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  1. 1. Photoacoustic sPectroscoPy By rajesh M KaMble T. P. No. 27
  2. 2. History  Photoacoustic effect discovered by Alexander Grahm Bell in 1880  Bell found that when light was focused on to thin diaphragm, sound was emitted  Bell also studied the sounds produced by the irradiation of various solid samples in a brass cavity sealed with a glass window
  3. 3. The original experiment carried out by Alexander Bell
  4. 4. Introduction  PAS or Optoacoustic spectroscopy was developed in 1973 which provides UV, Visible and IR absorption spectra of solids, semisolids and turbid liquids  Obtaining of spectra for above kind of samples by ordinary methods is usually difficult because of light scattering and reflection  It is non-destructive technique  Minimal or no sample preparation
  5. 5.  Applied for opaque and scattering samples  Used for qualitative and quantitative experiments  Used to detect defects on the surface of thin films
  6. 6. The Photoacoustic effect  PAS is based upon a light absorption effect  In PAS the gas to be measured is irradiated by a chopped beam of light of a pre-selected wavelength  The gas molecules absorb some of the light energy and convert it in to an acoustic signal which is detected by a microphone  If the frequency of the light coincides with an absorption band of the gas in the cell, then the gas molecules will absorb part of the light
  7. 7.  The higher the concentration of gas in the cell, the more light will be absorbed  As the gas absorbs energy, it is heated and therefore expands and causes a pressure rise  As the light is chopped, the pressure will alternately increase or decrease and an acoustic signal is thus generated  Only the absorbed light is converted to sound  The acoustic signal is detected by microphone  The electrical output signals from the microphone are added in an amplifier before they are processed
  8. 8. Photoacoustic effect in solids  In PAS studies of solids, the PAS effect is observed by from periodic heat flow from the solid to the surrounding gas  The periodic heat flow produces pressure fluctuation in the gas of the cell and are detected by the microphone  The power of resulting sound is directly related to the extent of absorption of light by solid  The analog signal from microphone recorded as a function of wavelength of incident light  The radiation reflected or scattered by the sample has no effect on microphone and thus does not interfere
  9. 9. Possible PA generating mechanisms
  10. 10. Conventional PAS setup
  11. 11. Instruments  A single beam PAS the spectrum from the lamp is first recorded digitally followed by the spectrum for the sample  The stored lamp data then used to correct the output from the sample for variations in the lamp output as a function of wavelength  Double-beam instrument is equipped with a pair of matched cells (and transducers), one contains a sample and the other reference material such as finely divided carbon
  12. 12. Applications  Bulk studies : - PAS provide optical data for solids which are not highly reflective, highly opaque or highly scattering (e.g. Insulator, semiconductor & metallic systems) -Many solid & semi solid biological systems can be studied by PAS  Surface studies  De-excitation studies
  13. 13. UV/visible PA spectroscopy  PAS permits spectroscopic studies of blood without separation of blood cells, protein & lipid molecules  The whole blood does not yield satisfactory spectra by conventional spectroscopy because of highly scattering properties of large molecules PAS spectra of smears of blood and blood components
  14. 14. Non-biological studies  Spectrum: (a) PAS of Cr2O3 powder in the region 200-900 nm (b) Optical absorption spectrum on a 4µ thick Cr2O3 crystal (c) Diffuse reflectance spectrum on Cr2O3 powder  The two crystal field bands of Cr3+ ion at 600 & 460 nm are clearly resolved in PA spectrum as they are in absorption spectrum of Cr2O3 crystal PAS spectra of Cr2O3
  15. 15. Surface studies  Adsorbed & chemisorbed molecular species on the surfaces of metals, semiconductors and insulators can be studied by PAS  PAS offers a simple & highly sensitive means for performing nondestructive compound identification directly on the TLC plates  Conventional spectroscopic techniques are unsuitable because of the opacity & light scattering properties of silica gel adsorbent on the TLC plates The compounds are: (A) p-nitroaniline (B) benzylidene acetone (C) salicylaldehyde (D) 1-tetralone and
  16. 16. De-excitation (Fluorescent)studies  PA effect measures non-radiative de-excitation processes in a system after it has been optically excited  This selective PAS technique applied to study of florescent (or phosphorescent) & photosensitive materials  Fluorescent Ho2O3: Ho3+ have strong fluorescent energy levels & these tend to de-excite through the emission of photon rather than phonon or heat excitation PAS spectra of Ho2O3
  17. 17. References 1. Principles of Instrumental Analysis, 5th Edn., Skoog and West 2. Photoacoustics and Photoacoustic Spectroscopy, Allan Rosencwaig (Chemical Analysis , Vol.57) 3. Rosencwaig A. Photoacoustic Spectroscopy: A New Tool for Investigation of Solids, Anal. Chem. 1975, 47(6), 592 A-604 A.
  18. 18. THANK YOU