Infrared Reflection Absorption Spectroscopy Data Research Articles

Configuration of ammonia on Cu{311}: Infrared spectroscopy and first-principles theory.

We describe Reflection Absorption Infrared Spectroscopy (RAIRS) and first-principles Density Functional Theory (DFT) studies of ammonia adsorption on the Cu{311} surface. Our experimental results indicate an upright chemisorbed species at low coverages, with at least one additional species accompanying this at higher coverages. Our high-coverage RAIRS data cannot be fully explained by DFT models containing only ammonia or its dissociation products, even allowing for molecular tilt and/or the formation of a bilayer. We therefore also consider urea and formamide as possible products of surface reaction with residual carbon monoxide, but these species are again not fully compatible with our observed spectra. The overlayer composition at high coverages remains mysterious.

Vibrational frequencies of CO bound to all three low-index cerium oxide surfaces: A consistent theoretical description of vacancy-induced changes using density functional theory.

The facet-dependent adsorption of CO on oxidized and reduced CeO2 single crystal surfaces is reviewed, with emphasis on the effect of CO coverage and the ability of state-of-the-art quantum-mechanical methods to provide reliable energies and an accurate description of the IR vibrational frequency of CO. Comparison with detailed, high-resolution experimental infrared reflection absorption spectroscopy data obtained for single crystal samples allows the assignment of the different CO vibrational bands observed on all three low-index ceria surfaces. Good agreement is achieved with the hybrid density functional theory approach with the HSE06 functional and with saturation coverage. It is shown that CO is very sensitive to the structure of cerium oxide surfaces and to the presence of oxygen vacancies. The combined theoretical-experimental approach offers new opportunities for a better characterization of ceria nanoparticles and for unraveling changes occurring during reactions involving CO at higher pressures.

Infrared Activities of Adsorbed Species on Metal Surfaces: The Puzzle of Adsorbed Methyl (CH3).

Reflection-absorption infrared spectroscopy (RAIRS) is widely used to identify molecular adsorbates on metals during surface chemical reactions, but the interpretation of RAIRS data can be difficult with experiment alone. Here, we reveal from first-principles calculations the origin of the contrasting RAIRS spectra of methyl adsorbed on Pt(111) and Ni(111). We find that the dynamic dipole associated with the symmetric C-H stretch vibration of CH3 along surface normal is significant on Pt(111) but negligibly small on Ni(111), explaining the strong IR activity in the former and the absence of any RAIRS peaks in the latter. This difference is correlated to different charge transfer patterns between metals and the adsorbate, which are determined by the different preferred adsorption sites of methyl on the two surfaces. This work highlights the need of electronic structure calculations in interpreting RAIRS spectra of adsorbates on metal surfaces.

Using Surface Science Techniques to Investigate the Interaction of Acetonitrile with Dust Grain Analogue Surfaces

Surface science methodologies, such as reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD), are ideally suited to studying the interaction of molecules with model astrophysical surfaces. Here we describe the use of RAIRS and TPD to investigate the adsorption, interactions and thermal processing of acetonitrile and water containing model ices grown under astrophysical conditions on a graphitic dust grain analogue surface. Experiments show that acetonitrile physisorbs on the graphitic surface at all exposures. At the lowest coverages, repulsions between the molecules lead to a decreasing desorption energy with increasing coverage. Analysis of TPD data gives monolayer desorption energies ranging from 28.8–39.2 kJ mol−1 and an average multilayer desorption energy of 43.8 kJ mol−1. When acetonitrile is adsorbed in the presence of water ice, the desorption energy of monolayer acetonitrile shows evidence of desorption with a wide range of energies. An estimate of the desorption energy of acetonitrile from crystalline ice (CI) shows that it is increased to ~37 kJ mol−1 at the lowest exposures of acetonitrile. Amorphous water ice also traps acetonitrile on the graphite surface past its natural desorption temperature, leading to volcano and co-desorption. RAIRS data show that the C≡N vibration shifts, indicative of an interaction between the acetonitrile and the water ice surface.

