Photoacoustic spectroscopy Piezoelectric transducer

The experimental design for the photoacoustic experiment is relatively simple. The apparatus is quite similar to that employed for nanosecond absorption spectroscopy with the major difference being that a piezoelectric transducer is used to monitor the acoustic waves rather than a photomultiplier tube to analyze the incident light. A representative schematic for PAC is shown in Fig. 2. 

It can sometimes be highly advantageous to detect the SRS signal indirectly. Thus in photoacoustic Raman spectroscopy (PARS), introduced by Barrett and Berry [37], a microphone or piezoelectric transducer is used to measure the acoustic wave amplitude generated in the sample when the vibrational or rotational excitation, created by the SRS process, relaxes non-radiatively into translational (heat) energy. Figure 5.8 shows the pure rotational PARS spectrum of CO2, taken from the article by J.J. Barrett, D.R. Siebert and G.A. West in Reference [4]. Although the resolution of -0.3 cm" is not remarkable, the... 

Rather than a more formal title such as laser Debye-Sears ultrasonic absorption spectroscopy we frequently refer to this experiment as simply "looking at sound." This has the advantage of drawing attention to the complementarity of this experiment to photoacoustic spectroscopy (5) that is aptly described as "listening to light," Piezoelectric transducers, lock-in amplifiers, and minicomputers play key roles in each of these two experiments. The details of the minicomputerization of our "looking at sound" experiment have been given elsewhere (6). 

Calorimetric techniques have also proved to be useful tools for determining energetic parameters in the photopolymerization process. For instance, photoacoustic spectroscopy (PA) has been used by Chance et al.(8>9) to study the reaction kinetics of 4BCMU. In a PA experiment the heat evolved as a result of a photochemical reaction as well as that due to photon absorption is detected by a microphone located in the gas surrounding the sample or by a piezoelectric transducer attached to the sample. The information extracted from this experiment is the reaction enthalpy for photopolymerization (per chain initiation event). 

The simplest sensing device is a thermistor that can directly measure the temperature changes (97). Thetemperaturechangeswere correlated with the quantum efficiency. A serious deficiency of this method is the slow equilibration time of the thermistor ( 1 sec.). The equilibration time can be considerably reduced by shifting to photoacoustic detection either in the form of a microphone (98) or a piezoelectric transducer (99). These developments follow the success that photoacoustic spectroscopy had in determining absorption spectra of solids (100) and efficiency of photovoltaic devices (101). 

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