ATR spectra

Around I960, Harrick [63] and Fahrenfort [64] demonstrated that ATR can be used for absorption measurements of thin films (the history of the method was well documented by Mirabella [65]). Since this time, the method has been extensively developed to study film on substrates with various optical properties (dielectrics, semiconductors, and metals) and shapes on bulk samples and on powders. The theory of ATR for thin layers is considered by Harrick [66] and Hansen [67] and has been reviewed in detail [68-72]. In this section, the experimental conditions necessary for the measurement of ATR in ultrathin films will be discussed in particular the effects of the materials for the IRE substrate as well as of the angle of incidence will be considered. This will allow the capabilities of the ATR method for a particular system to be estimated and, to a certain 

OPTIMUM CONDITIONS FOR RECORDING INFRARED SPECTRA OF ULTRATHIN FILMS 

The study of a layer buried at the interface between two different media is one of the most complex problems in applied spectroscopy. The difficulties arise from a masking of the analyzed material by the adjacent media and also from the fact that, as a rule, the thickness of such a layer is in the nanometer range. However, if at least one of the media is transparent at the IR absorption frequencies of the layer, then it is possible in principle to investigate the layer. 

One frequently examined interface is the solid-liquid interface, where the solid phase may be a dielectric, a semiconductor, or a metal. Species located at these interfaces are of primary importance in electtochemisny and in chemistry of surface-active substances (surfactants). Another common type of interface is the solid-solid interface, specifically dielectric-dielectric, dielectric-semiconductor, dielectric-metal, semiconductor-semiconductor, semiconductor-metal, and metal-metal interfaces. These structures have an extremely important role in such areas as microelectronics and the chemistry of composites. Furthermore, positioning an ultrathin film at the interface of two media, one can substantially increase surface sensitivity of all IR spectroscopic methods. 

When both bordering media are transparent, one can apply transmission spectroscopy in polarized radiation (Section 2.1) or, when there is a difference in the refractive indices of these media, the ATR method and IRRAS. For each type of solid-solid interface, except for the metal-metal interface, one can study the layers in the contact zone by IRRAS or ATR in the transparent spectral range of one of the media in the system. To choose the technique with which to investigate dielectric (semiconductor)-liquid, dielectric (semiconductor)-semiconductor, and dielectric-dielectric interfaces, several factors must be considered, including the region of transparency of the media under study and the relationship between their refractive indices. If the medium with the largest refractive index is the most transparent, one should use the ATR method otherwise IRRAS is more appropriate. 

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ATR spectra are similar, but not identical, to those obtained by measuring the transmission of radiation. 

Fig. 2 shows one application of ATR depth profiling. In this case, ATR spectra were obtained as a function of angle of incidence from a polymethylmethacrylate (PMMA) film of thickness 0.5 p.m that was deposited onto a germanium hemi-cylinder [4]. The solid line represents the ATR spectrum of PMMA while the squares represent the film thickness that was recovered from the infrared spectra using four different bands. It can be observed that the recovered film thickness was very close to the measured thickness. 

Papirer et al. used ATR, XPS, and SIMS to determine the effect of flame treatment on adhesion of polyethylene and polypropylene to styrene/butadiene (SBR) rubber [8]. Each flame treatment consisted of a 75-ms pass over a circular burner. The distance between the upper flame front and the polymer was kept fixed al 8 mm. A band was observed near 1720 cm" in the ATR spectra and assigned to carbonyl groups this band increased in intensity as the number of flame... 

Figure 14 Surface IR spectra of etched LDPE-ATR spectra recorded with a KRS-5 reflection element, at 45° angle of incidence. Times refer to chronic acid itch duration. (From Ref. 76.)...

FIG. 4 FTIR-ATR spectra of ethanol on a silicon oxide surface in ethanol-cyclohexane binary liquids at various ethanol concentrations 0.0, 0.1, 0.3, 0.5, 1.0, and 2.0 mol%. 

At higher ethanol concentrations, ATR spectra should contain the contribution from bnUc species, becanse of the long penetration depth of the evanescent wave, 250 nm. To examine the bulk contribution, the integrated peak intensities of polymer OH peaks of transmission (Ats) and ATR (Aatr) spectra are plotted as a function of the ethanol concentration in Figure 5. The former monitors clnster formation in the bulk liquid, and the latter contains contributions of clusters both on the snrface and in the bulk. A sharp increase is seen in A tr... 

