Photothermal experiments

Direct mirage effect — A deflection signal usually associated with photothermal experiments in which the deflected probe light beam passes on the same side of the photoilluminated -> interface [i]. See also - photothermal deflection spectroscopy. 

Fig. 3.1. Scheme of a photothermal experiment in which a small object (absorption cross-section a) is heated by a pump beam (power Pheat)- A probe beam (power Pprobe) illuminates the object and its close surroundings. The wave scattered by the temperature inhomogeneity slightly changes the detected probe power... 

Silva et al. had presented an alternate photothermal experiment in which pump and probe beams are collinear and the sample temperamre is determined from probe beam phase shifts rather than probe beam deflection (Silva et al., 2013a,b). Their experimental arrangement used a continuous-wave Gaussian (TEMqo with radius w ) pump beam and a weak collinear (i.e., zero lateral displacement) He-Ne laser probe beam (TEMqo with radius Wp), and they tested Yb -doped fluorozirconate (ZBLAN) and low-silica calcium aluminosilicate (LSCAS) glass. They measured the intensity of the central portion of the probe beam in the far field (pinhole/photodiode assembly) as a function of time, which is expected to follow... 

Rogers, J. A., and Nelson, K. A. 1996. A new photoacoustic/photothermal device for real-time materials evaluation An automated means for performing transient grating experiments. Physica B 56264 219-220. 

Figure 7 Electrochemical photothermal deflection spectroscopy experiment (0.5 mol dm-3 Na2S04 pH = 10.5, 20 mV s 1), illustrating the detection of the onset of dissolution of nuclear fuel (U02) (A) voltammetric response for scans to various anodic potential limits (B) and (C) probe beam deflection for each scan. The deflection of the probe beam is proportional to the dissolved uranium concentration, and deflection of the probe beam towards the electrode surface is an indication that dissolution is occurring (Reprinted from Ref. 2 with permission from Elsevier Science S.A.)...

The feasibility of thermic and calorimetric detection of the absorbed radiation has been mentioned in the context of grazing-incidence experiments. This is quite close to the class of photothermal techniques with which a number of different detection schemes is employed (Coufal, 1986). Out of these, photoacoustic spectroscopy (PAS) is frequently used in infrared spectroscopy (Graham et al., 1985 Urban et al., 1990 McClelland, 1992) while inspite of its potential, thermal beam deflection has not yet found as many applications as in other spectral ranges, possibly due to the lower availability of suitable lasers (Low and Morterra, 1985). 

The experiments described in the first part of this paper thus seek to extend our knowledge of the degradation processes which occur in PP, first, by linking photo and thermal processes through photothermal studies and second, by observing the effect of pre-Irradiation on subsequent thermal degradation. 

These experiments may also provide us with more information on the significance of gas-dynamic processes among the emitted fragments, where a substantial population of phenyl radicals appears to have been accelerated by entrainment, perhaps by reducing the density of the emitted N2 to where we might be able to see the direct phenyl radical time-of-flight. It is possible, as proposed in an earlier laser-polymer study [224], that both photochemical and photothermal mechanisms are occurring. 

The new concept that arises from these calculations is the difference in the temporal and spatial deposition of the available energy in photochemical and photothermal mechanisms. This concept provides the foundation to make specific comparisons with experiment and to explain experimental results as summarized below ... 

The photoacoustic sample cell/detector assemblies are inside a controlled atmosphere glove box. This glove box has two functions, firstly to allow experiments to be carried out in the anaerobic conditions expected in deep repositories, and secondly to act as containment for the radioactive materials. The nitrogen atmosphere glove box has been fitted with sufficient windows to allow ready adaptation of the LPAS system to carry out other photothermally-based spectroscopy techniques, namely thermal lensing spectroscopy and photothermal deflection spectroscopy. 

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