Crystallization of poly(ε-caprolactone) at the air-water interface studied by IRRAS and GI-WAXS

Poly (ε-caprolactone) (PCL) dissolved in chloroform was spread on the water surface of a Langmuir trough and after solvent evaporation, the surface pressure vs area (π-A) Langmuir isotherm was recorded. An extended plateau region indicates a 1st order phase transition related to PCL crystallization as also confirmed by Brewster angle microscopy (BAM). Infrared reflection absorption spectroscopy (IRRAS) and grazing incidence wide angle X-ray scattering (GI-WAXS) reveal details of the PCL crystallization process on a molecular level. The IRRAS and GI-WAXS experiments confirm that crystallization occurs at the plateau region of the π-A Langmuir isotherm. GI-WAXS also confirms that the polymer chains in the crystals are tilted by ≈ 21° with respect to the water surface normal direction. This value is consistent with IRRAS data at a maximum compression state of ≈ 2 Å2.

Mixed Self-Assembled Monolayers with Terminal Deuterated Anchors: Characterization and Probing of Model Lipid Membrane Formation.

We describe herein a series of self-assembled monolayers (SAMs) on gold designed for adjustable tethering of model lipid membrane phases. The SAMs consist of deuterated aliphatic anchors, HS(CH2)15CONH(CH2CH2O)6CH2CONH-X, where X is either -(CD2)7CD3 or -(CD2)15CD3, dispersed in a stable matrix of protein-repellent molecules, HS(CH2)15CONHCH2CH2OH. The mixed SAMs with variable surface densities of the anchors are thoroughly characterized before and after adsorption of phospholipids by means of ellipsometry, contact angle goniometry, and infrared reflection-absorption spectroscopy (IRRAS). In all cases, the bottom portions of the mixed SAMs (i.e., the h-alkyl thiol segments of the molecules) are arranged in a highly ordered all-trans conformation stabilized by a network of lateral hydrogen bonds. The terminal portions of the anchors (the oligo(ethylene glycol) spacer and deuterated alkyl segments, respectively), however, possess less ordered conformations in the mixed composition regime. For the SAMs containing the longer anchors (-(CD2)15CD3), the contact angle and infrared data point toward partial phase segregation. These findings are in excellent agreement with molecular dynamics simulations by Schulze and Stein. Upon analysis in air, the IRRAS data also indicate strong interaction between the adsorbed phospholipid molecules and the d-alkyl tails of the mixed SAM constituents. In such assemblies are the alkyl tails of the phospholipids aligned perpendicularly with respect to the supporting substrate, regardless of the anchor length. We also probed the in situ formation of a tethered bilayer lipid membrane (tBLM) via fusion of small unilamellar vesicles (SUVs) on the characterized SAMs using a quartz crystal microbalance with dissipation monitoring. Our experiments show that SUVs fuse efficiently of the two mixed SAMs with and without a pre-adsorbed lipid layer. Owing to the defined molecular composition and phase behavior, our SAM platform is attractive for detailed studies of tBLM formation and cell mimetic applications.

Rendering Photoreactivity to Ceria: The Role of Defects.

The photoreactivity of ceria, a photochemically inert oxide with a large band gap, can be increased to competitive values by introducing defects. This previously unexplained phenomenon has been investigated by monitoring the UV-induced decomposition of N2 O on well-defined single crystals of ceria by using infrared reflection-absorption spectroscopy (IRRAS). The IRRAS data, in conjunction with theory, provide direct evidence that reducing the ceria(110) surface yields high photoreactivity. No such effects are seen on the (111) surface. The low-temperature photodecomposition of N2 O occurs at surface O vacancies on the (110) surface, where the electron-rich cerium cations with a significantly lowered coordination number cause a local lowering of the huge band gap (ca. 6 eV). The quantum efficiency of strongly reduced ceria(110) surfaces in the photodecomposition of N2 O amounts to 0.03 %, and is thus comparable to that reported for the photooxidation of CO on rutile TiO2 (110).