Fluorescence emission spectra were recorded with Perkin-Elmer LS Luminescence Spectrometer, whereas IR (transmission and ATR spectra) with Perkin-Elmer 1710 IR Fourier Transform Spectrometer. 

The carbonyl index obtained from transmission and ATR spectra of PP films UV-irradiated (L2) in ozone shows that the concentration of carbonyl groups at the surface is 8 times higher than in a bulk. 

ATR spectra show that oxygen groups are located at the surface to the depth of 0.6 /l m. Kinetic curves of carbonyl groups formation in different experimental conditions are shown in Fig.4. 

Figure 4. Kinetics of carbonyl group formation at 1714 cm- in polypropylene samples (0) and ( ) ozone and UV light (L2) (4) and (A) ozone only (0) and ( ) ATR spectra (A) and (A) transmission spectra.

Four polarized ATR spectra can be recorded to characterize the three-dimensional (3D) orientation of a sample, p- and s-polarized spectra are recorded with the sample clamped with its Z- and X-axes sequentially aligned perpendicular to the incidence plane (that is, parallel to the s-polarized electric field). The absorbance measured in these different configurations is related to the anisotropic absorption indices of the sample, kj, as... 

Figure 6 Chain orientation along the hoop, length, and thickness directions measured at different positions along the length of a standard PET bottle. Polarized ATR spectra were recorded for (a) the outer and (b) the inner surfaces of the bottle, respectively. Reproduced with permission from Smith et al. [35]. Copyright Elsevier 2006.

The corresponding carbonyl absorption bands should appear at 1,790 and 1,740 cm-1, but did not show in the ATR spectra. 

The ATR technique is a commonly used infrared internal reflection sampling technique. It samples only the surface layer in contact with the ATR element the sampling depth probed is typically of the order of 0.3-3 pm [1]. Unless software corrected, compared with a transmission spectrum, the relative intensity of bands within an ATR spectrum increase in intensity with decreasing wavenumber. Several FTIR instrument companies now supply "ATR-correction" software developed to correct for the different relative intensities of bands observed between ATR and transmission spectra, so that ATR spectra can be more easily compared to and searched against transmission spectra. 

Caution should be applied in interpreting ATR spectra because this is a surface technique that only interrogates structure within a distance of a few... 

Fourier transform infrared (FTIR)-attenuated total reflection (ATR) spectra have been measured for LB films of stearic acid deposited on a germanium plate with... 

Fig. 22. FTIR-ATR spectra on silicon taken after deposition of one, three, and six monolayers C195]...

Attenuated total reflection (ATR) is the most common reflectance measurement modahty. ATR spectra cannot be compared to absorption spectra. While the same peaks are observed, their relative intensities differ considerably. The absorbances depend on the angle of incidence, not on sample thickness, since the radiation penetrates only a few micrometers into the sample. The major advantage of ATR spectroscopy is ease of use with a wide variety of solid samples. The spectra are readily obtainable with a minimum of preparation Samples are simply pressed against the dense ATR crystal. Plastics, rubbers, packaging materials, pastes, powders, solids, and dosage forms such as tablets can all be handled directly in a similar way. 

Figure 2 compares ATR spectra of the irradiated and unirradiated sides of a cellulose triacetate film after 24-hour radiation at 253.7 min vacuum. The ATR spectrum of a control cellulose triacetate film, which is identical with that of the unirradiated side in Figure 2, is given in Figure 3. Figure 4 shows the change of infrared absorptions of a cellulose film cast on a NaCl plate upon irradiation at 253.7 min vaccum. The spectra were recorded at 90°C. An increase in OH (3 microns) and a decrease in carbonyl (5.7 microns) absorption were noted. 

ATR spectra of stratified isotropic media can be calculated by use of the above formalism (18). This formalism has also been generalized for anisotropic layers (19), which makes it possible to carry out simulations of uniaxial layers (zZy = and... 

