1. Photoacoustic
sPectroscoPy
By
rajesh M KaMble
T. P. No. 27

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. The original experiment carried out by Alexander Bell

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.  Applied for opaque and scattering samples
 Used for qualitative and quantitative experiments
 Used to detect defects on the surface of thin films

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.  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. 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. Possible PA generating mechanisms

10. Conventional PAS setup

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. 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. 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. 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. 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. 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. 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. THANK YOU