Structural Changes in Acetophenone Fluid Films as a Function of Nanoscale Thickness

We report experimental observations of a developing fluid/solid interface by examining acetophenone films of varying thicknesses, supported on solid silver substrates. A dynamic wetting technique provides experimental control of fluid film thickness, as a function of rotational velocity. Ellipsometry and infrared reflection absorption spectroscopy data are analyzed to provide absolute film thickness and details of the changing chemical environment for varying film thickness. These data are compared to theoretical models that predict fluid film thicknesses, based on physical-chemical properties of the acetophenone/silver pair. As the velocity of the substrate is varied from 0.003 cm s-1 to 1.872 cm s-1, the fluid film's thickness changes from a ca. 200 nm to 2 μm. This increase in film thickness with increasing velocity follows a Landau trend, which is linear with respect to velocity2/3. Our data also show clear evidence of molecular orientation changes, as a function of film thickness, which occur as the thinner films are increasingly comprised of acetophenone molecules within a confined, interfacial environment. The spectral changes for the thinnest fluid films (<100 nm) are shown to exhibit features similar to transmission Fourier transform infrared (FTIR) data of frozen acetophenone, suggesting that these films are highly ordered, as a result of their nanometer-scale confinement.

Electropolymerisation of the natural monomer riboflavin and its characterisation

Electropolymerisation of riboflavin (Rf) was performed in different media in order to optimise the best conditions for the creation of a stable and electroactive polymerised riboflavin (PRf) film. Electropolymerisation was performed using cyclic voltammetry (CV) in solutions with various pH in different potential windows. Different electrode substrates were used for PRf deposition: pyrolytic graphite (PG) and highly oriented pyrolytic graphite (HOPG).The obtained films were characterised spectroscopically, microscopically and electrochemically applying Raman spectroscopy, reflection-absorption infrared spectroscopy (RAIRS), atomic force microscopy (AFM) and electrochemical methods such as cyclic voltammetry (CV), square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) in different media. The best film was obtained when PRf was electropolymerised from 0.1molL−1 PBS, pH 7.0. Raman and RAIRS data have indicated involvement of isoalloxazine ring III in the polymerisation process. The best response of such an electrode was in the same buffer solution. This electrode will be further applied in (bio)sensor construction.

Structure and function of the membrane anchoring self-assembled monolayers.

Structure of the self-assembled monolayers (SAMs) used to anchor phospholipid bilayers to surfaces affects the functional properties of the tethered bilayer membranes (tBLMs). SAMs of the same surface composition differing in the lateral distribution of the anchor molecule give rise to tBLMs of profoundly different defectiveness with residual conductance spanning 3 orders of magnitude. SAMs composed of anchors containing saturated alkyl chains, upon exposure to water (72 h), reconstruct to tightly packed clusters as deduced from reflection absorption infrared spectroscopy data and directly visualized by atomic force microscopy. The rearrangement into clusters results in an inability to establish highly insulating tBLMs on the same anchor layer. Unexpectedly, we also found that nanometer scale smooth gold film surfaces, populated predominantly with (111) facets, exhibit poor performance from the standpoint of the defectiveness of the anchored phospholipid bilayers, while corrugated (110) dominant surfaces produced SAMs with superior tethering quality. Although the detailed mechanism of cluster formation remains to be clarified, it appears that smooth surfaces favor lateral translocation of the molecular anchors, resulting in changes in functional properties of the SAMs. This work unequivocally establishes that conditions that favor cluster formation of the anchoring molecules in tBLM formation must be identified and avoided for the functional use of tBLMs in biomedical and diagnostic applications.

Near-Unity Reaction Probability in Olefin Hydrogenation Promoted by Heterogeneous Metal Catalysts.

