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Abstract:
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Photothermal effect refers to heating of a sample due to the absorption of
electromagnetic radiation. Photothermal (PT) heat generation which is an example of
energy conversion has in general three kinds of applications. 1. PT material probing
2. PT material processing and 3. PT material destruction. The temperatures involved
increases from 1-. 3. Of the above three, PT material probing is the most important
in making significant contribution to the field of science and technology.
Photothermal material characterization relies on high sensitivity detection techniques
to monitor the effects caused by PT material heating of a sample. Photothermal
method is a powerful high sensitivity non-contact tool used for non-destructive
thermal characterization of materials. The high sensitivity of the photothermal
methods has led to its application for analysis of low absorbance samples. Laser
calorimetry, photothermal radiometry, pyroelectric technique, photoacoustic
technique, photothermal beam deflection technique, etc. come under the broad class
ofphotothermal techniques. However the choice of a suitable technique depends upon
the nature of the sample, purpose of measurement, nature of light source used, etc.
The present investigations are done on polymer thin films employing photothermal
beam deflection technique, for the successful determination of their thermal
diffusivity. Here the sample is excited by a He-Ne laser (A = 6328...\ ) which acts as
the pump beam. Due to the refractive index gradient established in the sample surface
and in the adjacent coupling medium, another optical beam called probe beam (diode
laser, A= 6500A ) when passed through this region experiences a deflection and is
detected using a position sensitive detector and its output is fed to a lock-in amplifier
from which the amplitude and phase of the deflection can be directly obtained. The
amplitude and phase of the signal is suitably analysed for determining the thermal
diffusivity.The production of polymer thin film samples has gained considerable
attention for the past few years. Plasma polymerization is an inexpensive tool for
fabricating organic thin films. It refers to formation of polymeric materials under the
influence of plasma, which is generated by some kind of electric discharge. Here
plasma of the monomer vapour is generated by employing radio frequency (MHz)
techniques. Plasma polymerization technique results in homogeneous, highly
adhesive, thermally stable, pinhole free, dielectric, highly branched and cross-linked
polymer films. The possible linkage in the formation of the polymers is suggested by
comparing the FTIR spectra of the monomer and the polymer.Near IR overtone investigations on some organic molecules using local mode
model are also done. Higher vibrational overtones often provide spectral
simplification and greater resolution of peaks corresponding to nonequivalent X-H
bonds where X is typically C, N or O. Vibrational overtone spectroscopy of
molecules containing X-H oscillators is now a well established tool for molecular
investigations. Conformational and steric differences between bonds and structural
inequivalence ofCH bonds (methyl, aryl, acetylenic, etc.) are resolvable in the higher
overtone spectra. The local mode model in which the X-H oscillators are considered
to be loosely coupled anharmonic oscillators has been widely used for the
interpretation of overtone spectra. If we are exciting a single local oscillator from the
vibrational ground state to the vibrational state v, then the transition energy of the
local mode overtone is given by .:lE a......v = A v + B v2
• A plot of .:lE / v versus v will
yield A, the local mode frequency as the intercept and B, the local mode diagonal
anharmonicity as the slope. Here A - B gives the mechanical frequency XI of the
oscillator and B = X2 is the anharmonicity of the bond. The local mode parameters XI
and X2 vary for non-equivalent X-H bonds and are sensitive to the inter and intra
molecular environment of the X-H oscillator. |