Optical IRRAS

M.L. Hamilton, B.B. Perston, PW. Harland, B.E. Williamson, M.A. Thomson and P.J. Melling, Grazing-angle fiber-optic IRRAS for in situ cleaning validation, Org. Process Res. Dev., 9, 337-343 (2005). 

A great deal of success was attendant on the early application of PM-IRRAS to the gas/solid interface. Golden et ai (1981) reported the development of instrumentation, using conventional dispersive optics, able to record detailed infrared reflection-absorption spectra from molecules adsorbed on single-crystal Pt without any interference from the gas-phase species. 

In conclusion, strained surfaces can show very original structures and new catalytic properties. In order to associate the modified catalytic properties to the peculiar structures generated, one has to asume that these original structures are still present under the reactive mixture, at high pressure. Measurements under pressure of reactants are then necessary to measure both the surface structure and the surface species as reaction intermediates. Up to now, only very few data are available in that field. Recent developments around techniques such as STM [79-80], grazing X-ray Diffraction [81]. .. and optical vibrational spectroscopies such as IRRAS[82-83] using a polarized light and SFG [79] have demonstrated the possibility to realise such observations. 

A prerequisite for the development indicated above to occur, is a parallel development in instrumentation to facilitate both physical and chemical characterization. TEM and SPM based methods will continue to play a central role in this development, since they possess the required nanometer (and subnanometer) spatial resolution. Optical spectroscopy using reflection adsorption infrared spectroscopy (RAIRS), polarization modulation infrared adsorption reflection spectroscopy (PM-IRRAS), second harmonic generation (SFIG), sum frequency generation (SFG), various in situ X-ray absorption (XAFS) and X-ray diffraction spectroscopies (XRD), and maybe also surface enhanced Raman scattering (SERS), etc., will play an important role when characterizing adsorbates on catalyst surfaces under reaction conditions. Few other methods fulfill the requirements of being able to operate over a wide pressure gap (to several atmospheres) and to be nondestructive. 

In the present study, the water surface is being surveyed by two optical methods, i.e., Brewster Angle Microscopy [BAM] and Infrared Reflection-Absorption Spectroscopy [IRRAS], A schematic sketch of the BAM... 

Fig. 9.8 Optical setup for in-situ IRRAS based on the following windows (a) Cap2 equilateral prism (transmission range 76900-1100 cm" RAir/CaF,=0-03), (b) ZnSe hemisphere (transmission range 22200-700 cm" RAir/znSe=0-18).

One needs to know optical constants to calculate IRRAS spectra of molecules either adsorbed at the electrode surface or resident inside the thin-layer cavity. The isotropic optical constants of a given compound are usually determined from transmittance spectra. A pressed peUet, prepared by grinding the dispersion of the compound with a KBr or KCl powder, is typically used as a sample. Recently, Arnold et al. [41] have demonstrated that this method can yield non-reproducible results due to different histories of the sample preparation. In addition, the optical constants determined using the powder method can be quite different from those of the film at the metal/electrolyte interface because of the difference in the environment. 

A diagram of an experimental set-up used for PM IRRAS experiments is shown in Fig. 9.26 and a photograph of the spectroelectrochemical cell in Fig. 9.27. The set-up is built from components purchased from Newport and custom-machined parts in an external tabletop optical mount (TOM) box. A convergent infrared beam from the spectrometer enters a port of the TOM box where it is deflected by a flat mirror and focused onto the working electrode by a parabolic mirror (f=6 in, Nicolet). Before entering the cell, the beam passes through a static polarizer (diameter 1 in, with an anti-reflective coating -... 

Fig. 9.26 Experimental PM IRRAS setup (top view). FM, flat mirror PM, parabolic mirror SP, static polarizer PEM, photoelastic modulator OW, optical window L, lens ...

Fig. 9. 28 Experimental PM IRRAS setup (side view). D, detector FH, optical filter holder Ml, mount 1 LH, lens holder ...

In order to calculate A.I/ I) from the measured PM IRRAS spectra, one has to determine functions J2 and Jq in an independent experiment. A reliable method to measure the PEM response functions was described by Buffeteau et al. [69]. Below we describe a similar method that we adapted with minor changes to use for electrochemical systems [81]. The spectroelectrochemical cell is replaced by the dielectric total external reflection mirror (a Cap2 equilateral prism can be used for this purpose). The second polarizer is inserted just after the PEM and set to admit p-polarized light (identical setting to that of the first polarizer). The PEM is turned off and the reference spectrum is acquired. This spectrum gives the intensity of the p-polarized light Ip (cal), which passes through the whole optical bench. 

