RAIR spectra

RAIRS spectra contain absorption band structures related to electronic transitions and vibrations of the bulk, the surface, or adsorbed molecules. In reflectance spectroscopy the ahsorhance is usually determined hy calculating -log(Rs/Ro), where Rs represents the reflectance from the adsorhate-covered substrate and Rq is the reflectance from the bare substrate. For thin films with strong dipole oscillators, the Berre-man effect, which can lead to an additional feature in the reflectance spectrum, must also be considered (Sect. 4.9 Ellipsometry). The frequencies, intensities, full widths at half maximum, and band line-shapes in the absorption spectrum yield information about adsorption states, chemical environment, ordering effects, and vibrational coupling. 

Fig. 4.55. Experimental and calculated (dashed line) RAIR-spectra for poly(methyl methacrylate) films 3270 + 100 nm, 362 30 nm, and 78 + 15 nm thick (a) p-polarized light incident at 60° (b) s-polarized light incident at 60°, after [4.266].

If i = i — ik] and H2 = ns — are known as a function of wavelength, Eq. 12 can be used to calculate the entire RAIR spectrum of a surface film. Since transmission infrared spectroscopy mostly measures k, differences between transmission and RAIR spectra can be identified. Fig. 6 shows a spectrum that was synthesized assuming two Lorentzian-shaped absorption bands of the same intensity but separated by 25 cm. The corresponding spectrum of i values was calculated from the k spectrum using the Kramers-Kronig transformation and is also shown in Fig. 6. The RAIR spectrum was calculated from the ti and k spectra using Eqs. 11 and 12 and is shown in Fig. 7. 

The relative intensities of the bands in the transmission and RAIR spectra were used to determine the orientation of the long axis of the 4-MPP molecules with respect to the normal to the gold surface. It was found that this tilt angle was about 21°, a value that was similar to that obtained from molecular dynamics simulations [11]. 

Tsai et al. have used RAIR extensively in investigations of plasma polymerized acetylene films as primers for rubber-to-metal bonding [12]. Fig. 12 shows RAIR spectra of films having a thickness between about 5.7 and 90.0 nm. A strong band... 

Polished steel substrates primed with plasma polymerized acetylene films were immersed into a stirred mixture of these materials at a temperature of 155 5°C to simulate the curing of rubber against a primed steel substrate. During the reaction, the mixture was continuously purged with nitrogen to reduce oxidation. At appropriate times between 1 and 100 min, substrates were removed from the mixture, rinsed with hexane ultrasonically for 5 min to remove materials that had not reacted, dried, and examined using RAIR. The RAIR spectra obtained after reaction times of 0, 15, 30, and 45 min are shown in Fig. 13. 

When a plasma polymerized acetylene film on a steel substrate was reacted with the squalene-containing model rubber compound at 155°C for 15 min, a new band assigned to zinc stearate appeared near 1539 cm in the RAIR spectra... 

Fig. 13. RAIR spectra of model rubber compound reacted with plasma polymerized acetylene films on steel substrates for (A) 0, (B) 15, (C) 0 and (D) 45 min. Adapted by permission of Gordon and Breach Science Publishers from Ref. [12].

Observation of absorption bands due to LO phonons in RAIR spectra of thin, silica-like films deposited onto reflecting substrates demonstrates an important difference between RAIR and transmission spectra. Berreman has shown that absorption bands related to transverse optical (TO) phonons are observed in transmission infrared spectra of thin films obtained at normal incidence [17]. However, bands related to LO phonons are observed in transmission spectra of the same films obtained at non-normal incidence and in RAIR spectra. Thus, it is possible for RAIR and transmission spectra of thin films of some materials to appear very different for reasons that are purely optical in nature. For example, when the transmission infrared spectrum of a thin, silica-like film on a KBr disc was obtained at normal incidence, bands due to TO phonons were observed near 1060,790,and450cm [18]. 

The catalytic preformance of Co crystals with two surface conditions were compared annealed crystals with large atomically flat terraces and Ar+ ion sputtered surfaces which produced a high population of surface defects. A sequence of PM-RAIRS spectra are shown in Figure 3.2 during exposure of a sputtered Co (0001) surface to mixtures of H2 and CO, with the temperature and pressure for each spectrum indicated in the figure. 

