Time-resolved sum-frequency generation spectroscopy of cyclohexane adsorbed on Ni(111) under ultrashort NIR laser pulse irradiation

The adsorption of cyclohexane on Ni(111) was studied by infrared-visible sum-frequency generation (SFG) spectroscopy with and without near-infrared (NIR) pump pulse irradiation. Two adsorption states of cyclohexane were found in the monolayer region, a low-coverage state showing SFG peaks at 2740, 2815, and 2865 cm(-1), and a high-coverage state showing peaks at 2740, 2815, and 2905 cm(-1). Both states coexisted on the saturated Ni(111) surface. The broad peak at 2740 cm(-1) was due to the softened CH stretching mode of the axial CH groups of cyclohexane that point toward the Ni(111) surface. The peaks at 2815 and 2865 (or 2905) cm(-1) were due to the symmetric and asymmetric stretching modes of CH(2) groups, respectively, that were free from the surface. Irradiation with NIR pulses caused a temporary jump in temperature at the Ni(111) surface and enhanced the intensity of the 2905 cm(-1) peak, but weakened the other peaks. This indicates that the temperature jump excited the cyclohexane molecules from the low-coverage state to the high-coverage state. The dynamics of the structural change observed in the adsorbed cyclohexane on NIR irradiation is discussed.     

Alkyl chain conformation and the electronic structure of octyl heavy chalcogenolate monolayers adsorbed on Au(111)

Adsorption states of dioctyl dichalcogenides (dioctyl disulfide, dioctyl diselenide, and dioctyl ditelluride) arranged on Au(111) have been studied by X-ray photoelectron spectroscopy (XPS), infrared-visible sum-frequency generation (SFG), and ultraviolet photoelectron spectroscopy (UPS). XPS measurements suggest that dioctyl dichalcogenides dissociatively adsorbed on Au(111) surfaces to form the corresponding monolayers having chalcogen-gold covalent bonds. The elemental compositions of octanechalcogenolates on Au(111) indicate that the saturation coverages of the octyl heavy chalcogenolate (OcSe, OcTe) monolayers are lower than that of the octanethiolate (OcS) self-assembled monolayers (SAMs). The SFG observations of the CH(2) vibrational bands for the heavy chalcogenolate monolayers strongly suggest that a discernible amount of gauche conformation exists in the monolayers, while OcS SAMs adopt highly ordered all-trans conformation. The intensity ratio of the symmetric and asymmetric CH(3) stretching vibration modes measured by SFG shows that the average tilt angle of the methyl group of the OcSe monolayers is greater than that of the OcS SAMs. The larger tilt angle of the methyl group and the existence of a discernible amount of gauche conformation in the OcSe monolayers are due to the lower surface coverage of the OcSe monolayers compared with the OcS SAMs. The smaller polarization dependence in the angle-resolved UPS (ARUPS) spectra of the OcSe monolayers than that of the OcS SAMs is caused by the more disordered structures of the alkyl chain in the former. XPS, SFG, and ARUPS measurements indicate a similar tendency for the OcTe monolayers. The density of states (DOS) observed by UPS at around 1.3 eV for OcS adsorbed on Au(111) is considered to be the antibonding state of the Au-sulfur bond. Similar DOS is also observed by UPS at around 1.0 eV for the OcSe monolayers and at approximately 1.6 eV for the OcTe monolayers on Au(111).     

Interaction of coadsorbed CH3Cl and D2O layers on Pd(111) studied by sum frequency generation

Adsorption of methyl chloride and coadsorption of CH3Cl and D2O on Pd(111) surfaces at T=100 K have been studied under ultrahigh-vacuum conditions using femtosecond sum frequency generation (SFG) spectroscopy in the spectral regions of CH and OD bands. On the bare Pd(111) substrate, the CH3Cl coverage dependence of the resonant SFG signal is consistent with a progressive molecular rearrangement starting at half saturation followed by the growth of two ordered monolayers in which the molecular axes are perpendicular to the surface. When CH3Cl is adsorbed on top of predeposited D2O on Pd(111), the SFG signals as a function of the CH3Cl exposure indicate that methyl chloride is adsorbed onto D2O through hydrogen bonding. On the contrary when the adsorption order is reversed the strong decrease of the CH3 signal as a function of the D2O exposure is explained by assuming that water molecules penetrate inside the CH3Cl layers, leading to the formation of disordered CH3Cl clusters. In all cases a nonresonant contribution due to molecular adsorption is observed and it shows a dependence upon surface structure and coverage significantly different from that of the resonant vibrational bands.     