The kinetics of the hydrogenation of ethylene on platinum surfaces was studied by using high-flux effusive molecular beams and reflection-absorption infrared spectroscopy (RAIRS). It was determined that steady-state ethylene conversion with probabilities close to unity could be achieved by using beams with ethylene fluxes equivalent to pressures in the mTorr range and high (≥100) H2:C2H4 ratios. The RAIRS data suggest that the high reaction probability is possible because such conditions lead to the removal of most of the ethylidyne layer known to form during catalysis. The observations from this study are contrasted with those under vacuum, where catalytic behavior is not sustainable, and with catalysis under more realistic atmospheric pressures, where reaction probabilities are estimated to be much lower (≤1 × 10(-5)).

Spectroscopic and structural characterization of the formation of olefin metathesis initiating sites on unsupported β-Mo2C

Reflection absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS) and thermal desorption spectrometry (TDS) measurements were used to follow the preparation of surface alkylidenes on β-Mo2C. Treating the molybdenum carbide surface with acrolein and acetone respectively formed propylidiene and 2-propylidene groups. The propylidiene functionalised surface was found to be active for a ring opening insertion reaction with 1,3,5,7-cyclooctatetraene (COT). Helium ion microscopy (HIM) was used to determine the topological features of the surface. The surface region giving the XPS and RAIRS data is composed of large grains of several microns in diameter. In turn, the surface of these grains shows finer structure in the form of small grains suggesting that the olefin metathesis reactivity is related to the morphology of the sample.

Electron beam irradiation of dimethyl-(acetylacetonate) gold(III) adsorbed onto solid substrates

Electron beam induced deposition of organometallic precursors has emerged as an effective and versatile method for creating two-dimensional and three-dimensional metal-containing nanostructures. However, to improve the properties and optimize the chemical composition of nanostructures deposited in this way, the electron stimulated decomposition of the organometallic precursors must be better understood. To address this issue, we have employed an ultrahigh vacuum-surface science approach to study the electron induced reactions of dimethyl-(acetylacetonate) gold(III) [AuIII(acac)Me2] adsorbed onto solid substrates. Using thin molecular films adsorbed onto cooled substrates, surface reactions, reaction kinetics, and gas phase products were studied in the incident energy regime between 40 and 1500 eV using a combination of x-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RAIRS), and mass spectrometry (MS). XPS and RAIRS data indicate that electron irradiation of AuIII(acac)Me2 is accompanied by the reduction in AuIII to a metallic Au0 species embedded in a dehydrogenated carbon matrix, while MS reveals the concomitant evolution of methane, ethane, carbon monoxide, and hydrogen. The electron stimulated decomposition of AuIII(acac)Me2 is first-order with respect to the surface coverage of the organometallic precursor, and exhibits a rate constant that is proportional to the electron flux. At an incident electron energy of 520 eV, the total reaction cross section was ≈3.6×10−16 cm2. As a function of the incident electron energy, the maximum deposition yield was observed at ≈175 eV. The structure of discrete Au-containing deposits formed at room temperature by rastering an electron beam across a highly ordered pyrolytic graphite substrate in the presence of a constant partial pressure of AuIII(acac)Me2 was also investigated by atomic force microscopy.

Self-assembled monolayers of benzylmercaptan and para-cyanobenzylmercaptan on gold: surface infrared spectroscopic characterization

Self-assembled monolayers (SAMs) on gold substrates were prepared from benzylmercaptan (BM) and para-cyanobenzylmercaptan (pCBM), and the resulting surfaces were investigated using conventional infrared reflection-absorption spectroscopy (IRRAS) as well as polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). IRRAS data are analyzed by comparison with transmission IR spectra and theoretical (DFT) simulations. The spectroscopic results indicate the presence of well-ordered monolayers of BM and pCBM with an orientation perpendicular to the surface. IRRAS and PM-IRRAS data are compared to each other and the respective merits of both methods are discussed.

