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Photoacoustic Spectroscopy

Volume 21, Issue 9, pp. 14-16

David W. Ball
There is a form of spectroscopy known as photoacoustic spectroscopy. As with many forms of spectroscopy, its name is descriptive. The prefix "photo" makes sense for a form of spectroscopy, but "acoustic"? Doesn't that mean "relating to sound"? Is there really a form of spectroscopy that uses sound?

Yes, there is.

Figure 1: Number of articles published using the key phrase "photoacoustic spectroscopy" since the early 1970s.
In 1880–1881, Alexander Graham Bell (1–3) found that when a thin disk was exposed to mechanically chopped sunlight, sound was emitted. In addition, he noted a similar effect when infrared or ultraviolet light was used. A plot of the loudness of the sound versus, for example, the wavelength of the light used, is called a photoacoustic spectrum. According to Haisch and Niessner (4), this effect essentially was forgotten until researchers led by Allen Rosencwaig (ironically) at Bell Labs rediscovered the behavior and provided a theoretical basis (5). To indicate the growth of the field, Figure 1 shows the number of publications listed by the Chemical Abstracts Service under the subject "photoacoustic spectroscopy".

Photothermal Techniques

Photoacoustic spectroscopy is part of a class of photothermal techniques, in which an impinging light beam is absorbed and alters the thermal state of the sample. This "thermal state" can manifest itself as a change in temperature, density, or other measurable property of the sample. One method of detection is to experimentally measure the temperature or density of the absorbing material. This is referred to as thermometric detection.

However, if the incoming light is modulated, the absorbing sample warms and cools in a cycle. If the cycle is so fast that the sample does not have time to expand and contract in response to the modulated light, a change in pressure develops. This pressure "wave" can lead to the production of a sound wave. These sound waves can be detected by a sensitive microphone, piezoelectric devices, or optical methods (for example, deflection of highly collimated light reflecting from the surface). These techniques are more properly called photoacoustic techniques. It has been argued that photo-acoustic spectroscopy is as much a type of calorimetry as it is spectroscopy (5).

Photoacoustic Spectroscopy: Gases

Figure 2: General experimental setup for performing photoacoustic spectroscopy on a gas.
One advantage to photoacoustic spectroscopy is that it can be performed on all phases of matter. Figure 2 shows a general setup for the photoacoustic spectroscopy of a gas sample. When a species absorbs some of the incoming light, one of several mechanisms of de-excitation is intermolecular colliding, which ultimately leads to increases in translation energy of the gas particles — that is, heating. 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.

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