Why is metallic Pt the best catalyst for methoxy decomposition?

The decomposition of methoxy on Cu(111), Ag(111), Au(111), Ni(111), Pt(111), Pd(111), and Rh(111) has been studied in detail by the density functional theory calculations. The calculated activation barriers were successfully correlated with the coupling matrix element V 2 ad and the d-band center (ε d ) for the group IB metals and group VIII metals, respectively. By comparison of the activation energy barriers of the methoxy decomposition on different metals, it was found that Pt is the best catalyst for methoxy decomposition. The possible reason why the metallic Pt is the best catalyst has been analyzed from both the energetic data and the electronic structure information, that is, methoxy decomposition on Pt(111) has the largest exothermic behavior due to the closest p-band center of the CH 3 O among all metals after the adsorption.     

Methane dissociative adsorption on the Pt(111) surface over the 300-500 K temperature and 1-10 Torr pressure ranges

The dissociative adsorption of methane on the Pt(111) surface has been investigated and characterized over the 1-10 Torr pressure and 300-500 K temperature ranges using sum frequency generation (SFG) vibrational spectroscopy and Auger electron spectroscopy (AES). At a reaction temperature of 300 K and a pressure of 1 Torr, C-H bond dissociation occurs in methane on the Pt(111) surface to produce adsorbed methyl (CH(3)) groups, carbon, and hydrogen. SFG results suggest that C-C coupling occurs at higher reaction temperatures and pressures. At 400 K, methyl groups react with adsorbed C to form ethylidyne (C(2)H(3)), which dehydrogenates at 500 K to form ethynyl (C(2)H) and methylidyne (CH) species, as shown by SFG. By 600 K, all of the ethylidyne has reacted to form the dissociation products ethynyl and methylidyne. Calculated C-H bond dissociation probabilities for methane, determined by carbon deposition measured by AES, are in the 10(-8) range and increase with increasing reaction temperature. A mechanism has been developed and is compared with conclusions from other experimental and theoretical studies using single crystals.     

Observation of a methoxy species on Ni(111) by high-resolution electron energy-loss spectroscopy

The chemisorption and decomposition of methanol on Ni(111) has been studied by high-resolution electron energy-loss spectroscopy. We isolate and identify a methoxy species (CH₃O) which forms as a quasi-stable surface intermediate during the thermal decomposition of chemisorbed methanol. The methoxy species is bonded with the oxygen end nearest to the surface and the methyl group inclined at an oblique angle to the surface.     

Kinetic mechanism of methanol decomposition on Ni(111) surface: a theoretical study

The decomposition of methanol on the Ni(111) surface has been studied with the pseudopotential method of density functional theory-generalized gradient approximation (DFT-GGA) and with the repeated slab models. The adsorption energies of possible species and the activation energy barriers of the possible elementary reactions involved are obtained in the present work. The major reaction path on Ni surfaces involves the O-H bond breaking in CH(3)OH and the further decomposition of the resulting methoxy species to CO and H via stepwise hydrogen abstractions from CH(3)O. The abstraction of hydrogen from methoxy itself is the rate-limiting step. We also confirm that the C-O and C-H bond-breaking paths, which lead to the formation of surface methyl and hydroxyl and hydroxymethyl and atom hydrogen, respectively, have higher energy barriers. Therefore, the final products are the adsorbed CO and H atom.     

Transmission infrared spectroscopy of methyl- and ethyl-terminated silicon(111) surfaces

Transmission infrared spectroscopy (TIRS) has been used to investigate the surface-bound species formed in the two-step chlorination/alkylation reaction of crystalline (111)-oriented Si surfaces. Spectra were obtained after hydrogen termination, chlorine termination, and reaction of the Cl-Si(111) surface with CH(3)MgX or C(2)H(5)MgX (X = Cl, Br) to form methyl (CH(3))- or ethyl (C(2)H(5))-terminated Si(111) surfaces, respectively. Freshly etched H-terminated Si(111) surfaces that were subsequently chlorinated by immersion in a saturated solution of PCl(5) in chlorobenzene were characterized by complete loss of the Si-H stretching and bending modes at 2083 and 627 cm(-1)(,) respectively, and the appearance of Si-Cl modes at 583 and 528 cm(-1). TIRS of the CH(3)-terminated Si(111) surface exhibited a peak at 1257 cm(-1) polarized perpendicular to the surface assigned to the C-H symmetrical bending, or "umbrella" motion, of the methyl group. A peak observed at 757 cm(-1) polarized parallel to the surface was assigned to the C-H rocking motion. Alkyl C-H stretch modes on both the CH(3)- and C(2)H(5)-terminated surfaces were observed near 2900 cm(-1). The C(2)H(5)-terminated Si(111) surface additionally exhibited broad bands at 2068 and 2080 cm(-1), respectively, polarized perpendicular to the surface, as well as peaks at 620 and 627 cm(-1), respectively, polarized parallel to the surface. These modes were assigned to the Si-H stretching and bending motions, respectively, resulting from H-termination of surface atoms that did not form Si-C bonds during the ethylation reaction.     