Fig. 6. Calculated ATR spectra (angle of incidence 45°) for a monolayer adsorbate (thickness tZ3 = 3 A) on a 20-nm-thick metal film in contact with a solvent as a function of the complex refractive index of the metal film. Sohd line parallel polarized light dotted line perpendicular-polarized light. The appropriate complex refractive index n2 is given at the top of each spectrum. The vertical bars indicate the scale for the absorbance, which is different for each spectrum. Parameters ni = 4.01 (Ge), n4 = 1.4 (organic solvent), rfs = 3 A, He = 1.6, S = 280000cm , Vo = 2000cm , y = 60cm . The parameters correspond to adsorbed CO. The calculations were performed by using the formalism proposed by Hansen (76), and the results are given in terms of absorbance A = —logio(7 /7 o), where 77 is the reflectivity of the system Ge/Pt/ adsorbate/solvent and Rg is the reflectivity of the system Ge/Pt/solvent (7S).

Propanol oxidation under mild conditions takes place with high selectivity. No products other than acetone were observed in the ATR spectra recorded in situ. The situation is more complex for the oxidation of primary alcohols such as ethanol. The first oxidation step produces acetaldehyde, which is prone to further reactions, as is apparent in the ATR spectra. Figure 20, left, shows ATR spectra recorded in situ during ethanol oxidation. Figure 20, right, shows some signals as a function of time. The experiment was performed in a manner similar to that of the one... 

Fig. 21. ATR spectra (a) recorded during flow of a solution of 0.056 niol/L cinnamyl alcohol in argon-saturated toluene over a Pd/ALOs catalyst. The time between the first (bottom) and last (top) spectrum was 17 min. (b) Spectra recorded during subsequent flow of an identical solution—except that it was saturated with air—over the same sample. The time between the first (top) and last (bottom) spectrum was 2 min. (c) Spectra recorded during subsequent flow of dissolved CO (0.5% in argon) over the same catalyst. Time between first (bottom) and last (top) spectrum was 10 min. The background for the spectra shown in (a) and (b) was recorded before admitting the alcohol to the sample. For the spectra in (c), the background was recorded before admitting CO (46).

Fig. 22. Demodulated ATR spectra representing adsorption/desorption of TBHP at room temperature on Ti-Si aerogels with various Ti contents and Si reference sample (OTi) lOTi (10% T1O2). 20Ti (20% TiO2). The TBHP concentration in cyclohexane was modulated between 0 and 3mmol/L (JO).

Fig. 23. ATR spectra (left) of a 5% Pd TiO catalyst. The powder catalyst was deposited on a ZnSe IRE. Toluene saturated with various gases then flowed over the sample. At the beginning the palladium was oxidized. First, toluene saturated with argon flowed over the catalyst (bottom left, bottom spectrum). Then the flow was switched to hydrogen-saturated toluene, which led to reduction of the palladium (bottom, left). Afterwards, the flow was switched to oxygen-saturated toluene (top. left). The right graph shows the absorbance at 1700 cm" as a function of time during the treatments 49).

Fig. 25. ATR spectra recorded during epoxidation of cyclohexene catalyzed by a Ti-Si aerogel with TBHP as the oxidant under the influence of forced modulation of the cyclohexene concentration (a) time-resolved spectra (reference recorded before modulation) (b) difference spectra obtained by subtracting one (arbitrarily chosen) spectrum (c) phase-resolved (demodulated) spectra. The data set for the spectra in (a)-(c) is the same (SO).

Adsorption of CD on palladium showed distinct differences from that on platinum, which is already apparent from the spectra compared in Fig. 33. The ATR spectra did not show any indication of the presence of a species covalently bound by Pt C ci-bond (a-hydrogen abstraction, species 2 in the case of platinum). The strongly adsorbed 7i-bonded flat species (1 and T) were found to be more favored on platinum, whereas on palladium the tilted, nitrogen lone-pair-bound species (3 ) were dominant. Furthermore, the comparative study showed that CD is more strongly adsorbed on platinum than on palladium, indicated by the more prominent shift of the vibrational modes with respect to the free molecule on platinum than on palladium. 

Fig. 35. Effect of phase behavior on palladium-catalyzed oxidation of benzyl alcohol to benzaldehyde in supercritical CO2 characterized by transmission- and ATR-IR spectroscopy combined with video monitoring of the reaction mixture (102). The figure at the top shows the pressure dependence of the reaction rate. Note the strong increase of the oxidation rate between 140 and 150 bar. The in situ ATR spectra (middle) taken at 145 and 150 bar, respectively, indicate that a change from a biphasic (region A) to a monophasic (B) reaction mixture occurred in the catalyst surface region in this pressure range. This change in the phase behavior was corroborated by the simultaneous video monitoring, as shown at the bottom of the figure.

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