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We report the formation processes of the self-assembled layer of 1,3,5-tris(4-bromophenyl)benzene (TBB) molecules on the Ag(111) and Si(111) (√3 × √3)-Ag surfaces by STM measurements and density functional theory (DFT) calculations. The self-assembled layers on the surfaces show characteristic structures controlled by the interplay between the intermolecular interaction and the molecule-substrate interaction. Through the cooperative interplay between the molecule-substrate interaction and the intermolecular halogen bond (XB), the periodic arrangement of TBB molecules appears on the Ag(111) surface. On the other hand, the two types of TBB arrangement appear on the Si(111) (√3 × √3)-Ag surface (phases 1 and 2). Phase 1 is the periodic arrangement of the TBB molecules and is derived from the cooperative interplay between the molecule-substrate interaction and the intermolecular van der Waals (vdW) interaction and the hydrogen bond (HB), and phase 2 is a random arrangement and is derived from the competitive interplay between the molecule-substrate interaction and the intermolecular XB and HB. Our present study specifies the role of the substrate in the molecular self-assembly of the substrate. Although the structure of the molecular self-assembly is controlled by the choice of the substrate, the cooperative interplay between the molecule-substrate interaction and the intermolecular interaction is necessary to realize the ideal periodic arrangement.

The electromagnetic response of Dirac semimetals in the infrared and terahertz frequency ranges is attracting growing interest for potential applications in optoelectronics and nonlinear optics. The interplay between the free-carrier response and interband transitions in the gapless, linear dispersion relation plays a key role in enabling novel functionalities. Here we investigate ultrafast dynamics in thin films of a photoexcited Dirac semimetal Cd$_3$As$_2$ by probing the broadband response functions as complex quantities in the multiterahertz region (10-45 THz, 40-180 meV, or 7-30 $\mu$m), which covers the crossover between the inter and intraband response. We resolve dynamics of the photoexcited nonthermal electrons which merge with originally existing carriers to form a single thermalized electron gas and how it is facilitated by high-density excitation. We also demonstrate that a large reduction of the refractive index by 80% dominates the nonequilibrium infrared response, which can be utilized for designing ultrafast switches in active optoelectronics., Comment: 20 pages

Hydrogen spillover is a crucial process in the selective hydrogenation reactions on Pd/Cu single atom alloy catalysts. In this study, we report the atomic-scale perspective of these processes on the single atom alloy catalyst Pd/Cu(111) based on the experimental and theoretical results, including infrared reflection absorption spectroscopy (IRAS), temperature programmed desorption (TPD), high-resolution X-ray photoelectron spectroscopy (HR-XPS), and density functional theory (DFT) calculations for core-level excitation. The hydrogen spillover onto Cu(111) was successfully observed in real time using time-resolved IRAS measurements at 80 K. The chemical shifts of Pd 3d

We investigated mass transports during hydrogen adsorption and absorption processes of PdCu alloys that has advantages of higher hydrogen diffusivity and economically lower-cost than the other Pd-alloys. The research was made with a comparison of the well-known ordered phase of the bcc structure (the B2 phase) and a mixed phase of the fcc and B2 structures. In-situ ultrahigh vacuum X-ray photoelectron spectroscopy (UHV-XPS) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) using synchrotron radiation were carried out to trace the chemical states of the Pd and Cu atoms as a function of temperatures. It is newly elucidated that the initial adsorption and absorption processes were similar in the two phases, but a hydrogen diffusion rate to the bulk was apparently higher in the ordered phase than in the mixed one. We found the dynamics of the Pd and Cu atoms during the hydrogen adsorption/absorption processes largely depends on temperature. In the hydrogen atmosphere, the Pd atoms segregate at the surface below 373 K and Cu atoms segregate at the surface above 373 K, showing a reversal behavior. The present results agree well with the previous theoretical calculations and, thus, provide appropriate inputs toward developments of the industrial hydrogen permeation materials.

The reaction of carbon dioxide (CO2) on Zn-deposited copper surfaces was systematically investigated by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). In the presence of 0.8 mbar CO2 a...