Evidence for epitaxial arrangement and high conformational order of an organic monolayer on Si(111) by sum frequency generation spectroscopy

The structure of an octadecyl monolayer formed on a hydrogen-terminated Si(111) surface in neat octadecene was studied by infrared-visible sum frequency generation (SFG) spectroscopy. The SFG spectra in the CH vibration region were dominated by peaks corresponding to those of the methyl group, confirming that the monolayer is essentially in the all-trans conformation. The shapes of the spectra were strongly dependent on the azimuthal angle, and the strength of the asymmetric vibration mode obtained from the theoretical fitting shows threefold symmetry with respect to the azimuthal angle, suggesting the epitaxial arrangement of the monolayer with the Si(111) substrate. The orientation angle of the methyl group estimated from SFG anisotropy was in good agreement with the theoretical prediction.     

Ultrafast vibrational dynamics and spectroscopy of a siloxane self-assembled monolayer

Time and frequency domain sum-frequency generation (SFG) were combined to study the dynamics and structure of self-assembled monolayers (SAMs) on a fused silica surface. SFG-free induction decay (SFG-FID) of octadecylsilane SAM in the CH stretching region shows a relatively long time scale oscillation that reveals that six vibrational modes are involved in the response of the system. Five of the modes have commonly been used for the fitting of SFG spectra in the CH stretching region, namely the symmetric stretch and Fermi resonance of the methyl group, the antisymmetric stretch of the methyl, as well as the symmetric and antisymmetric stretches of the methylene group. The assignment of the sixth mode to the terminal CH(2) group was confirmed by performing a density function theory calculation. The SFG-FID measures the vibrational dephasing time (T(2)) of each of the modes, including a specific CH(2) group within the SAM, the terminal CH(2), which had never been measured before. The relatively long (～1.3 ps) dephasing of the terminal CH(2) suggests that alkyl monolayer structure is close to that of the liquid condensed phase of Langmuir Blodgett films.     

Molecular dynamics simulation of liquid methanol. II. Unified assignment of infrared, Raman, and sum frequency generation vibrational spectra in methyl C-H stretching region

Vibrational spectra of methyl C-H stretching region are notoriously complicated, and thus a theoretical method of systematic assignment is strongly called for in condensed phase. Here we develop a unified analysis method of the vibrational spectra, such as infrared (IR), polarized and depolarized Raman, and ssp polarized sum frequency generation (SFG), by flexible and polarizable molecular dynamics simulation. The molecular model for methanol has been developed by charge response kernel model to allow for analyzing the methyl C-H stretching vibrations. The complicated spectral structure by the Fermi resonance has been unraveled by empirically shifting potential parameters, which provides clear information on the coupling mechanism. The analysis confirmed that for the IR, polarized Raman, and SFG spectra, two-band structure at about 2830 and 2950 cm(-1) results from the Fermi resonance splitting of the methyl C-H symmetric stretching and bending overtones. In the IR spectrum, the latter, higher-frequency band is overlapped with prominent asymmetric C-H stretching bands. In the depolarized Raman spectrum, the high frequency band at about 2980 cm(-1) is assigned to the asymmetric C-H stretching mode. In the SFG spectrum, the two bands of the splitted symmetric C-H stretching mode have negative amplitudes of imaginary nonlinear susceptibility χ(2), while the higher-frequency band is partly cancelled by positive imaginary components of asymmetric C-H stretching modes.     

Computational study of the adsorption energetics and vibrational wavenumbers of NH3 Adsorbed on the Ni(111) Surface

The structure and stabilities of NH(3) adsorbed on different sites of a Ni(111) surface are compared based on density functional, plane-waves calculations within a periodic framework. The surface has been modeled by 4- and 5-layer slabs with 2 x 2 and 3 x 3 unit cells. Calculated results are in good agreement with available experimental data, confirming the atop adsorption site to be the most favorable, with no preferred azimuthal orientation for the H atoms. For NH(3) adsorbed at the atop site, the one-dimensional potential energy profiles along the N-H and N-Ni bonds and the coupling between adjacent N-H bond oscillators have been calculated and fitted to an analytical expression using an accurate anharmonic potential model. Variational calculations have been performed to obtain frequencies for the N-H and N-Ni stretching vibrations and N-H stretching line widths. The model for calculating line widths has also been tested with CO adsorbed at the hcp hollow of the Ni(111) surface.     

