Photoacoustic spectroscopy

An introduction to photoaccoustic spectroscopy.

1. PHOTOACOUSTIC SPECTROSCOPY MOHAMED NIJAS V S9 No:11 I.S.P.

2. CONTENTS Introduction. Physical Mechanisms. Detection of the photoacoustic effect. Photoacoustic spectroscopy. Photoacoustic signal linearization. Advantages. Limitations. Applications. References

3. PHOTO ACOUSTIC EFFECT Phenomenon which deals with the interaction of light (optics) and sound (acoustics), discovered by Brillouin in the year 1922. Actually the formation of sound waves following light absorption in a material sample. The light intensity must vary, either periodically (modulated light) or as a single flash (pulsed light). The light intensity must vary, either periodically (modulated light) or as a single flash (pulsed light). Much of the reported research and applications is concerned with the near ultraviolet/visible and infrared spectral regions. These measurements can be used to determine certain properties of the studied sample.

4. PHYSICAL MECHANISMS. There are several different mechanisms that produce the photoacoustic effect: Photothermal mechanism, Change in the material balance of the sample, Change in the molecular organization. PHOTOTHERMAL MECHANISM: (1) Conversion of the absorbed pulsed or modulated radiation into heat energy. (2) Temporal changes of the temperatures at the loci where radiation is absorbed – rising as radiation is absorbed and falling when radiation stops and the system cools. (3) Expansion and contraction following these temperature changes, which are "translated" to pressure changes.

5. CONTINUED ………... The pressure changes can be sensed by a sensor coupled directly to it. Commonly, for the case of a condensed phase sample (liquid, solid), pressure changes are rather measured in the surrounding gaseous phase (commonly air), formed there by the diffusion of the thermal pulsations. The main physical picture, in this case, envisions the original temperature pulsations as origins of propagating temperature waves ("thermal waves"), which travel in the condensed phase, ultimately reaching the surrounding gaseous phase. If its propagation distance in the condensed phase is not too long, its amplitude near the gaseous phase is sufficient to create detectable pressure changes.

6. DETECTION OF THE PHOTOACOUSTIC EFFECT In applying the photoacoustic effect there exist various modes of measurement. The useful applicable time-scale in this case is in the millisecond to sub-second scale. Light is continuously chopped or modulated at a certain frequency (mostly in the range between ca. 10–10000 Hz) and the modulated photoacoustic signal is analysed with a lock-in amplifier for its amplitude and phase. The photoacoustic signal, obtained from the various pressure sensors, depends on the physical properties of the system, the mechanism that creates the photoacoustic signal, the light-absorbing material, the dynamics of the excited state relaxation and the modulation frequency or the pulse profile of the radiation, as well as the sensor properties.

7. PHOTOACOUSTIC SPECTROSCOPY This spectroscopic technique can b e broadly classified into two, i.e.; for gases and for condensed medium. Photoacoustic Spectroscopy: Gases According to the various gas laws, an increase in the temperature of the gas leads to an increase in the pressure of an isochoric (constant-volume) sample. If the incoming light is modulated — modulation frequencies can vary from single to several thousand hertz — the gas pressure increases and decreases accordingly, creating sound. Varying the wavelength of the incoming light will change the amount of light absorbed, the amount of pressure changes occurring, and the amount of sound produced, and a spectrum of loudness versus wavelength can be produced.

8. CONTINUED ………... The laser beam is sent through the absorber cell. If the laser is tuned to the absorbing molecular transition part of the molecules in the lower level will be exited into upper level by collisions with other atoms or molecules in the cell. these exited molecules may transfer their excitation energy completely or partly into translational, rotational, or vibration energy of the collision partners.

9. CONTINUED ………... When the laser beam is chopped at frequencies Ω < 1/T, where T is the mean relaxation time of the exited molecules, periodic pressure variations appear in the absorption cell, which can be detected with a sensitive microphone placed inside the cell. The output signal S(volt) of the microphone is proportional to the pressure change Δ P induced by the absorbed radiation power ΔW. If saturation can be neglected, the absorbed energy per cycle is given by ­­­­ Is proportional to the density N i [cm -3 ] of the absorbing molecules in level li > , the absorption cross section σ ik , the absorption path length Δx, the cycle period Δt, and the incident power P L . the signal decreases with increasing quantum efficiency η k , unless the fluorescence is absorbed inside the cell and contributes to the temperature rise.  

