Photoacoustical Measurements

While transient photocurrent and photoinduced discharge techniques are the conventional methods for measuring photogeneration efficiencies, these cannot be readily employed in the presence of trapping. A further limitation is that it is difficult to separate the field dependence of the photogeneration process from field dependencies associated with an injection or interfacial process (Seki, 1970, 1972a). As such, it is difficult to apply the method to dispersions or two-phase materials. Experimental methods that may avoid these limitations are 

The principal advantage of photoacoustical techniques is that the measurements are not affected by trapping. The limitation is that the method is based on the assumption that the absorption of radiation either produces heat or free carriers. In the presence of pther processes, it is difficult to measure photogeneration efficiencies on a quantitative basis. A final limitation is experimental complexity. Although of considerable potential relevance, the techniques have not been widely used. 

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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. 

Figure 5. Schematic diagram of the adaptation of a Nicolet 7199 FT-IR spectrometer for photoacoustic measurements on solid samples.

One may be somewhat disappointed by the error bars in A0bs7/ and Ar/7, which are larger then most obtained by classical calorimetric methods, such as reaction-solution or combustion calorimetry. However, the comparison is unfair because PAC deals with species that have lifetimes smaller than a microsecond, not amenable to those classical methods. Although the quality of the photoacoustic measurements is rather good, as shown by the correlations obtained (see figure 13.7), a realistic error in the ratio of the slopes (0bs) is 1-2%, which implies an uncertainty in A0bsH of 4 to 8 kJ mol-1. This error bar is particularly serious when the reaction quantum yield is low (recall equation 13.15). 

Laser systems have been applied to spectroscopic studies of expls (Ref 40). A novel expl vapor detector, based on photoacoustic measurements, has been used to characterize absorption coefficients of expl reaction products (Ref 63). Acoustic spectra from underwater TNT explns have also been measured and analyzed with respect to refracted pulse behavior (Ref 15)... 

Although this artefact is extremely welcome to scientists, working on powdered substrates, it inevitably has its effects on the quantitative aspect of photoacoustic measurements. This is probably the main disadvantage of photoacoustic spectroscopy of powders. 

Although C02 lasers offer limited line-tunability from 9 to 11 xm wavelength, the significant output power makes them amenable to some methods of explosives detection. C02 lasers also see application as light sources in some photoacoustic measurement schemes (Section 4.4.). For example, Chaudhary [20] has used a C02-based photoacoustic technique to detect ppb (by weight) amounts of TNT and RDX. McKnight et al. [21] has used the acoustic pulse from a focused C02 laser employing different spot sizes to identify buried objects. 

Changes in the catalytic activity of activated alumina surfaces have also been probed by Temperature Programmed Desorption (TPD) and FTIR photoacoustic measurements [74]. Ammonia TPD allows a fast and convenient determination of the overall acidity of a solid surface. The desorption profiles provide information on the distribution, the amount and strengths of the acid surface sites, since molecules adsorbed at weaker sites desorb at lower temperatures than those adsorbed at stronger acidic sites. The activation of y-alumina with CHC1F2 resulted in a sig-... 

Field-enhanced fluorescence quenching techniques have been widely used to study photogeneration during the past two decades (Menzel and Popovic, 1978 Popovic and Menzel, 1979 Popovic, 1982, 1983, 1984 Yokoyama and Mikawa, 1982 Popovic etal., 1985, 1987 Popovic andHor, 1988, 1995 Weiss and Bufbeny, 1988 Niimi and Umeda, 1993 Popovic and Mesbah, 1993 Hor and Popovic, 1994. Fluorescence quenching is not affected by trapping. As with photoacoustical measurements, a fundamental limitation of the technique is... 

A new rheo-photoacoustic Fourier transform infrared cell has been developed to perform stress-strain studies on polymeric materials. The rheo-photoacoustic measurements lead to the enhancement of the photoacoustic signal and allow one to monitor the effect of elongational forces on the molecular structure of polymers. Propagating acoustic waves are detected as a result of infrared reabsorption and the deformational and thermal property changes upon the applied stress. 