Adsorbate coverages and surface reactivity in methanol oxidation over Cu(110): An in situ photoelectron spectroscopy study

The adsorbate species present during partial oxidation of methanol on a Cu(110) surface have been investigated in the 10(-5) mbar range with in situ x-ray photoelectron spectroscopy and rate measurements. Two reaction intermediates were identified, methoxy with a C 1s binding energy (BE) of 285.4 eV and formate with a C 1s BE of 287.7 eV. The c(2x2) overlayer formed under reaction conditions is assigned to formate. Two states of adsorbed oxygen were found characterized by O 1s BE's of 529.6 and 528.9 eV, respectively. On the inactive surface present at low T around 300-350 K formate dominates while methoxy is almost absent. Ignition of the reaction correlates with a decreasing formate coverage. A large hysteresis of approximately 200 K occurs in T-cycling experiments whose correlation with adsorbate species was studied with varying oxygen and methanol partial pressures. The two branches of the hysteresis differ mainly in the amount of adsorbed oxygen, the methoxy species, and a carbonaceous species. Methoxy covers only a minor part of the catalytic surface reaching at most 20%. Above 650 K the surface is largely adsorbate-free.     

甲醇和甲氧基在Cu(111)表面吸附的第一性原理研究

Adsorption of methanol and methoxy at four selected sites(top,bridge,hcp,fcc)on Cu(111)surface has beeninvestigated by density functional theory method at the generalized gradient approximation(GGA)level.The cal-culation on adsorption energies,geometry and electronic structures,Mulliken charges,and vibrational frequenciesof CH_3OH and CH_3O on clean Cu(111)surface was performed with full-geometry optimization,and compared withthe experimental data.The obtained results are in agreement with available experimental data.The most favoriteadsorption site for methanol on Cu(111)surface is the top site,where C-O axis is tilted to the surface.Moreover,the preferred adsorption site for methoxy on Cu(111)surface is the fcc site,and it adsorbs in an upright geometrywith pseudo-C_(3v) local symmetry.Possible decomposition pathways also have been investigated by transition-statesearching methods.Methoxy radical,CH_3O,was found to be the decomposition intermediate.Methanol can be ad-sorbed on the surface with its oxygen atom directly on a Cu atom,and weakly chemisorbed on Cu(111)surface.Incontrast to methanol,methoxy is strongly chemisorbed to the surface.     

Total-internal-reflection broad-bandwidth sum frequency generation spectroscopy of hexadecanethiol adsorbed on thin gold film deposited on CaF2.

Sum frequency vibrational spectra for hexadecanethiol (HDT) adsorbed on thin gold film deposited on the surface of a CaF2 prism have been measured using total-internal reflection broad-bandwidth sum frequency generation (TIR-BBSFG) spectroscopy. The bands attributed to the CH3 symmetric and asymmetric stretching vibrational modes were observed in the sum frequency vibrational spectra. The orientation of the methyl groups was analyzed using the ratio of the intensities of the two modes. The methyl groups of HDT on the thin gold film were much more randomly orientated than those on Au( 111).     

甲氧基在Rh(111)表面吸附的密度泛函研究

采用密度泛函理论(DFT)的B3LYP方法,以原子簇Rh13(9,4)为模拟表面,在6-31G(d,p)与Lanl2dz基组水平上,对甲氧基在Rh(111)表面的四种吸附位置(fcc、hcp、top、bridge)的吸附模型进行了几何优化、能量计算、Mulliken电荷布局分析以及前线轨道的计算。结果表明,当甲氧基通过氧与金属表面相互作用时,在bridge位的吸附能最大,吸附体系最稳定,在top位转移的电子数最多;吸附于Rh(111)面的过程中C—O键被活化,C—O键的振动频率发生红移。     

A Periodic Density Functional Study on Adsorption Properties of Methoxy on Au（111） Surface