Buffeteau and coworkers [69] demonstrated that signals ( ) and AJ have to be corrected further to take into account the fact that the ratio of the optical throughputs of the experimental set-up for p- and s-polarized light y is not equal to unity. When y yf 1, the corrected PM IRRAS spectram can be calculated with the help of the formula [36]... 

The correction of the measured PM IRRAS signal for the PEM response functions was introduced in the studies of the potential-induced reorientation of a film formed by 4-pentadecyl pyridine [81], one year later. For the first time, the absorbance of a Langmuir film adsorbed at the electrode surface was determined in that paper. The theoretical spectrum of a film of randomly oriented molecules was calculated from independently measured optical constants, and tilt angles of the pentadecyl chain and the pyridine moiety were determined using the absolute method and Eq. (44). This work laid the methodological foundations for future PM IRRAS studies on monolayers and bilayers formed by amphiphilic molecules at electrode surfaces. 

One common problem when examining ultrathin films on various surfaces and at various interfaces by IR spectroscopy is that of selecting both the best IR method [transmission, IR reflection-absorption spectroscopy (IRRAS), ATR, DT, or DR] and the best experimental geometry (optical configuration) for this method. For films on plane substrates, this can be done using the optical theory introduced in Chapter 1 (Sections 2.1-2.6). In the case of powdered substrates, the optimum conditions are chosen based on the general theoretical and empirical regularities (Section 2.7). 

The IRRAS method can be used to obtain information about ultrathin films not only at metals but also on semiconductor and dielectric (including liquid) substrates. This class of problem is applicable to many areas, including thin-fihn optics, electronic and electroluminescent devices [27] (Chapter 5), sensors and transducers [28], flotation technology [29] (Section 7.4.4), and biomedical problems [30, 31]. Although the sensitivity is much lower than when metallic substrates are used, the waiving of the metal selection rule allows both s- and /7-polarized spectra to be measured and thus a more thorough investigation of molecular orientation within the layer. 

IRRAS spectra of transparent layer with optical constants 02 = 1.5, fC2 = 0, and 0I2 = 10 nm on quartz surface. 

By making the buffer layer optically thin (< 70 nm thick) but chemically thick, the absorption due to perpendicular modes of ultrathin films on dielectrics can be measured in the whole IR range with surface sensitivity above that in the transparent IRRAS but below that in metallic IRRAS. 

The effect on the band intensities of the angle of incidence of radiation, the radiation polarization, and the optical constants of the layer, immersion media, and the metal was analyzed in Refs. [40, 79, 80]. Figures 2.28-2.30 demonstrate how the refractive index of the input medium affects the band intensities in the IRRAS of ultrathin films of weakly and strongly absorbing material. The calculations show virtually no absorption of j-polarized radiation by the layer (for an explanation of this effect, see Section 3.2.2). For p-polarized radiation, the band intensity (1.5.4°) depends strongly on the angle of incidence and the... 

Figure 2.31. Calculated p-polarized IRRAS spectra of SIO2 layer of thickness (1)1, (2) 5, (3) 20, and (4) 100 nm located at Ge-AI Interface. Spectra were obtained for values of (a) 30°, (fa) 60°, and (c) 85°. Optical constants of SIO2 were specified In Ref. [2] and of Al in Ref. [75, nee = 4.0.

Figure 2.32. Experimental spectra of AI2O3 layer on Al mirror heated at t = 550°C in air for 0.5 h, using p-polarized IRRAS at = 60° (1) sample in air, (2) sample in contact with immersion medium (KRS-5). Reprinted, by permission, from V. P. Tolstoy and S. N. Gruzinov, Opt. Spectrosc. 63, 489-491 (1987), p. 490, Fig. 4. Copyright 1988 Optical Society of America.

A theoretical analysis of the system consisting of either the ZnSe or Ge IRE, an Fe or hematite substrate layer, an adsorbate layer, and a solution of methylene chloride has been performed by Loring and Land [88], The system consisting of the ZnSe IRE, AI2O3 intermediate layer, the sputtered Si substrate layer, and water has been analyzed within the framework of the Fresnel formalism by Sper-line et al. [89], Calculations also reveal that within a narrow wavelength range and at a certain ratio of the optical refractive indices, enhancement of intensities is observed in the spectrum of a given layer in an arbitrary two-layer structure located on an IRE. The ATR spectra of such structures were considered in detail in Ref. [66], This enhancement is attributed to interference of the radiation, as is the enhancement of the reflectivity in BML-IRRAS (Section 2.3.3). 

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