PHOTOACOUSTIC
Vítězslav Otruba


The spectrophone
2010
prof. Otruba
2
In 1881 A.G. Bell proposed a spectrophone “for the purpose of examination of the absorption spectra
of bodies in those portions of the spectrum that are invisible”.

Photoacoustic effect
•The photoacoustic effect is referred to as the occurrence of acoustic effects in the sample under
investigation as a result of its localized periodic heating induced by incident amplitude modulated
light radiation.
•The modulation frequency can take values over a very wide range of 10 to 108 Hz, depending on the
type of substance to be studied and the type of acoustic detector. Modern microphones in
combination with quality electronics can detect acoustic effects corresponding to temperature
changes of the order of 10-6 to 10-7 ° C.
•The phenomenon occurs in gaseous, liquid and solid substances and according to the way in which
heat is distributed from the source, two functional modes are distinguished - thermoacoustic and
thermoelastic.
•
2010
prof. Otruba
3
Výsledek obrázku pro photoacoustic

(Photoacoustic Spectroscopy - PAS)
•The level of acoustic effects depends on the amount of radiation absorbed - by measuring the
photoacoustic effect at different wavelengths of light to determine the absorption spectrum of the
test substance. PAS has some advantages over conventional optical spectroscopy:
•transmitted, reflected or elastically scattered light is not detected and does not interfere with
the measurement itself
•allows to measure highly optically permeable materials (gases) containing a small amount of
absorbing component (10-15 methane in nitrogen)
•Optical absorption can be measured even with high scattering materials (powders, amorphous
substances, gels, colloidal suspensions etc.) or even with optically impermeable substances
•analysis of subsurface layers without sample destruction
•offers the possibility of measuring over a wide range of frequencies with the same detector
2010
prof. Otruba
4

Instrumentation requirements
•Source of intense radiation
•Laser is optimal
•Classic power supplies are also possible (eg. Xe lamp)
•Sensitive detection
•Microphones
•Hydrophones
•Piezo detectors
•Strain gauges
•Fiber detectors
•Strain gauge
•Piezoelectric
•Analysis of output signal of detectors
•FT
•Transition signal analysis
•Correlation functions
•
2010
prof. Otruba
5

Process flow diagram in OA (gas phase)
2010
prof. Otruba
6
Photochemical reactions
Absorption of radiation
Luminiscence
Modulated gas heating
Absorption by cuvette window
Generation of acoustic oscillations
Boundary conditions
Acoustic wave propagation
Detection
processing

Application 1
•Measurement of low absorbance
•Amin ≈ 10-10 cm-1 (pro l = 10 cm), typically cca 10-3 cm-1 (pro l = 1 cm)
•Low concentrations
•Transitions with low probability
•Vibration and rotational spectra of high n
•Trace content detection
•In gas up to ≈ 10-13
•Small size with semiconductor lasers
•Detectors in gas chromatography
•Field inspection devices
•Sensors for process control
•Examples:
•NH3, SO2 ≈ 10-10
•NO2 ≈ 10-11
•CH4 ≈ 10-12
•CO ≈ 10-8
2010
prof. Otruba
7

Application 2
•Absorption from excited states
•Chemical reactions in the gas phase
•Intermediate concentration measurement
•Spectra IR, UV, Raman of intermediates
•Reaction schemes, e.g.:
• CH3I → CH3* +  I
•
•                          2P1/2 ↔ 2P3/2  ?
•Doppler-free spectrometry
•Spectrometry of condensed phases
2010
prof. Otruba
8

Rare Earth Powder Oxide Spectrum
2010
prof. Otruba
9

Absorption spectra of water
•Absorption optoacoustic spectra of water and heavy water measured by dye lasers (solid line) in
comparison with works of other authors.
2010
prof. Otruba
10

Doppler-free photoacoustic spectrometry
•Description :
•1 – Ar+ laser
•2 – single-mode dye laser
•3 – beam splitter
•4 – spectrum analyzer
•5 – reference absorption cell
•6 – spectrometer
•7 – 50% beam splitter
•8 – electric motor
•9 – microphone
•10 – acoustic cuvette
•11 – base frequency
•f1 (751 Hz) + f2  (454 Hz)
•12 – lock-in amplifier
•13 – output
•14 – data evaluation
•
2010
prof. Otruba
11
The frequency equal to the sum f1 + f2 will be generated only if both opposite beams saturate the
same particles, ie particles at zero velocity with respect to the beams.

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2010
prof. Otruba
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2010
prof. Otruba
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Resonant optoacoustic cuvettes
•A – longitudinal resonance
•B – azimuthal resonance
•C – radial resonance
•D – Helmoltz resonator
•
•(A0 perpendicular cross-sectional area of the tube, l0 – tube length)
2010
prof. Otruba
14
A
B
C
D

2010
prof. Otruba
15
Photoacoustic cell applied for temperature-dependent investigations on fatty acids. The temperature
of the water bath was varied between 278 and 350K with a cold finger and two immersion heaters,
while the temperature of the microphone
was kept constant with a cooling/heating device.

Resonant photoacoustic
2010
prof. Otruba
16

Photoacoustic soot sensor
2010
prof. Otruba
17

2010
prof. Otruba
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2010
prof. Otruba
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2010
prof. Otruba
20

Photoacoustic Microscopy - PAM
•Because light radiation is very easy to focus on an area of the order of 10-6 m, it is possible to
perform a photoacoustic phenomenon in microscale and to obtain information about the
two-dimensional distribution of the photoacoustic signal by sample scanning.
•By computer processing of this information, an image can also be created on the screen where
different blackening areas correspond to different intensity of the photoacoustic signal. However,
it is a complex function not only of light absorption in the sample but also of its local
structure, its elastic and thermal properties, and also the morphology and perfection of the sample
surface.
•It always depends on factors influencing not only absorption but also light scattering.
2010
prof. Otruba
21

Principles of PAM
•The amplitude-modulated laser light radiation is focused on a spot of about 1µm in diameter on the
surface of the sample and the sample is scanned in two dimensions. The resulting photoacoustic
signal is detected by a transducer in direct contact with the sample (9) or microphone (6). The
electrical signal is further processed and used to create an image on the monitor screen.
•As a transducer, piezoelectric grinding is usually used, which can work with high modulation
frequencies, allows faster scanning and has less sensitivity to ambient noise.
•Electret miniature microphones with high sensitivity and frequency range up to MHz are used as
microphone.
•
2010
prof. Otruba
22

PAM applications
•The method provides visual information in a microscale similar to conventional optical microscopy
and further allows e.g.:
•to determine the local optical absorption spectrum or its differences in different places of the
surface
•obtain information about local thermal and elastic properties (detection of layered structures on
and under the surface)
•detect photovoltaic processes in semiconductor devices (the presence of short circuits or losses
can be detected in a timely and non-destructive manner)
•investigate photochemical processes
•measure thin film thickness (by analyzing amplitude and phase of photoacoustic signal)
•determine the depth profile of the examined material (by changing the wavelength of the incident
light or by changing the modulation frequency, the depth of the optical penetration and the depth
at which the photoacoustic signal is produced can be changed)
•detect the presence of fluorescent dopants (since fluorescence reduces the PAM signal) and
possibly their absorption band if it is possible to change the wavelength of the modulated
excitation light.
2010
prof. Otruba
23