1INTRODUCTION Methoxy(CH3O)has been identified as the first intermediate in the decomposition of methanol on extensive list of clean transition metal surfaces,such as Ni(100)[1],Cu(100)[2,3],Cu(111)[4],Ag(111)[5],Au(110)[6],Pd(111)[7]and Ru(0001)[8].The electronic structure of the metal is a determining factor in OH bond scission.In fact,group IB clean surfaces have shown very low activity towards this reaction,al-though there are reports on low amounts of methoxy formed on clean Cu(…     

Vinyl acetate formation by the reaction of ethylene with acetate species on oxygen-covered Pd(111)

The reaction pathway of vinyl acetate synthesis is scrutinized by reacting gas-phase ethylene (at an effective pressure of 1 x 10-4 Torr) with eta2-acetate species (with a coverage of 0.31 +/- 0.02 monolayer) on a Pd(111)-O(2x2) model catalyst surface in ultrahigh vacuum. It is found that the 1414 cm-1 infrared feature due to the symmetric OCO stretching mode of the acetate species decreases in intensity due to reaction with gas-phase ethylene, while temperature-programmed desorption experiments demonstrate that vinyl acetate is formed. The formation of ethylidyne species is detected when almost all of the acetate species have been removed. The experimental removal kinetics are reproduced by a model in which adsorbed acetates react with an ethylene-derived (possibly ethylene or vinyl) species, where ethylene adsorption is blocked by the acetate present on the surface.     

Reactions of trimethylindium on TiO2 nanoparticles: experimental and computational study

This article reports the results of an experimental and computational study on the reaction of trimethylindium, In(CH3)(3), adsorbed on TiO2 nanoparticle films. Experimentally, Fourier transform infrared (FTIR) spectra have been measured by varying In(CH3)(3) dosing pressure, UV irradiation time in the absence and presence of oxygen, and surface annealing temperature on both "clean" and HO-covered TiO2 nanoparticle films. Computationally, adsorption energies, molecular structures, and vibrational frequencies of possible adsorbates have been predicted by first-principles calculations based on the density functional theory (DFT) and the pseudopotential method. Three important reactions involving CH3 elimination, CH4 elimination, and CH3 migration from the adsorbed trimethylindium have been elucidated in detail. CH(3 migration is the only exothermic process with the lowest reaction barrier. On the basis of experimental and computational results, the two sharpest peaks at 2979 and 2925 cm(-1), detected in the dosage and UV irradiation experiments in the absence of oxygen, are attributable to the asymmetric and symmetric C-H vibrations of methyl groups in In(CH3)3(a) and its derivatives, (H3C)2In(a), H3CIn(a), and H3CO(a). In the UV irradiation experiment in the presence of oxygen, the methyl groups attached to the In atom were quickly oxidized to the methoxy with the C-H vibrations at 2925 and 2822 cm(-1) and to the carboxyl group with vibrations at 2888 cm(-1) (vs(CH)), 1577 cm(-1) (va(OCO)), 1380 cm(-1) (delta(CH)), and 1355 cm(-1) (vs(OCO)). Finally, from the computed energies with vibrational analysis, the adsorbed structure of the carboxyl group was confirmed to involve two oxygen atoms doubly adsorbed on two surface Ti atoms.     

Detection of OH stretching mode of CH3OH chemisorbed on Ni3+ and Ni4+ by infrared photodissociation spectroscopy

Structures of nickel cluster ions adsorbed with methanol, Ni3+ (CH3OH)m (m = 1-3) and Ni4+ (CH3OH)m (m = 1-4) were investigated by using infrared photodissociation (IR-PD) spectroscopy based on a tandem-type mass spectrometer, where they were produced by passing Ni3,4+ through methanol vapor under a multiple collision condition. The IR-PD spectra were measured in the wavenumber region between 3100 and 3900 cm-1. In each IR-PD spectrum, a single peak was observed at a wavenumber lower by approximately 40 cm-1 than that of the OH stretching vibration of a free methanol molecule and was assigned to the OH stretching vibrations of the methanol molecules in Ni3,4+ (CH3OH)m. The photodissociation was analyzed by assuming that Ni3,4+ (CH3OH)m dissociate unimolecularly after the photon energy absorbed by them is statistically distributed among the accessible modes of Ni3,4+ (CH3OH)m. In comparison with the calculations performed by the density functional theory, it is concluded that (1) the oxygen atom of each methanol molecule is bound to one of the nickel atoms in Ni3,4+ (defined as molecular chemisorption), (2) the methanol molecules in Ni3,4+ (CH3OH)m do not form any hydrogen bonds, and (3) the cross section for demethanation [CH4 detachment from Nin+ (CH3OH)] is related to the electron density distribution inside the methanol molecule.