The ceria-based catalyst incorporated with Cr and a trace amount of Rh (Cr0.19Rh0.06CeOz) was prepared and the reversible redox performances and oxidation catalysis of CO and alcohol derivatives with O2 at low temperatures (

Complete H–D exchange of butene was reported on a D-absorbed Pd–Au alloy surface. However, its reaction mechanism has not been clarified yet. Here we investigate the reaction of cis-2- and 1-butene on Pd70Au30(110) by means of high-resolution electron energy loss spectroscopy. We demonstrate that both butene isomers are in the π-bonded state and that 1-butene irreversibly converts to cis-2-butene around 250 K. This isomerization accounts for previously observed similar product distributions in the H–D exchange reaction of cis-2- and 1-butene on the D-absorbed Pd70Au30(110) surface. Since the reverse change from cis-2- to 1-butene is not observed at any temperature, we conclude that the H–D exchange proceeds by the dissociative mechanism via vinylic and π-allylic intermediates rather than the Horiuti–Polanyi mechanism.

Electronic states and growth modes of the Zn-deposited Cu(111) surface at 300 K were quantitatively studied using core-level and valence photoelectron spectroscopies. Both Cu 2p and Zn 2p core-levels shifted to higher binding energy with increasing the amount of deposited Zn up to multilayer. The origin of the core-level shift of Cu 2p was further investigated by density functional theory calculations; the shift of the Cu 2p peak results from the change in the effective electrostatic potential (initial state effect) caused by the formation of Zn-Cu surface alloy, and the increase of coordination numbers of surface Cu atoms by Zn overlayer. The observed valence photoelectron spectra show the formation of the two atomic-layer Zn-Cu alloy up to the Zn coverage of 1 ML, followed by the formation of three-dimensional Zn islands on the alloyed surface at 300 K.

The vibrational modes of chemisorbed CO on a Si(001) surface are investigated by means of transmission Fourier-transform infrared absorption spectroscopy. We observed the three components corresponding to the stretching vibration of the terminal-site CO adsorbed on the down-dimer sites of a Si(001) surface. The symmetric stretching vibration and asymmetric stretching vibration are observed separately. This assignment is consistent with the polarization dependence of the incident light and with the vibrational modes obtained by performing theoretical calculations. We found that both the intradimer row and interdimer row coupling should be considered to explain the appearance of the three components.

The chemistry of simple alkene molecules on the Si(100) surface is reviewed with the newly-produced visual presentation by theoretical calculations. The early pioneering studies by the Kyoto Group and Pittsburgh group reported the di–σ bond formation and the precursor-mediated chemisorption for acetylene and ethylene on Si(100), respectively. Thereafter, these studies have been stimulating various studies of organic molecules on Si surfaces. Our recent studies have observed the precursor states for alkene chemisorption and elucidated the microscopic mechanisms of the di–σ bond formation (cycloaddition) with the help of theoretical calculations; the site-, stereo- and regio-selective chemisorption of simple alkene molecules on Si(100)c(4 × 2) has been established.

The nature of C-H···M (M = metal surface) interactions is reviewed based mainly on our recent investigations of cyclohexane on Rh(111). Infrared reflection-absorption spectroscopy measurements at low temperature (∼20 K) have shown that the softened CH stretching band consists of several sharp peaks. At temperatures above 80 K, each peak is broadened, most probably by anharmonic coupling with thermally excited low-energy frustrated translational modes. The origin of fine structure in this band and its similarity to that in hydrogen bond systems are discussed. In addition, novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) using temperature programmed desorption, ultraviolet photoelectron spectroscopy, and spot profile analysis low-energy electron diffraction. The desorption energy of deuterated cyclohexane (C6 D12 ) is lower than that of C6 H12 (inverse kinetic isotope effect). In addition, the work function change by adsorbed C6 D12 is smaller than that by adsorbed C6 H12 . These results indicate that C6 D12 molecules are slightly more distant from the surface than C6 H12 molecules (vertical geometric isotope effect). A lateral geometric isotope effect was also observed for the two-dimensional cyclohexane superstructures as a result of the repulsive interaction between interfacial dipoles (= work function change). These isotope effects are ascribed to the quantum nature of hydrogen involved in the C-H···M interaction.

