icelike bilayer. Water desorption from Rh(111) was a half-order process. The activation energy and the preexponential factor of desorption are estimated to be 60 kJ/mol and 4.8×1016 ML½/s at submonolayer coverage, respectively. With an increase in water coverage, the flat up domain becomes a zigzag layer, like an ice bilayer. At the saturation coverage, the amount of down species is 1.3 times larger than that of the up species. In addition, the activation energy and the preexponential factor of desorption decrease to 51 kJ/mol and 1.3×1014 ML½/s, respectively. [ABSTRACT FROM AUTHOR]

NO photodesorption from the oxygen-covered, oxidized, and sulfur-saturated Ni(111) surfaces was studied. Temperature programmed desorption effectively discriminates between NO species adsorbed on Ni metal sites and on Ni sites modified by O or S, making the study of photodesorption from each site possible. Oxygen- and sulfur-modified Ni(111) surfaces both exhibit enhanced photodesorption cross sections compared to a clean Ni surface. Both the direct observation of NO(g) photodesorption and postirradiation thermal desorption measurements of NO coverage have been carried out. The photodesorption process is first order in photon flux and in NO coverage within a given chemisorption state. The photon-energy dependence of the NO photodesorption cross section from oxidized and S-saturated Ni(111) surfaces was measured in detail over a photon energy range of 1.5–3.8 eV. For both oxidized and sulfur-saturated Ni(111), the threshold for NO photodesorption is ∼1.5 eV. In addition, the photodesorption cross sections are dependent on the adsorption states of NO on the modified Ni(111) surfaces. The mechanisms of NO photodesorption are discussed and it is proposed that NO photodesorption is probably induced by the valence electron excitation from the occupied NO 2π*-d state to the unoccupied NO 2π*-d state. However, a hot electron model originating from the substrate cannot be excluded. The quenching of electronically excited NO on the clean Ni(111) surface is efficient. [ABSTRACT FROM AUTHOR]

The reaction of CO on the vicinal Cu(997) surface at 340 K under CO gas pressure of 0.8 mbar was investigated by ambient pressure X-ray photoelectron spectroscopy. A main reaction product on the surface was identified as carbonate (CO), based on estimation of the composition ratio of oxygen to carbon. CO was produced on the surface through the reaction of CO with oxygen formed from CO dissociation. The amount of adsorbed CO was increased and saturated as time elapsed. After saturation of adsorbed CO, atomic oxygen appeared on the surface, indicating that CO dissociation into CO and O continued to take place. The present study shows the importance of CO intermediate in the CO chemistry on stepped Cu surfaces. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the valence electronic states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) on the 2-methylpropene chemisorbed Si(100)(2×1) surface using valence photoelectron spectroscopy. Since the electron affinity of condensed F4-TCNQ is 5.24eV and the energy from the valence band maximum of the 2-methylpropene saturated Si(100)(2×1) surface to the vacuum level is 4.1eV, spontaneous charge transfer would be expected in the present system. At sub-monolayer coverage of F4-TCNQ, characteristic peaks are observed at 1.1 and 2.5eV below Fermi energy. The former peak is assigned to a singly occupied affinity level, and the latter is ascribed to a relaxed highest occupied molecular orbital of adsorbed F4-TCNQ. The work function change is increased up to +1.3eV as a function of F4-TCNQ coverage. These results support the occurrence of charge transfer into F4-TCNQ on the 2-methylpropene saturated Si(100)(2×1) surface. [Copyright &y& Elsevier]

The anisotropic correlation between buckled dimers on Si(100) was investigated by scanning tunneling microscopy. A bidentate ligand molecule was used to pin two neighboring dimers at 300 K. The chemically pinned dimer induces antiferromagnetic interaction along the dimer rows. Observed results agree well with Monte-Carlo simulations semi-quantitatively. [ABSTRACT FROM AUTHOR]