One useful aspect of photoacoustic spectroscopy of gases is that very bright light sources — lasers — can be used to detect
very tiny concentrations of a particular gas, on the order of parts per trillion. This makes photoacoustic spectroscopy very useful in following the concentrations of trace gases in mixtures, like soot
in diesel exhaust or NOx in the atmosphere. 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. Saturation effects can also cause problems
Photoacoustic Spectroscopy: Condensed Phases
The mechanism of the photoacoustic effect in condensed samples is not as straightforward as it is for gas samples. For example,
in 1973, Parker (7) noticed a photoacoustic signal apparently coming from the windows of the sample cell, which should have
been transparent to the incoming radiation. Further (and this might be unique to this particular form of spectroscopy), the
exact mechanism of spectrum production depends on the type of detector used. The commonly accepted mechanism for the photoacoustic
effect is called RG theory, after its developers Rosencwaig and Gersho (8,9). The main source of the acoustic wave is the
repetitive heat flow from the absorbing condensed-phase sample to the surrounding gas, followed by propagation of the acoustic
wave through the gas column to microphone-based detector.
However, a photoacoustic signal also can be detected piezoelectrically. 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.
One advantage of photoacoustic spectroscopy is that it is nondestructive to the sample; the sample does not have to be dissolved
in some solvent or embedded in a solid-state matrix. Samples can be used "as is." Another advantage is the potential for performing
depth profiles of analytes in optically transparent media. Figure 3 shows how this is possible: essentially, the slower the
modulation rate, the farther the heat can diffuse before the incoming light is cut off. Figure 4 shows examples of infrared
photoacoustic spectra of poly(vinylidene chloride) films using different modulations, showing differences in the spectra due
to differences in penetration depth.
Figure 3: How photoacoustic spectroscopy can perform depth profiling.