Oligo(ethylene oxide) Self-Assembled Monolayers with Self-Limiting Packing Densities for the Inhibition of Nonspecific Protein Adsorption

We have created a molecule that forms self-assembled monolayers (SAMs) on Au, possessing the characteristics for inhibition of nonspecific protein adsorption, i.e., uniformly distributed, loosely packed, conformationally mobile, hydrated ethylene oxide (EO) chains of near optimal packing densities. SAMs of the bipodal molecule CH(3)O(CH(2)CH(2)O)(5)CH(2)CON(CH(2)CH(2)CH(2)SCOCH(3))(2) [N,N-(bis-3'-thioacetylpropyl)-3,6,9,12,15,18-hexaoxanonadecanamide (BTHA)] on polycrystalline Au are described. Spectroscopic ellipsometry (SE) and reflection-absorption infrared spectroscopy data indicate that BTHA SAM thickness and EO chain disorder closely match that of partially formed monothio-(EO)(5-6)CH(3) SAMs when they exhibit maximum inhibition of protein adsorption. However, in contrast to the monothio-(EO)(5-6)CH(3) SAMs, the BTHA SAM thickness and EO chain disorder remain constant in the presence of unbound molecules because of the structurally imposed upper limit of one EO chain per two Au occupancy sites. SE data indicate high resistance to protein adsorption for bovine serum albumin, fibrinogen, and a mixture of the two, suggesting uniform EO surface coverage on a length scale at least equal to the smallest dimension of these proteins.

1-(1-Naphthyl)Ethylamine Adsorption on Platinum Surfaces: On the Mechanism of Chiral Modification in Catalysis

The adsorption of 1-(1-naphthyl)ethylamine (NEA) on platinum surfaces has been characterized by reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) both under ultrahigh vacuum and in situ from liquid solutions. The main focus of this study was to identify the mechanism by which single enantiomers of NEA bestow chirality on the platinum surface. Evidence was acquired for both of the prevailing explanations available in the literature for the NEA behavior: formation of supramolecular chiral templates and complexation of individual modifiers with the reactant. Indeed, TPD titrations of NEA-modified Pt(111) using propylene oxide (PO) as a chiral probe point to a relative enhancement in the adsorption of one enantiomer over the other at intermediate NEA coverages, which is the behavior expected from the templating mechanism. However, a difference in adsorption energetics was also observed. Both the TPD and RAIRS data suggest possible interactions between the adsorbed NEA and adjacent PO that differ according to the relative chirality of the two compounds. The NEA uptake from solution displays additional enantioselectivity, in particular when the adsorption of enantiopure compounds is compared with that of racemic mixtures, and also points to possible adsorption changes induced by ethyl pyruvate, a common reactant in chiral hydrogenation processes.

Activation of double and triple bonds in C6 unsaturated hydrocarbons by the Ru(001) surface: an overview

AbstractThe decomposition patterns of hexene and hexyne isomers on clean Ru(001), under ultra‐high vacuum, are overviewed and correlated with functionality and isomerism. The key surface intermediates and products were identified by reflection absorption infrared spectroscopy (RAIRS). The results evidence the relevant role of the unsaturation position over its nature. The effects of steric hindrance and geometrical isomerism are apparent only in the inhibition of some decomposition pathways. The RAIRS data show that 1‐hexene chemisorbs at low temperature (∼90 K) and coverage as a di‐σ complex, whereas 1‐hexyne forms a di‐σ/π complex. By thermal activation, both these species dehydrogenate (in C1), yielding hexylidyne [µ3‐η1‐C(CH2)4CH3], which further decomposes (at 280–290 K) into surface metallocycles, [Ru3C(CH2)4CH2‐Ru] and [Ru3C(CH2)4CRu3]. Eventually, at 300 K, complete CC bond breaking occurs, yielding just adsorbed methylidyne [µ3‐η1‐CH]. The hexene and hexyne isomers with the unsaturation between secondary carbons may follow two surface‐assisted decomposition mechanisms. At low temperatures, they adsorb as the corresponding alkyne di‐σ/π complex, which implies a rehybridization of the sp2 (or sp) carbons with reduction of the bond order, plus, for alkenes, dehydrogenation at the same carbons. These complexes decompose by breaking the CC bonds adjacent to the surface anchors: C1C2 and C3C4 in the case of the 2‐isomer, yielding methylidyne, ethyne di‐σ/π complex [µ3‐η2‐CHCH] and propylidyne [µ3‐η1‐CCH2CH3], and C2C3 and C4C5 in the 3‐isomer, with the formation of the ethyne di‐σ/π complex and ethylidyne [µ3‐η1‐CCH3]. The second decomposition path occurs upon direct adsorption at the reaction temperatures. It involves the scission of the multiple bond, with the formation of shorter chain alkylidynes: propylidyne (for 3‐hexyne and Z‐3‐hexene), ethylidyne and butylidyne [µ3‐η1‐C(CH2)2CH3] (for 2‐hexyne). The reactivity of Z‐2‐hexene revealed to be different, since no evidence was found for the second decomposition path. This was ascribed to a reduced accessibility of the double bond to the surface, due to a steric hindrance effect of the alkyl chain. The influence of geometrical isomerism was particularly clear in the decomposition of E‐3‐hexene, which has a remarkable stability. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Application of band-target entropy minimization (BTEM) and residual spectral analysis to in situ reflection–absorption infrared spectroscopy (RAIRS) data from surface chemistry studies