We have investigated the valence electronic states of α-sexithiophene (α-6T) on three passivated Si(0 0 1) surfaces, oxidized Si(0 0 1), water-adsorbed Si(0 0 1) and ethylene-adsorbed Si(0 0 1), using ultraviolet photoelectron spectroscopy (UPS). At a thickness of α-6T layer below 0.5 nm, clear features of the π states are observed for water-adsorbed Si(0 0 1) and ethylene-adsorbed Si(0 0 1), whereas broad features are observed for oxidized Si(0 0 1). This difference is attributed to the formation of well-ordered stacking on water-adsorbed Si(0 0 1) and ethylene-adsorbed Si(0 0 1).

We investigated the adsorbed states of CO on Pt(997) using high-resolution X-ray photoelectron spectroscopy (HR-XPS) with synchrotron radiation. Four kinds of adsorbed surfaces were prepared: (1) the saturation at 200 K, (2) the saturation at 310 K, (3) “on-top at terrace” and “on-top at step” CO species, and (4) only step-CO species. HR-XPS measurements of these surfaces were carried out at 90 K or 310 K. In the case of the saturation coverage at 310 K, three sharp peaks were clearly observed at binding energies of 285.91, 286.24, and 286.64 eV in the C 1s region. These peaks were assigned to the “bridge at terrace”, “on-top at step” CO and “on-top at terrace”, respectively. In addition, the Pt 4f 7/2 spectra were measured under the same conditions. In the case of the Pt(997) clean surface, three components, i.e., step, terrace and bulk Pt atoms, were identified at 70.6, 70.8, and 71.1 eV in the Pt 4f 7/2 spectrum, respectively. For the CO saturated Pt(997) surface at 200 K, three shifted surface components were newly observed at 71.5, 72.1 and 71.8 eV. They are related to the “bridge at terrace,” “on-top at terrace,” and “on-top at step” CO species, respectively. We discuss the charge transfer between CO and surface Pt atoms based on the chemical shifts of both C 1s of CO and Pt 4f 7/2 . The charge transfer occurs not only from the step Pt atom but also from surrounding terrace Pt atoms in the case of the “on-top at step” CO species.

We fabricated iron and iron silicide microstructures on an Si(1 0 0) clean surface via electron beam induced process of Fe(CO)5 multilayer and subsequent annealing. The fabricated microstructures were in situ analyzed by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). We successfully analyzed the coverage and chemical states of the artificial deposited iron structure area-selectively by AES. The artificial iron structure was fabricated after heating to above 350 K to desorb residual Fe(CO)5 species. The artificial structure was observed even after 1190 K annealing by SEM, but AES measurements showed it to be covered by Si atoms. We concluded that the buried iron silicide microstructure was formed by the present process.

We observed four kinds of adsorbed NO molecules on Pt(9 9 7) at 11 K using infrared reflection absorption spectroscopy (IRAS). The peaks at 1690, 1484 and 1615 cm−1 are assigned to the N–O stretching modes of the on-top site and the hollow site on the terrace and the bridge site at the step, respectively. The 1385 cm−1 peak is observed below ∼70 K. We assign the 1385 cm−1 peak to the hollow site of the (1 1 1) microfacet at the step or the lower-terrace hollow site nearest to the step. By heating, site-to-site hopping to the more stable site occurs and the relative stability of four adsorption sites can be determined.

Using infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM), we investigated the adsorption states of NO on the Pt(9 9 7) step surface. At 90 K, we observe three N–O stretching modes at 1490 cm −1 , 1631 cm −1 and 1700 cm −1 at 0.2 ML. The 1490 cm −1 and 1700 cm −1 peaks are assigned to NO molecules at fcc-hollow and on-top sites of the terrace, respectively. The 1631 cm −1 peak is assigned to the step NO species. In the present STM results, we observed that NO molecules were adsorbed at the bridge sites of the step as well as fcc-hollow and on-top sites of the terrace. To help with our assignments, density functional theory calculations were also performed. The calculated results indicate that a bridge site of the step is the most stable adsorption site for NO, and its stretching frequency is 1607 cm −1 . The interactions between NO species at different sites on Pt(9 9 7) are also discussed.

