Setup for Resonant Photoacoustic Spectroscopy

In Fig. 5 a typical setup for resonant photoacoustic spectroscopy as used in the authors laboratory is shown. The radiation of a cw laser is intensity modulated by a mechanical chopper of high precision. An electro-optic modulation device may also be employed or the laser beam is modulated directly by modulation of its power supply. As already discussed, vibrational excitation with an IR laser, for example, causes a modulated pressure change in the resonator via fast vibrational relaxation. This acoustic signal is detected with a microphone, because these devices provide the highest sensitivity. Detectors employed in calorimetry to measure the heating of a mpte such as thermistors or thermophi are less sensitive and p( sess a slower rise 

Setup for resonant photoacoustic spectroscoj with a cylindrkal resonator, a He—Ne laser, and simultaneous recording of the chopping frequency variation and the acoustic signal 

The acoustic resonator must be constructed with great care. The resonant cavity should possess a nearly ideal metry. Perturbations caused by windows, gas inlet, and microphone should be carefully minimized. The acoustic resonator is characterized by the quality factor Q, which is defined as the ratio of the resonance frequency to the frequency bandwidth between halfpower points. The amplitude of the microphone signal is l/ /2 of the maximum amplitude at these points, because the energy of the standing wave is proportional to the square of the induced pressure. The acoustic Q should be independent of the level of excitation. In a high quality resonator, Q s of several thousand may be achieved. 

Many laser experiments have been performed with cylindrical resonators (see Fig. 6) possessing a size of 10-15 cm in length and diameter For these 

For a spherical resonator no resonance experiments with laser excitation have been reported so far. However, acoustic excitation of radial modes with a second microphone was accomplished just recently and, as expected, very high Q factors were obtained in a high quality spherical resonator The goal of these experiments was the measurement of thermophysical properties with high precision. A spherical resonator is also an interesting candidate for photoacoustic investigations. 

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