Mirror velocity

Depth profiling of a solid sample may be performed by varying the interferometer moving-mirror velocity (modulated IR radiation). By increasing the mirror velocity, the sampling depth varies, and surface studies may be performed. Limitations do exist, but the technique has proven to be quite effective for solid samples [21]. In addition, unlike diffuse reflectance sampling techniques, particle size has a minimal effect upon the photoacoustic measurement. 

Depending on the source of the graphite, one obtains distinctly different IR/PA spectra (frequently caused by adsorbed species) and the response of the DTGS detector of an IR spectrometer turns out to be a more accurate measure of variable source intensity (12). A normalization technique (13) requiring measurement of the spectrum at two different mirror velocities and corrected by black body spectra taken at the same two velocities appears to be the best normalization method reported thus far. 

Each interferogram is an average of 32 scans and contains 4 096 data points, giving a resolution of 4 cm The mirror velocity was 0.693 cm/s. [Courtesy M.RNadler, Mlchelson Laboratory, China Lake, CAJ... 

What is the difference between true (conventional) detonation and such pseudo-propagation with externally imparted (by the motion of the mirror) velocity ... 

All of the infrared experiments were performed on a Digilab FTS-40 Fourier transform infrared (FT-IR) spectrometer equipped with a narrow-band liquid-nitrogen-cooled mercury-cadmium-telluride (MCT) detector. The spectrometer was operated at a nominal resolution of 4 cm-1 using a mirror velocity of 1.28 cm/s. The data collected using the gas chromatography (GC) IR software were measured at 8 cm-1 resolution. Protein assays for all the experiments were measured on a Beckman DU-70 UV-visible spectrophotometer. 

The typical polymer or rubber sample would be classified as optically transparent or opaque and thermally thick except possibly for the strongest bands. In this case the signal intensity would be proportional to the product of the optical absorption coefficient (P) and the thermal diffusion length and show a - 3/2 dependence on the modulation frequency (to). The angular modulation frequency is a product of the interferometer mirror velocity and the wavenumber ... 

Sometimes the mirror velocity must be calculated from what the instrument manufacturer specifies as the He-Ne laser modulation frequency. 

A wide variety of FTIR instruments have been used to obtain PA-FTIR spectra. A cursory examination of the literature reveals some of the instruments used JEOL JIR-5500 IBM IR-95, IR-98,9195 Bruker IFS 66, IFS 88, IFS 113V Perkin Elmer 1750,1760-X, 1800 Bomen DA 3.02 Nicolet 20 SX, 20 DXB, SX-170, 7199, 740, 800 Mattson Cygnus 100, Sirius 100, Polaris Digilab FTS-10, FTS-10M, FTS-11, FTS-15, FTS-20, FTS-20E, FTS-60, FTS-65, FTS-6000 Laser Precision Analytical (Analect) RFX-75 and Analect FX-260. The Laser Precision Analytical Instrument required extensive modification. The list is only intended to indicate the wide variety of different instruments which have been used and may not be complete. The normal adjustments for maximum performance should be made, beam splitter alignment, etc. The source aperture, if any, should be opened fully for maximum source intensity. The gain control and other adjustments for best operation at each of the different mirror velocities to be used should be made. The... 

A discussion of surface analysis requires a review of the depth being sampled during PA-FTIR spectroscopy. The depth being sampled during PA-FTIR analyses of rubbers is the thermal diffusion depth (Dt). This is a function of the thermal diffusivity of the sample, the wavenumber, and the mirror velocity. 

One type of sample to be considered is one which has a bloom or similar surface layer. This was an area first explored by Carter and co-workers [60]. They reported finding a bloom of dimorpholinyl thione and zinc stearate formed from ingredients used during the vulcanisation of the NR sample. As the mirror velocity was increased from 0.316 cm/s to 1.216 cm/s the bands associated with the bloom increased. Figure 2.12 shows the PA-FTIR spectrum of a zinc stearate bloom on an amber coloured silica-filled SBR. 

Table 2.2 Thermal diffusion depth as a function of mirror velocity (for thermal diffusivity = 1.3 x 10 3) ...

The 1539 cm 1 zinc stearate band can be ratioed to the 966 cm"1 SBR band and plotted as a function of mirror velocity (depth) to show that it is increasing in concentration as the surface is approached. This is shown in Figure 2.13. 

Laminates have been extensively studied by PA-FTIR and are the subject of extensive investigations by the new much more complex step-scan PA-FTIR techniques. Kapton film is often used to illustrate the analysis of this type of sample. Kapton 200FN919 film is a DuPont product which consists of a 25 pm layer of polyimide laminated between 12.7 pm layers of Teflon. If a high mirror velocity is used then a spectrum of only the Teflon layer is obtained. On the other hand if a slow mirror velocity is used a composite spectrum of the Teflon and polyimide layer is obtained. The difference spectrum (low mirror velocity spectrum minus high mirror velocity spectrum) represents that of the polyimide layer. As illustrated in Figure 2.16 the difference spectrum closely resembles the PA FTIR spectrum of polyimide except for the regions where the strong C-F bands of the Teflon are present. 

Assuming the mirror velocity is constant, equal to F (centimeters per second), then X — Ff, and... 

CCD gain, e /ADU (analog-to-digital converter unit) mirror velocity in an interferometer, cm sec ... 

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