Figure 3.2. Sequence of PM-RAIR spectra taken on a sputtered Co(0001) surface. The starting conditions were lOOmbar of CO at 298 K 200mbar of H2 was then added and the temperature increased stepwise to 490 K. It can be seen that the absorption signal due to CO attached to defect sites is removed in an irreversible process. The lowest curve was obtained at room temperature and under vacuum conditions and the hydrogen desorbs from the Co, while the CO and hydrocarbons remain. (After Beitel et al. 1997.)...

The interpretation of HREELS is similar to that of RAIRS (pp. 41 et seq.), although RAIRS spectra are usually expressed in wavenumbers, while HREELS are in meV (1 meV = 8.065 cm-1). The resolution in RAIRS is typically 0.25meV, while it is about 20meV for HREELS (despite the HREELS acronym ). 

Figure 3.7. In-situ reflection-absorption infrared (RAIRS) spectra as a function of catalyst temperature from a Pd(lll) single-crystal surface in the presence of a NO + CO gas mixture (240mbar, Pco/Pno = 1-5) [66]. The data clearly show the appearance of an isocyanate-related band at 2256 cm-1 at temperatures above 500 K. In-situ spectroscopic experiments such as these have proven indispensable to detect and identify key reaction intermediates for the catalytic reduction of NO on metal surfaces. (Figure provided by Professor Goodman and reproduced with permission from the American Chemical Society, Copyright 2003).

Figure 8.7 RAIRS spectra show that lateral interactions force CO to leave the twofold adsorption sites on palladium (IR frequency of about 1920 cm 1) when NO is coadsorbed, and push it to the on top site (adsorption frequencies above 2000 cm-1). Adsorbed NO gives rise to the absorption peaks below 1800 cm 1 (from Raval et al. [22]).

Figure 13.6 Reflection-absorption infrared (RAIR) spectra of DICH (1,6-diisocyanohexane) films on Au as a function of dosing conditions. Immersion in (a) 0.001 M DICH for24h (b) 0.1 M DICH for 2min and (c) 0.1 M DICH for24h [24].

The SAM was obtained by immersing the clean substrate for 48 h in an ethanol solution of ferrocenylhexyl isocyanide. The strong v(N=C) peak at 2147cm" observed in FTIR spectra of free ferrocenylalkyl isocyanide (on a KBr plate) is not present in the RAIR spectra of this isocyanide on a nickel surface. Considering the surface selection rules for RAIR spectroscopy, the absence of a v(N=C) peak in the RAIR spectrum indicates that the chemisorbed isocyanides are bonded through both their carbon and nitrogen atoms, and they adopt an orientation in which the N=C bond is parallel to the surface. 

Figure 7.12 In situ PM-RAIRS spectra of a microcrystalline sample of [Pd(Me)(OTf)(dppp)]. (a) At room temperature (b) under 500 mbar CO (c) under 2 mbar CO and 333 mbar of ethene (d) under subsequent exposure to 750 mbar CO (e) during subsequent polymerisation under 666 mbar of ethene/CO (evolution of the spectrum at 15 min intervals).

In many respects gold is more convenient than sihcon for characterizing polymer brush films. For instance, the RAIR spectra from planar gold are usually superior to that from sihcon wafers, especially for the SAM characterization. In addition, the S - Au bond is easily cleaved by treatment in a dilute solution of iodine. This allows for the isolation of the tethered polymer and the determination of Mn and PDI. The SAMs are easily deposited from dilute solutions of thiol or disulfide precursors. The only major disadvantage to gold is the instability of the weak S - Au bond, which is unstable above 60 °C, and imder UV irradiation in air. 

During the past decade a very considerable literature has developed concerning the generation and reactivity of alkyl and alkylidene groups adsorbed on metal single-crystal surfaces produced via the photochemical or thermal decomposition of adsorbed alkyl halides or nitrogen-substituted alkanes. In this review, we concentrate on publications which exhibit VEEL or RAIR spectra of the hydrocarbon groupings that can be used as reference spectra for the identification of such species in spectra of species derived from hydrocarbon chemisorption. Reviews of such work cover the kinetic as well as spectroscopic aspects of this area of research (142-144). 

Two results appear immediately from the study of CO on Mo2C. First, the TPD data show that CO only partially dissociates on Mo2C. Dissociation on Mo2C leads to high temperature recombinative desorption of CO. However, the ratio of the area of the latter peak to the saturation area of the 325 K feature is 0.2. Second, the RAIRS spectra display a single v(CO) stretching mode with a frequency characteristic of on-top bonded... 

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