# Photoacoustic Spectroscopy

Photoacoustic spectroscopy (PAS) uses acoustic waves produced from materials which are exposed to light to measure its concentration. PAS is unique in that it combines heat measurements with optical microscopy. Gases have been the ideal samples used but more research has been increasing gradually to use PAS efficiently for solid and liquid samples. When measuring a sample, it takes measurements directly through looking at the internal heat instead of the effects of the light on the surroundings. This makes PAS highly accurate and useful for sensitive detectors. Interest sparked after Alexandar Graham Bell wrote about his findings when he discovered the acoustic effect in 1880.

Bell accidently stumbled onto this effect as he was experimenting with this invention of the photophone. He noticed that a clear acoustic sound formed whenever the sunlight hitting the sample was interrupted. Bell realized that the absorption of light by the material caused the sound wave which is now known as the photoacoustic effect. The ultraviolet and infrared spectra also worked and experimented by Bell. However, the apparatus was not sophisticated enough to show any promise in accurate results and the development of PAS was put on a halt. It was not until the introduction of more sophisticated equipment did the development of PAS start again. Today, most set ups use other sources than sunlight and microphones instead of the ear to measure the waves emitted from a material accurately.

## Theory

Generally, when a material absorbs light, there are many paths the energy can go on. Light is always conserved as shown by the equation,

$1 + \alpha +T+R \label{1}$

where is $$\alpha$$ the absorbance, $$T$$ is the transmittance, and $$R$$ is the reflectance. Light that hits the sample must either be absorbed, transmit through the material, or reflect off of the material. PAS focuses on the light path that is absorbed as that is where heat is released. As light strikes the sample, the photons are absorbed and electrons are excited from the energy created. This energy is then released as heat and as the heat expands, acoustic waves are formed. This process is explained below and shown in figure 1.

As light is absorbed electrons are excited either electronically or vibrationally. When looking at electronic excitation, electrons jump to a higher energy level. As they drop back to its ground state, the extra energy is given off as heat. Collision deactivation, another form of heat formation, involves the colliding of atoms. The collision of atoms give off energy in the form of heat. However in the case of electronic excitation, the energy can also be dissipated through chemical reactions or radiative emissions as seen in figure 1. Chemical reactions involve any reactions with its surroundings as energy is used to initiate those reactions. Radiative emissions involve the energy given off as photons, rendering it useless for PAS which requires heat. This reduces the amount of heat formed as energy is spent somewhere else. It is possible to have chemical reactions form heat, but only a portion of the energy absorbed goes towards heat. On the other hand, with vibrational energy, chemical reactions and radiative emissions have little effect. The lifetimes of the vibrations are long enough prevent chemical reactions and radiative emissions from interfering. Therefore, the atoms have as much time as needed to complete the process of collision deactivation which will effectively use the full amount of energy to transfer to heat.

With the formation of heat, thermal expansion also occurs. The expansion of heat creates localized pressure waves and in turn, can be measured as an acoustic wave. However, as with the case of the formation of energy, heat can also be lost through the surroundings. Heat diffusion lowers the temperature around the emitted energy source which in turn lessens the pressure fields. With acoustic waves sent after every pulse of light, a sensor can then measure the waves. By adjusting the wavelength of each pulse of light, the corresponding acoustic wave can be measured and plotted to form a spectrum of the material.

With the advancement in technology, amplifiers, light sources, and sensors have significantly improved. Figure 3 shows a common set up of the inside of a photoacoustic spectrometer. Light sources typically use infrared lasers or wire filaments such as tungsten that produce high intensities of light. To send pulses of light to hit the sample, the light source is either turned off and on to produce the pulsing effect or a rotating disc with openings to control the pulses of light going through. A mirror directs the pulses of light to hit a set of filters, which can change to alter the wavelength of the light hitting the sample.

Once the light passes through the filter, it hits the contact window, which is where the sample is held. Two microphones are placed inside to pick up the acoustic waves and is sent to measure its electrical signal. Different wavelengths are tested and a spectrum for the sample is created.

## Importance of Photoacoustic Spectrometry

#### Differences in PAS

Though PAS may seem similar to other infrared techniques such as Fourier transform infrared spectroscopy (FTIR), it has many unique aspects. PAS does not measure the effect relative to the background but directly from the sample, making it extremely accurate. Samples with multiples gases can be singled out and measured. Samples can also be in tiny amounts as the sample case is small. However, it can currently only measure samples as small as a few milliliters, which is still small compared to other techniques.

#### Applications

Because of the high sensitivity and accuracy of PAS, it is ideal to use it in gas detectors. Gas levels in the atmosphere can be measured and provide details on any dangers of rising toxic gases. It is also useful in determining the materials of an unknown samples. Each material has its own unique spectrum and by observing the acoustic waves produces, one can match the waves to specific profiles of materials. PAS is also used for high resolution imaging by analyzing the topography of the sample. Using the topography and the profiles of electric signals, one can create an image with shaded colors to indicate the different materials. The cost of creating these devices have decreased and is gradually being used more widely in gas detectors and sample analysis in labs.

### Questions

1. Light hits a sample of aluminum. The data collected shows that the sample had a transmittance 0.12 and reflectance value of 0.8196. Assuming light is conserved, what is the absorbance of the material?
2. Does vibrational or electronic excitation of electrons produce more heat? Why is that?
3. Why does PAS require pulses of light instead of a continuous steady source of light to hit the sample?

1. Using equation 1 stated above, light must be conserved and so 1-0.12-0.8196= α

α = 0.0604

2. Vibrational excitation of electrons produce better heat as all the energy from the absorbed light transfers into heat. This is because the vibrational lifetimes of the atoms is long enough so that the transfer of energy to heat is not interrupted by other processes.

3. To obtain a signal, it must come in the form of a wave. Having a constant source of light will prevent waves from forming and the spectrum will only show up as a straight line. The point of the pulsing light is to have the material absorb the energy, convert it to heat, send out the acoustic waves, and let it rest before allowing it to absorb another amount of energy.

## References

1. Hummel, Rolf E. Electronic Properties of Materials: An Introduction for Engineers. Berlin: Springer-Verlag, 1985. Print.
2. Miklos, Andras, Stefan Schafer, and Peter Hess. "Photoacoustic Spectroscopy, Theory."AccessScience (n.d.): n. pag. Academic Press. Web. 24 Nov. 2015.
3. Svelto, Orazio. "Nonradiative Decay and Energy Transfer." Principles of Lasers. 4th ed. New York: Plenum, 1976. 50-56. Print
4. Zharov, V. P., and V. S. Letokhov. "Optoacoustic Spectroscopy of Condensed Media." Laser Optoacoustic Spectroscopy. Berlin: Springer-Verlag, 1986. 45-58. Print.

## Contributors

• Kevin Quan, Materials Science and Engineering Department, UC Davis undergraduate