Scanning interferometers

On metals in particular, the dependence of the radiation absorption by surface species on the orientation of the electrical vector can be fiilly exploited by using one of the several polarization techniques developed over the past few decades [27, 28, 29 and 30], The idea behind all those approaches is to acquire the p-to-s polarized light intensity ratio during each single IR interferometer scan since the adsorbate only absorbs the p-polarized component, that spectral ratio provides absorbance infonnation for the surface species exclusively. Polarization-modulation mediods provide the added advantage of being able to discriminate between the signals due to adsorbates and those from gas or liquid molecules. Thanks to this, RAIRS data on species chemisorbed on metals have been successfidly acquired in situ under catalytic conditions [31], and even in electrochemical cells [32]. 

Figure 2.50 Potential difference infrared (PDIR) spectra for adsorbed azide at the silver-aqueous interface in the asymmetric N-N-N stretch region. The reference (base) potential was -970 mV vs. SCE sample potentials as indicated. The solution contained 0.01 M NaNj/0.49 M NaCI04. The spectra are the average of 1024 interferometer scans at each potential. From Corrigan and Weaver (1986), Copyright 1986 American Chemical Society.

NaN3 + 0.1 M NaClO. Base (reference) potential was -0.97 V vs. SCE sample potentials (mV vs. SCE) are as indicated. Spectra were obtained by acquiring 1024 interferometer scans at the base and sample potentials, the potential being altered after every 32 scans (see references 7 and 10 for further details). 

Figure 5. Schematic drawing of a high throughput pulsed slit jet FTIR setup involving a 600 mm nozzle that is synchronized to the interferometer scans [154],...

Figure 1 - Photoacoustic FTIR spectra of fibers from new cotton cloth (lower Spectrum) and approximately 700 year old cotton (upper spectrum). The aged cotton fiber was sample 3019 (circa A.D. 1250-1300). Both spectra were obtained by averaging 1000 interferometer scans. Note the presence of new absorption bands in the C=0 region in the aged sample.

Photo-ions are then detected as a function of interferometer delay. The result is an in-terferogram that upon Fourier transformation yields a Raman spectrum whose resolution does not depend on the bandwidths of the Raman excitation sources but, instead, on the delay range of the interferometer scanned in the experiment. Mass-selective IDSRS and FT-IDSRS have been employed in a number of studies, including one of the benzene dimer (Henson et al., 1991). For examples of IDSRS spectra see Sec. 6.1.4.5. 

The time necessary for one interferometer scan depends on the required resolution. Typically c. 0.3 s or less is needed for a resolution of 8 cm . However, in order to diminish the electronic noise and make use of the multiplex advantage, several interferograms are scanned, added, and averaged, in a couple of minutes. 

Fig. 15. FTIR spectra for adsorbed CO at different coverage degrees obtained by dosing different amounts of CO on Pt(lll) at -0.25 V vs. SCE in 0.1 M HCIO4. Spectra were obtained by means of 30 interferometer scans and subtracting another set taken at 0.5 V after total oxidation of CO. (After (53]). Reprinted by permission of Journal of Chemical Physics AIP.

The solution loss band which is very pronounced at pH 2.8 indicates that no replenishment of these ions occurs in the thin layer within the time necessary to collect 1000 interferometer scans (c. 30 s) and to execute some software work (c. 8 s). The potential-dependent negative band located between 1230 and 1260 cm and the shoulder near 1190cm" are adsorbate features. 

Fitting the data to this simple phenomenological model affords a substantial dimensionality reduction while preserving most of the original fidelity. Each data matrix (intensity vs. frequency and time) is reduced to two vectors, namely temperature and area versus time. A representative example of the temperature and area curves are presented in Fig 5. The standard error in the area was about ten times greater than temperature, and is periodically displayed in Fig 5. Initial temperatures were 1700-2000 K and often followed an exponential decay with rates between 0.91-1.24 s Examining Fig 2, we note that errors in the residuals due to temporal aliasing contribute less than 0.5% since the interferometer scanned at least 16 times faster than the temperature decay rate. 

Instrumentation. Spectra were acquired with a Nicolet 60SX FTIR spectrometer, continuously purged with dry air and equipped with a liquid-nitrogen-cooled, wideband mercury-cadmium telluride detector. Coaddition of 100 interferometer scans at 8-cm 1 resolution was employed. The location of absorption maxima was confirmed by spectra taken at l-cm 1 resolution. All spectra were converted into Kubelka-Munk units prior to use. Integration of peak areas was accomplished by using software available on the Nicolet 60SX. All peak areas were normalized to the 1870-cm-1 Si-O-Si combination band (15). 

Figure 2.45. (a) Diffuse reflectance and (b) normalized diffuse transmittance of 1.3-mm-thick samples of Cab-O-Sil silica powders entirely covered by (4) DMB and (S) DMP.CN siloxy substituents, (a) Number of coadded scans 250, resolution 4 cm" interferometer scanning speed 0.5 cm s". (b) Photopyroelectric signals normalized to spectrum of empty sample holder/detector. Number of coadded scans 75, resolution 4 cm" interferometer scanning speed 0.03 cm s". Adapted, by permission, from F. Boroumand, J. E. Moser, and H. Vandenbergh, Appl. Spectrosc. 46, 1874 (1992), pp. 1883 (Fig. 7) and 1885 (Fig. 10). Copyright 1992 Society for Applied Spectroscopy.

It is always advisable to adhere closely to the manufacturer s instructions, especially in regard to the use of a dry purge gas. Failure to do this will result in data dominated by the spectrum of water vapor. Photoacoustic measurements are based on thermal processes, which are intrinsically slow, and in order to maximize sensitivity, slow interferometer scan rates are recommended for best performance. Note that the sample size is normally limited to a few millimeters. Always docmnent the purge gas that is used and the scan speed of the instrument because these impact the intensity and quality of the final spectrum. 

Figure 20.2 Schematic illustration of the relationship between the interferometer scan and data acquisition in time-resolved measurements using a step-scan FT-IR spectrometer.

Apodization is an operation that is necessary because of the finite length of the interferogram. Monochromatic radiation produces a sinusoidal interferogram, but the length of this is limited by the maximum path difference in the interferometer scan. The Fourier transform of such a signal is a line of finite width with oscillations on either side. This line has the form of a sine function ((sin a/a)) (Figure 12). The oscillations (or side-lobes) that... 

---