The chemical reactivity of the buckled dimer on Si(1 0 0) is reviewed from the viewpoint of physical properties of the substrate and the molecules, which have been obtained by several experimental techniques including valence and core-level photoelectron spectroscopy, high-resolution electron energy loss spectroscopy, scanning tunneling microscopy as well as theoretical calculations. The up dimer atom ( S u ) of the buckled dimer on Si(1 0 0) is electron-rich and the down dimer atom ( S d ) is electron-deficient. Thus, S u is nucleophilic and S d is electrophilic, and one can expect that the S u acts as a Lewis base, and the S d acts as a Lewis acid. This is a simple guiding principle and works well to understand and predict Si surface chemistry. Typical adsorption systems including trimethylamine, ammonia, water, BF 3 , ethylene and vinylbromide on Si(1 0 0) c (4 × 2) are discussed mainly based on our recent experimental investigation at the atomic-scale.

The atomic configuration of C 6 H 8 molecules chemisorbed on Si(0 0 1) surface is elucidated by the first-principles molecular dynamics method. The characteristic pattern in the STM image at ∼0.6 ML coverage is found to be reproduced by considerable thermal fluctuation of the molecules including the change in the molecular conformation. The reaction path and growth mechanism of the chemisorption region is also discussed based on static calculation of the activation barrier.

We investigated the adsorption states and the reaction of vinyl bromide (CH2=CHBr) on Si(100)c(4×2), using high-resolution electron energy loss spectroscopy. We found that the chemisorption of this molecule occurs via a precursor state and a vinyl bromide molecule is di-σ bonded to the Si dimer on Si(100)c(4×2) in the chemisorption state. The precursor species are observed in the temperature range from 58 to 90 K. Judging from the vibrational spectra, we conclude that the structure of the precursor state is a three-atom intermediate state (π complex). The activation energy and the preexponential factor of the reaction from the precursor state to the chemisorption state are estimated to be 27.3 kJ/mol (283 meV) and 1.5 × 1013 s-1, respectively. The activation energy decreases with increasing the vinyl bromide coverage owing to the steric repulsion.

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.

Adsorption of trimethylamine (TMA) on Si(100)c(4×2) has been studied using scanning tunneling microscopy (STM), valence photoelectron spectroscopy (PES), and high-resolution electron energy spectroscopy (HREELS) between 80 and 300 K. The TMA molecule is adsorbed nondissociatively only on the down dimer atom and appears as a triangle shaped bright protrusion in the occupied state STM image. The saturation coverage of TMA on Si(100) is estimated to be ∼0.25 ML (ML = one molecule per surface Si atom). The PES and HREELS results suggest that TMA is adsorbed through the N lone pair by forming an Si−N dative bond. The HREELS spectra indicate that the symmetry of adsorbed TMA is Cs and the molecular C3 axis is tilted from the surface normal. The stretching mode of the SiN dative bond is observed at 525 cm-1. Thus the combined STM, PES, and HREELS results indicate that the chemisorption of TMA on Si(100)c(4×2) occurs selectively on the down dimer atom via dative bonding. Hence, the TMA adsorption on Si(100)c(4×2)...

We have investigated the C 1s photoelectron spectra of ethylene on the Si(1 0 0)(2 × 1) surface, using high resolution photoelectron spectroscopy. The vibrational structure has been observed in the C 1s spectra where the C–H stretching mode is dominant. The vibrational splittings of C 1s ionized state are very similar to those in the ground state. It is found that linear coupling model is applicable in this system and this excitation process is a Frank–Condon process. The curvature of potential energy surface of C–H and C–D bond is almost unchanged upon core ionization.