The band-target entropy minimization (BTEM) curve resolution technique has been used to analyze in situ reflection–absorption infrared spectroscopy (RAIRS) data of CO chemisorption on Ni(1 1 1) single crystal surfaces. The bilinearity assumption for pRAIRS data, that is, negative logarithm to the base 10 of raw reflectance RAIRS data, was found to be sufficiently valid for the test data. A total of 11 real pure component pRAIRS spectra were elucidated via BTEM in tandem with an iterative residual spectral data analysis. Furthermore, 2 abstract pure component right singular vectors were found to account for all the pRAIRS non-linearities, baseline drifts and other spectral noise. In total, 100.2% of the pRAIRS signals were accounted for by these 13 spectral components. The 11 real pure component pRAIRS spectra and their corresponding relative concentration kinetic sequences correlate with 6 well-known adsorbed CO domain structures. Moreover, amongst the BTEM resolved spectra were five new bands that were not previously observed using conventional visual identification methods adopted by surface chemists. These new bands engendered new understanding to the mechanism of CO chemisorption on Ni(1 1 1). The combination of BTEM with residual spectral analysis was thus demonstrated to be efficacious for curve resolution of in situ RAIRS data obtained from surface chemistry studies.

Comparative studies of O2and N2in pure, mixed and layered CO ices

We present laboratory data on pure, layered and mixed CO and O2 ices relevant for understanding the absence of gaseous O2 in space. Experiments have been performed on interstellar ice analogues under ultra high vacuum conditions by molecular deposition at 14 K on a gold surface. A combination of reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) is used to derive spectroscopic and thermodynamic properties of the ices. It is found that for pure ices the desorption energy of O2 is larger than that of CO and N2. TPD spectra reveal similar desorption processes for all examined CO-O2 ice morphologies. The different amorphous and crystalline components of pure 13CO RAIR spectra are analyzed. The RAIRS data of the 13CO stretching vibration show a significant difference between layered and mixed CO-O2 ices: layered CO-O2 ices resemble that of pure 13CO whereas the spectra of mixed ices are broadened. The experiments also show that the sticking probabilities of O2 on CO and O2 on O2 are close to unity. These new results are compared with recently analyzed data of CO-N2 ices. The differences in the TPD and RAIRS spectra of the CO-N2 and CO-O2 ice systems are explained by differences in quadrupole intermolecular interactions and by different crystallization processes of these ices.

A quantitative comparison between reflection absorption infrared and sum-frequency generation spectroscopy

Experimental and theoretical investigations of the C–O stretch vibration of CO on a stepped platinum surface (Pt(5 3 3)) reveal distinct differences between spectra obtained with reflection absorption infrared spectroscopy (RAIRS) and sum-frequency generation (SFG). The SFG data reveal large variations in the C–O stretch intensity with increasing coverage, whereas the RAIRS data show a continuous increase. The data can be quantitatively reproduced using a model that accounts for the different dependence of SFG and RAIRS on the dipole–dipole coupling.