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Photoacoustic spectroscopy (PAS) is a tool to study thermal emission resulting from non-radiative relaxation followed by absorption of radiation. PAS was studied primarily by Alexander Graham Bell, John Tyndall, Wilhelm Rontgen and Lord Rayleigh in 1884. For the history of PAS, was not until 1975 that photoacoustic spectroscopy started to be used as a wide range of different applications. This technique provides the following main advantages over the known types of spectroscopy.
• Detection and identification of samples in ppm or ppb concentrations, samples ranging from solids to gases.
• Non-destructive method of detecting samples
• Studies of the optical and thermal properties of the sample at lower concentration,
• Providing
The heat released from sample material due to absorption of laser radiation
2. Generation of acoustic and thermal wave from the sample material
3. Determination of PA signal in the detector
HEAT RELEASE IN THE SAMPLE MATERIAL:
The interaction of light with the material cause a series of effect namely excitation in the rotational, vibrational, electronic energy levels. Then from excited state, it loses energy by radiation process like spontaneous emission, stimulated emission; and by non-radiative processes which gives off heat energy. The heat released by the process can be explained by a rate equation. It is generally an assumption that the absorption of the photon is expressed by a linear relaxation process, so the heat released can be determined by solving the rate equation. The heat released can be delayed by some gases mixtures when the excess of energy is somehow taken out by collisions to a long lifetime of other molecules. In cases of very short laser pulses there is a possibility of optical saturation, in that case the heat released is no more a linear function of the intensity of light and the absorption coefficient.

Fig 1: Elementary process occurring during generation of PA
In pulsed mode two parameters are important to learn firstly the transit time of the sound through the heated volume given by τs and the response time of a PA detector denoted by τd. Generally the value of τs is in the μs range, because the laser beam diameter usually is in the range of few milli-metres. The outward propagating wave front is usually detected by high-frequency pressure sensors. In regular practise, the time evolution of the signal is dependent on the sensitivity of the

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