10. CONTINUED ………... Since the absorbed energy ΔW is transferred into kinetic or internal energy of all N molecules per cm 3 in the photoacoustic cell with the volume V, the temperature rise ΔT is obtained from, ΔW = ½ fVNkΔT Where f is the number of degrees of freedom that are accessible for each of the N molecules at temperature T[k]. If the chopping frequency of the laser is sufficiently high, the heat transfer to the walls of the cell during the pressure rise time can be neglected. From the equation of state, we may obtain that Δp = NkT = (2ΔW)/fv The output signal s from the microphone is then        Where the sensitivity S m of the microphone not only depends on the characteristics of the microphone but also on the geometry of photo-acoustic cell.

11. CONTINUED ………... The mechanism of the photoacoustic effect in condensed samples is not as straightforward as it is for gas samples. in 1973, a scientist noticed a photoacoustic signal apparently coming from the windows of the sample cell, which should have been transparent to the incoming radiation. Instead of being dissipated as heat, the absorbed radiant energy also can be transferred through the solid-state vibrational modes, or phonon modes, of the sample. The motions of these phonon modes are nondissipative (unlike heating), limited only by the size of the sample. A piezoelectric detector physically connected to the sample can detect absorbed energy in this manner. Although piezoelectric detection is about 100 times less sensitive than microphone detectors, it can be preferable for large samples or for samples that do not efficiently convert absorbed light to heat.

12. PHOTOACOUSTIC SIGNAL LINEARIZATION The phase of the photoacoustic signal saturates at higher values of the absorption coefficient This allows calculation of a “linearized” spectrum that has a linear dependence extending to higher values of the absorption coefficient. This reduces the truncation of strong absorbance peaks which are troubling to classical spectroscopists but turn out to actually have little influence when modern chemometrics are use in both quantitative and qualitative analyses. Linearization, however, is very useful in reducing sampling depth and increasing surface specificity

13. CONTINUED ………... Sampling depth, L, depends on the interferometer’s OPD mirror velocity, v, the sample’s thermal diffusivity, D, and the wave number, ѵ, at a particular point in a spectrum

14. CONTINUED ………... L is defined as the reciprocal of the thermal wave decay coefficient, as =(πf/D)1/2 With step-scan FTIRs sampling depth is constant across the spectrum and controlled by the phase modulation frequency, fm , and given by L=(D/π fm ) ½ L only defines the sampling depth if the sample’s absorption coefficient, α, in the spectral region of interest, is low enough to allow light to penetrate deeper than L. If this is not the case, the sampling depth is determined by the decay of light waves rather than of thermal waves and is given by 1/α

15. WHY WE NEED IT??? Considering the photothermal mechanism alone, the photoacoustic signal is useful in measuring the light absorption spectrum, particularly for transparent samples where the light absorption is very small. Dividing the signal spectrum by the light intensity spectrum can give a relative percent absorption spectrum, which can be calibrated to yield absolute values. This is very useful to detect very small concentrations of various materials. Also useful for the opposite case of opaque samples, where the absorption is essentially complete. Very bright light sources — lasers — can be used to detect very tiny concentrations of a particular gas, on the order of parts per trillion.

16. LIMITATIONS Perfect implementation is available only for gaseous sample. Photoacoustic spectroscopy can also be limited because laser light is not very broad in bandwidth; the analyte molecule must absorb some light from the source in order to be detectable.

17. APPLICATIONS One advantage to photoacoustic spectroscopy is that it can be performed on all phases of matter. In calculating the concentrations of trace gases in mixtures, like soot in diesel exhaust or NO x in the atmosphere. Analysis of textile dyes. The photoacoustic effect is used to study biological samples such as blood, skin, eye lenses, tumours, and drug-laced tissues. Several studies are available in which photoacoustic spectroscopy has been used to identify different types of bacteria.

18. REFERENCES Laser spectroscopy: basic concepts and instrumentation. ---------- W Demtroder . Photoacoustic and Photoacoustic Spectroscopy ----------- (R.E. Krieg Publishing Company, Malabar, Florida, 1980 Performance and Selected Applications of an Acousto-Optic Spectrometer ----------- Martin Hühne , Ursula Eschenauer , and Heinz W. Siesler (OSA PUBLISHING)

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