The adsorbed states of 1,4-cyclohexadiene molecules on Si(1 0 0)(2 × 1) at 300 K have been investigated as a function of coverage in real space by means of scanning tunneling microscopy (STM). Two kinds of intermolecular interaction have been studied (i) between the neighboring molecules on the neighboring dimer rows and (ii) between the neighboring molecules on the same dimer row. By analyzing the observed arrangements of adsorbed molecules, we have concluded that the intermolecular interaction between the molecules on the neighboring dimer rows is very small. On the other hand, the intermolecular interaction between the neighboring molecules on the same dimer row becomes repulsive when the distance between two adsorbates is shorter than 2a (a=3.85 A: the distance between the Si dimers on the same row). Various types of arrangements are discussed, based on the steric repulsion. The proposed models can explain the alternate height variations in consecutive molecular protrusions in STM images and the saturation coverage of 0.6 molecules per Si dimer at 300 K.

We used the energy-selected-photoelectron photoion coincidence (ESPEPICO) method to study site-specific fragmentation caused by C:1s photoionization of 1,1,1-trifluoro-2-propanol-d1 [CF3CD(OH)CH3, TFIP-d1] on a Si(100) surface. High-resolution electron energy loss spectroscopy showed that TFIP-d1 is dissociatively chemisorbed like (CF3)(CH3)CDO–Si(100), and different chemical shifts at the three carbon sites were observed by photoelectron spectroscopy. The site-specific fragmentation evident in the ESPEPICO spectra of the sub-monolayer at room temperature indicates that the TFIP-d1 there has an O–Si bond oriented in the trans position with respect to the C–CF3 bond. Here we discuss the fragmentation processes in light of the results obtained with the ESPEPICO method and the Auger-electron photoion coincidence method.

Surface reactions between N16O and 18O2 coadsorbed on Pt(111) have been studied by temperature‐programmed desorption (TPD), low energy electron diffraction and infrared reflection absorption spectroscopy (IRAS). When the surface covered with N16O and 18O2 is annealed, 18O16O desorbs at 155 K and N18O at 145, 310, and 340 K. In addition, a new absorption line at 1658 cm−1 due to N18O appears above 265 K. However, there is no indication of oxidation products of NO in the TPD and IRAS measurements. Thus, only oxygen‐exchange reactions take place on the coadsorbed surface. Reaction yields and mechanisms of the oxygen‐exchange strongly depend on the adsorption states of oxygen. At ∼145 K, molecularly adsorbed oxygen and N16O (νNO=∼1930 cm−1) directly interacting with the oxygen molecule are involved in the exchange reaction. Above 270 K the exchange reaction takes place between atop N16O (νNO=∼1720 cm−1) and the metastable oxygen adatoms that do not possess a long‐range order. On the contrary, the oxygen‐excha...

Thermal excitation of adsorbed oxygen species is found to initiate the CO oxidation on Pt(111). We have prepared three different coadsorption systems to study the reactivity of different oxygen species; (1) CO on the O2 preadsorbed Pt(111) surface, (2) CO on the nearly perfect Pt(111) p(2×2)‐O surface, and (3) CO on the disordered atomic oxygen‐preadsorbed Pt(111) surface. Four CO2 desorption peaks (α‐CO2 at 125 K, β3‐CO2 at ∼225 K, β2‐CO2 at ∼260 K, and β1‐CO2 at 320 K) are observed. The desorption temperatures of CO2 strongly depend on the adsorbed states of oxygen species. We have shown that the α‐CO2 state, β2,3‐CO2 states, and β1‐CO2 state are correlated with adsorbed O2, disordered oxygen atoms, and p(2×2) oxygen atoms, respectively. The difference in CO2 desorption temperature is related to thermal excitation of each oxygen species, which is derived from the structural information of coadsorbed states during thermal evolution by means of low‐energy electron diffraction and infrared reflection absor...

