Photoacoustic saturation

Equation 4 is applicable to the Type I experiment where there is circular polarization modulation at u)C. The relation between Aq and the sample s absorptivity and thermal properties is seen in Figure 3. A linear dependence on 3 and thus on g is found for < as (uB > us). At > as (ue < us) saturation occurs and the PACD magnitude decreases as the absorptivity increases. Thus photoacoustic saturation will lead to anomalously low g values, since the photoacoustic magnitude approaches a constant value for 3 > as (14). The PACD phase angle (Figure 4) is a function of as and 3 and varies from -90° (3 as) to 0° (3 as). 

Figure 3. A log-log plot of the FACT) magnitude normalized to the coefficient K of Equation 2 vs. log B, where B = ft/as. ft is the optical absorption coefficient (cm 1) and a is the thermal conduction coefficients (cm 1). The onset of photoacoustic saturation occurs at logB = 0. B = ft/as as = 100 cm 1 ( 19).

Representative results are shown in Figure 12.3, with ATR spectra of the same polymers being shown for comparison [8], although it should be noted that the ATR spectra were measured at higher resolution than the photothermal spectra. The distortion of the stronger bands in the ATR spectra of the more polar polymers is caused by the effect of anomalous dispersion when an internal reflection element (IRE) with a relatively low refractive index, presumably ZnSe, was used. Remarkably the highest quality of photothermal spectrum was measured in the case of polypropylene, which has a relatively weak spectrum, and the lowest quality spectrum was measured in the case of Nylon 6, where the effect of photoacoustic saturation [10] is clearly evident. It is interesting to speculate on whether this spectrum and that of polycarbonate would have been improved had the velocity of the interferometer mirror been increased. Spikes in some of the spectra at 1082 and 1804 cm were attributed to supply frequency harmonics. 

The values of the linear absorption coefficient a(v) and the optical absorption depth pp at the peak of the strongest bands in three spectral regions are also given in Table 20.2. Clearly, > pp for each of these bands, which is the condition for photoacoustic saturation for spectra taken at the lowest optical velocity, so that these three bands in the PE spectmm all have about the same intensity. 

Figure 20.13. PA spectra of a sample consisting of a 5-pm layer of a silicone on a polycarbonate substrate measured with three superimposed phase modulation frequencies (a), 360 Hz (b), 60 Hz (c), 10 Hz. Many bands in the spectra of silicones have very high absorptivity, so that the strong silicone bands exhibit photoacoustic saturation even for such a thin layer. The bands of the polycarbonate (marked with arrows) can be seen to increase in intensity as the phase modulation frequency gets lower.

In case 2a and 2b, since most of the radiation is absorbed with a length that is smaller than the thermal diffusion length, photoacoustic saturation sets in. 

PAS spectra are similar to those obtained using ordinary Fourier transform infrared (FTIR) spectroscopy except truncation of strong absorption bands which occurs due to photoacoustic signal saturation. PAS allows the structure to be studied at different thicknesses because the slower the frequency of modulation, the deeper the penetration of IR radiation. 

It is useful to briefly discuss some of the common and, perhaps, less common experimental approaches to determine the kinetics and thermodynamics of radical anion reactions. While electrochemical methods tend to be most often employed, other complementary techniques are increasingly valuable. In particular, laser flash photolysis and photoacoustic calorimetry provide independent measures of kinetics and thermodynamics of molecules and ion radicals. As most readers will not be familiar with all of these techniques, they will be briefly reviewed. In addition, the use of convolution voltammetry for the determination of electrode kinetics is discussed in more detail as this technique is not routinely used even by most electrochemists. Throughout this chapter we will reference all electrode potentials to the saturated calomel electrode and energies are reported in kcal mol. ... 

Step-scan photoacoustic data are presented that prove the ability of the technique to successfully isolate the infrared signature on the top layer from the infrared spectrum of the bulk material, proving the sub-micron resolution capability of the method. Results are shown that underline the fact that the most serious problem in photoacoustic spectroscopy is saturation at high absorptivities. 12 refs. 

Ozone concentrations are measured with the same photoacoustic system using the 9P28 and 9P24 CO2 laser lines (a = 8.7atm cm at 1039.369cm and a = 0.83 atm cm at 1043.163 cm" respectively Harren et al. 1990b). A sensitivity of 20 pl/1 is obtained. It has been checked that no saturation occurs for ozone. 

Saturation effects, which can occur for stronger absorption bands, are the main limitation of photoacoustic spectroscopy. These distortions can be overcome by using thin samples [93] or by analyzing the phase of the photoacoustic signal... 

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