Figure 23. Photogeneration quantum yield for H2Pc-based photoreeeptors with 33 wt.% hydrazone (DEH) CTM, as a function of applied field. Open-circle data were obtained by xerographic discharge and filled-circle data from photoacoustic measurements. (Reprinted with permission from Ref [36y].)...

Photoacoustic Measurement of absorbed power as sound intensity -h... 

N. A. Garcia, G. Rossbroich, S. E. Braslavsky, H. Durr, and C. Dorweiler, Photoacoustic measurements and MINDO/3 calculations of energy storage by short-lived species the spiro[l,8-a]dihydroindolizine-betaine system, J. Photochem. 37,297-305 (1985). 

The first observations of energy transfer were made in gas phase atomic systems. There have been many publication dealing with similar studies on polyatomic systems in the gas phase, liquid phase and in molecular crystals. Only very little, however, is known about transfer processes and mechanisms when donor and acceptor molecules are adsorbed at the surface of a solid (1-3). In this paper we present some photoacoustic measurements and compare the results with fluorescence decay time measurements. As model substances we have chosen four different donor dyes and two acceptor dyes. The dyes were adsorbed on silica. 

The influence of acceptor concentration on energy transfer can be demonstrated if fluorescence quantum yield is plotted against the reduced acceptor concentration. Quantum yields of adsorbed molecules can be derived from photoacoustic measurements... 

Materials Photoacoustic measurements were made on a component-assembled PAS spectrometer consisting of a 9W argon ion laser (Spectra Physics), a 0.5 cc internal volume PAS cell equipped with a sensitive electret microphone (Radio Shack, 3.2 mV/Pa). Circular polarization modulation was achieved with a special low frequency (220 Hz) photoelastic modulator (15) (Hinds International). Signals were detected and processed with a vector tracking lock-in amplifier (PAR model 5204), and intensity modulation was done with a 30-slot blade mechanical chopper (Ortec). Syntheses of all compounds were by well established literature methods. 

Very revently a paper by Burkey and Heilweil reported photoacoustic measurements providing data concerning the energetics and dynamics of the ring closure of 485 and 486 to 487 and 488, respectively (Scheme 93). 

This research Is supported by the Flat-Plat Solar Array Project sponsored by the Department of Energy by agreement with the National Aeronautics and Space Administration. The authors would also like to thank Dr. Susan D. Allen for her assistance In carrying out some of the photoacoustic measurements. 

With colloidal photocatalysts, increased diffusiveness of light caused by scattering in turbid waters can lead to enhanced photolysis rates. For example, a study of the effects of clays on the rate of a uv photolysis showed an initial increase in the photolysis rate on addition of clay followed by a decrease when the clay concentration finally produced offsetting light attenuation (] ). The light available for photochemistry should be measured by thermal lensing and photoacoustic measurements. This has been demonstrated to be effective for waters containing humic colloids and suspended fine sands (1 ). 

Fig. 7. H-type photoacoustic cells, a) Helmholtz r nator with separated sample diamber and d dion chamber for jdioteacou spectroscopy on solids, b) H- pe cylindrical cell for res>nant photoacoustic measurements in gases. In the tube section a coaxial cylindrical microphone is installed (Rrf. >)...

After selective absorption of radiation, excited molecules may relax either by emission of radiation or by non-radiative processes (cf Chapter 3, Fig. 3.1). In photoacoustic measurements, the conversion of absorbed radiation into thermal energy is utilized. This type of conversion results in changes in the sample s thermodynamic parameters such as temperature or pressure. Changes in pressure generate acoustic waves, which eventually will be transferred to the surroundings of the sample (Fig. 5.12) where they can be measured by a sensitive microphone see Fig. 5.13 (photoacoustic spectroscopy (PAS)). 

Mauzerall et al. employed pulsed photoacoustic measurements on the proteins involved in bacterial photosynthesis and applied the Drude-Nernst equation to estimate... 

Figure 2. Instrumental setup of a combined Chi fluorescence and photoacoustic measurement. See text for further explanation.

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