Photothermal deflection method

Figure 3.35 Ethylene produced by oxidative dehydrogenation of ethane over Mo-V-Nb and Ni-Ce-Ta oxide catalyst libraries. The detection of ethylene was performed in a scanning mass spectrometer using a photothermal deflection method. Inactive Mo-V-Nb oxide catalyst (a) active Ni-Ce-Ta oxide library (b) [7] (by courtesy of Kluwer Academic Publishers).

Combinatorial methods were also applied in the discovery of new catalysts for the low-temperature oxidation213 and oxidative dehydrogenation of propane.214 A 144-member catalyst library was screened by photothermal deflection spectroscopy and mass spectrometry to find the most active compositions of V-Al-Nb and Cr-Al-Nb oxides for the oxidative dehydrogenation of ethane.215 The ternary combination V(45)-Sn(45)-Mo(10)-O selected by laser-induced fluorescence imaging gave much higher yield than did V205 in the selective oxidation of naphthalene to naphthoquinone.216... 

Where R is the reflectivity and d is the thickness. Very accurate values of R and T are needed when the absorptance, (id, is small. The technique of photothermal deflection spectroscopy (PDS) overcomes this problem by measuring the heat absorbed in the film, which is proportional to ad when ad 1. A laser beam passing just above the surface is deflected by the thermal change in refractive index of a liquid in which the sample is immersed. Another sensitive measurement of ad is from the speetral dependence of the photoconductivity. The constant photocurrent method (CPM) uses a background illumination to ensure that the recombination lifetime does not depend on the photon energy and intensity of the illumination. Both techniques are capable of measuring ad down to values of about 10 and provide a very sensitive measure of the absorption coefficient of thin films. 

One way of measuring true absorption is to employ photothermal deflection spectroscopy (see Fig. 9-1). In this case the absorbed light, leading to a slight heating up of the sample, is detected by the temperature-dependent deflection of a laser beam passing close by the sample. The obvious advantage of this method is that it really measures the absorption coefficient (a). 

Measurements of the deflection of a light beam caused by local heating at the solid/solution interface as a function of the wavelength of light illuminating this interface are an alternative possibility the resulting method is called photothermal deflection spectroscopy (PDS) [127, 128]. The experimental setup as depicted in Fig. 5.30 closely resembles the arrangement for probe beam deflection studies treated in Sect. 5.1.9. 

Boccara A.C., Fournier D., Jackson W., Amer N.M. Sensitive photothermal deflection technique for measuring absorption in optically thin media. Opt. Lett. 1980 5 377 Bornholdt C., Krauser J., Nolting H.-P. Novel simple method for determination of refractive index of optical monomode slab waveguides. Electron. Lett. 1985 21 706-708 Brandt G.B. In-plane scattering in glass and niobium oxide waveguides. Opt. Eng. 1981 20 150-152... 

PTD. See photothermal deflection technique pulling-up-sphere method, 242 pulse sequences used for solid state NMR 2D MQ MAS NMR, 39 cross-polarization, 39 one pulse, 39 REDOR, 39 spin-echo, 39 pump absorption, 361 PVP (polyvinylpyrrolidone), 302 pycnometer, 193... 

The technique employed is IR-FT photothermal beam deflection spectroscopy (PBDS). It is an off-shoot of photoacoustic spectroscopy (PAS) [1] and is based on the "mirage detection of the photothermal effect invented by Boccara et al. [2] and shown to result in a spectroscopic technique of remarkable versatility and utility. Some applications of "mirage spectroscopy," mainly in the visible, and theoretical treatments, have been described [3 6]. The method has now been developed for use in the IR. The spectrometer and techniques are described in detail elsewhere [7], but it will be useful to give a brief outline of the principles. 

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