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The Influence of an Inactive Layer on the Photoacoustic Spectra. Mirosław Maliński Faculty of Electronics Technical University of Koszalin Poland

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Introduction n There are two main measuring methods of the photoacoustic or photothermal spectra: the microphone and piezoelectric methods. n There are two main models of the physical structure of the sample used in the field of the PHA and PPT spectroscopy: a single layer model & the inactive layer one. n The inactive layer influences strongly the experimental PHA and PPT spectra. n The inactive layer influences the frequency characteristics too. n The interpretation of the spectra depends on the physical model of the sample.

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Presentation of the models of samples. A single layer sample ( an ideal case). All thermal and optical parameters are the same in the whole volume of the sample - the sample is homogeneous. A sample with an inactive layer.( a real sample) The uppermost layer of the sample exhibits a lower thermal conductivity and a thermal diffusivity. The thermal reflection coefficient from the layer to the sample R = -1. Optical parameters are the same.

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The main features of the microphone and piezoelectric measuring methods n The microphone signal is proportional to the temperature of the surface ( front or rear of the sample. n The piezoelectric signal is proportional to the temperature spatial distribution in the sample. n As a consequence: the inactive layer influences in a different way the amplitude and phase spectra in the microphone and in a piezoelectric measuring methods.

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The idea of the inactive layer R=1 R= -1

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Theoretical PPT spectra of silicon. (an ideal sample) M.Maliński Phys. Stat. Sol (a), 198 (1), 169,(2003)

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The PPT experimental spectra of silicon l=0.07 cm, f= 200 Hz

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Experimental PPT spectra of silicon M.Maliński, A.Memon, T.Ikari Archives of Acoustics 28(2),139, (2003)

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Experimental PPT spectra of silicon

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Explanation

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Temperature distributions Low absorption region High Absorption region

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PHA phase spectra PHA phase spectra of Zn 1-x-y Mg y Be x Se M.Maliński, L.Bychto, S.Łęgowski, J.Szatkowski, J.Zakrzewski Analytical Science 17,133,(2001)

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PHA amplitude spectra PHA amplitude spectra of Zn 1-x-y Mg y Be x Se

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Frequency amplitude PPT characteristics. (amazing) Cd 0.49 Mg 0.51 Se d=0.15 cm, alfa=0.04 cm 2 /s, E exc =3.7 eV Theoretical curves for R=1,0,-1. Crosses -experimental results. Theoretical curves: solid R=1, dash R= 0, dots R= -1, d= 0.0015 cm.

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Frequency amplitude PPT characteristics. (amazing) CdSe d=0.108 cm, alfa=0.02 cm 2 /s, E exc =3.7 eV Theoretical curves for R=1,0,-1. Crosses -experimental results. Theoretical curves: solid R=1, dash R= 0, dots R= -1, d= 0.0015 cm.

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Temperature distribution formula used for the computations derived with the thermal wave interference method. M.Maliński Archives of Acoustics 27(3), 217,(2002).

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CONCLUSIONS The inactive layer strongly changes the piezoelectric and photoacoustic spectra and frequency characteristics. In the case of the piezoelectric spectra it influences mainly the amplitude spectra in the high absorption region. In the case of the photoacoustic spectra it influences mainly the phase spectra in the low absorption region. The formulae for the PHA and PPT signals describing the influence of an inactive layer are presented and discussed. The full analytical model is under construction now.

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