We describe an independently driven four-probe method to investigate local channel mobility in organic field-effect transistors (OFETs). In OFET devices, probe-organic contact resistance affects device characteristics even in four-probe measurement because a change in contact resistance at the source probe induces a change in channel potential, resulting in different local carrier density. To overcome this problem, we introduced a feedback circuit between the source probe and a channel voltage probe to keep the channel potential constant. We demonstrate four-probe I-V measurement on a pentacene thin film (50 nm thick) under controlled channel potential. The feedback successfully enables us to separate contact resistance and channel resistance even under different contact conditions. We also measured four-probe resistance as a function of gate bias and channel probe position. The present results were in good agreement with two-dimensional model calculation by arranging four probes in a defect-free area; the mobility of the pentacene single grain was evaluated to be 0.25 cm(2)/(V s).

The adsorption of N2 on Ni(111) at 89–115 K under steady‐state (quasiequilibrium) conditions has been studied using Fourier‐transform infrared reflection absorption spectroscopy (FT‐IRAS) and low‐energy electron diffraction (LEED). At very low coverage, a single N 3/4 N absorption band is observed at 2218 cm−1 which is assigned to a singleton N2 adspecies. With increasing N2 pressure, two other bands develop at 2212–2208 and 2204–2203 cm−1, and a faint ( 7/8 × 7/8 )R30° LEED pattern is observed. At even higher pressure, a sharp ( 7/8 × 7/8 )R30° LEED pattern develops together with a relatively broad single band at 2194 cm−1. The enthalpy of adsorption of N2 on Ni(111) is estimated to be −8.4 kcal/mol at zero coverage from isothermal IRAS measurements. Slight attractive interactions between N2 adspecies are detected. The origins of the vibrational features which are related to island formation will be discussed in detail.

Photodesorption of CO, NO, and O2 from modified Ni(111) surfaces has been studied. Both direct observation of photodesorption and postirradiation thermal desorption measurements have been carried out. In each case, the photodesorption process is first order in photon flux and first order in the coverage within particular chemisorption states. The photon energy dependence of the photodesorption cross section was measured in the energy range from ∼1.4 to 5.4 eV using a filtered Hg lamp. The CO photodesorption was observed only on the oxidized Ni(111) surface: no CO photodesorption was observed on the clean Ni(111) surface nor Ni(111) containing chemisorbed O and S. The photon energy dependence of the cross section for CO and O2 photodesorption resembles the optical absorption spectrum of NiO. We propose that, in the case of chemisorbed CO and O2 on NiO, photodesorption is induced by the NiO band gap interband transition. However, NO photodesorption occurs not only on the oxidized Ni(111) surface but also on the Ni(111) surfaces containing chemisorbed O and S. This observation suggests that NO photodesorption is induced by valence electron excitation in the NO‐metal complex, but does not exclude the possibility of a contribution also from hot electron excitation in the substrate.

A miniature effusion cell for the vacuum deposition of volatile organic solids with low vapor pressures has been fabricated and used in surface science studies. The effusion cell is designed to operate at up to 200 degrees C under ultrahigh vacuum (UHV) conditions. The size of this cell is so small that it is attached to the top of a transfer rod and can be introduced from a subchamber into a main UHV chamber (retrievable). In addition, the small heat capacity of this cell means rapid heating and cooling rates. The advantages of this evaporator are its simplicity of design and ease of fabrication, assembly, and operation.

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 ...

Photon‐induced desorption studies of chemisorbed CO on Ni(111), Ni(111) with chemisorbed oxygen, and oxidized Ni(111) surfaces have been carried out. Only the oxidized Ni(111) surface exhibits measurable CO photodesorption in the energy range from 1.8 to 4.1 eV. Both direct observation of CO photodesorption and post‐irradiation thermal desorption of CO have been performed. The photodesorption process is first order in photon flux and in CO coverage. At a photon energy of 4.1 eV, the cross section for CO photodesorption is measured to be 5×10−18 cm2. The measured threshold energy, 2.7±0.5 eV, is in good agreement with a model involving the O2−2p→Ni2+3d interband transition in NiO as the excitation process leading to photodesorption.

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 A; an incoming water molecule on Rh(111) is trapped with eight postcollision hops on the average.

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 (square root 3 x square root 3)R30 degrees 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 x 10(16) ML(1/2)/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 x 10(14) ML(1/2)/s, respectively.