V. GREGORIOU

Polaroïd Corporation

The origins of Photoacoustic Spectroscopy (PAS) date back to the discovery of the photoacoustic effect by Alexander Graham Bell in 1880. Bell found that when light was focused onto thin diaphragms, sound was emitted. In latter experiments, Bell studied the sounds produced by the irradiation of various solid samples in a brass cavity sealed with a glass window.

Practical use of the photoacoustic effect for condensed phase materials had to wait for advances in instrumentation and theory. In 1973, PAS was rediscovered simultaneously by A. Rosencwaig at Bell Laboratories and by A. G. Parker at Johns Hopkins University. A general theory for the photoacoustic effect was developed by Rosencwaig and Gersho and is commonly referred to as the RG Model.

Photoacoustic spectroscopy is now commonly used in the analysis of a variety of materials. It is a non-destructive technique that is applicable to almost all types of samples. It offers minimal or no sample preparation, the ability to look at opaque and scattering samples, and the capability to perform depth profiling experiments. PAS can be used for both qualitative and quantitative analysis. In particular, depth profiling experiments are also useful for the characterization of surface-coated and laminar materials and for studies of weathering, aging, curing, and the diffusion of species into or out of a polymer matrix.

Introduction to Theory of PAS

In PAS the transformation of an optical event to an acoustic one takes place. Initially, modulated light is absorbed by the sample located in a sealed cell. The non-radiative decay of this absorbed light produces a modulated transfer of heat to the surface of the sample. This modulated thermal gradient produces pressure waves in the gas inside the cell that can be detected by the attached microphone. This microphone signal, when plotted as a function of wavelength, will give a spectrum proportional to the absorption spectrum of the sample. One main advantage of PAS is the ability to get information about the depth in the sample of the absorption. The amount of the sample contributing to the PA signal is proportional to the thermal diffusion depth. This thermal diffusion depth m, is inversely proportional to the modulation frequency f. For example, for a typical organic polymer, a high modulation frequency will probe a thin layer (f = 1 KHz; m ~ 4 mm,) while at a low modulation frequency, a much thicker layer is probed (f = 100 Hz; m ~ 16 mm).

Figure 1 shows a depiction of the PA signal generation as it relates to depth profiling. The model sample illustrated in this figure has a thermally thin surface layer (thickness << m) on a bulk substrate. After the light has been absorbed, the heat has to diffuse from the point of absorption to the surface of the sample to be detected. Since this thermal diffusion is a slow process relative to the light absorption and non-radiative decay, an absorption in the bulk will have a phase lag between the time of absorption and the thermal signal. However, a surface absorption should not have a phase lag since the heat doesn't have far to travel to generate the detected pressure change in the transfer gas.

Experimental Considerations

For PAS, a FT-IR spectrometer capable of operating in both continuous scan mode and in step-scan mode can be used. As will be shown in the next section, the step-scan mode works very well for depth-profiling experiments. A photoacoustic cell is generally available as an accessory from most major suppliers of FT-IR instruments. Helium gas is used as the transfer medium in the cell and to purge water vapor and carbon dioxide. A 60% carbon black-filled polymer is often used as a reference sample.

Continuous Scan Mode

Most commercially available FT-IR spectrometers utilize the continuous-scan mode of operation, where the moving mirror is scanning at constant velocity. This type of scanning works very well for routine measurements. In the continuous-scan mode of interferometry the laser fringe counter is used to sense the accuracy of the scanning velocity. If a deviation is sensed, correction signals are generated that assure the proper operation (constant velocity). The consequence of this mode of operation is that each infrared wavelength (l), is modulated at its own particular Fourier frequency, given by the following Equation 1:

f(l) = 2v/l (1)

where v is the mirror velocity. Continuous-scan FT-IR is the technique of choice when static spectral properties are determined. Co-addition of successive scans increases the signal-to-noise ratio (S/N) by a factor proportional to Ö t, where t is the time that the signal is averaged at each collection point.

Step-Scan Mode

In step-scan FT-IR data are collected while the retardation is held constant or is oscillated about a fixed value. Two types of experiments are possible with step-scan interferometry. One type is the time-domain or time resolved experiments where data are collected as a function of time at each mirror position. The other type of experiment is the so-called frequency-domain or synchronous modulation experiments. In these experiments, either amplitude modulation (AM) or phase modulation (PM) can be used to modulate the intensity of the infrared radiation in order to generate step-scan interferograms.

For PAS, phase modulation is used. Phase modulation is accomplished by oscillating one of the interferometer mirrors about each step position at a specified amplitude and frequency. To detect the phase modulated signal, either a lock-in amplifier or a digital signal processor (DSP) is required. The advantages of step-scan operation include the ability to apply virtually any modulation frequency to the infrared radiation and to carry out multiple modulation experiments. Since the frequency of modulation is not a function of any retardation velocity (e.g., mirror scan speed), there is no dependence on radiation wavelength. An advantage of lock-in amplifier or DSP detection is the easy retrieval of the signal phase. This is possible due to the fact that the beamsplitter (instrumental) phase is identical for the in-phase and quadrature (90° out of phase) components of the signal. These components are easily obtained as the in-phase (I) and quadrature (Q) outputs of a two-phase lock-in amplifier. As a result, not only the magnitude M, but also the phase F can be easily obtained by following Equations 2 to 5:

(2)

F = arctan(Q/I) (3)

I = M cosf (4)

Q = M sinf (5)

 

 

 

The advantages of Step-scan vs. continuous Scan for PAS

In step-scan FT-IR spectroscopy, the same modulation frequency can be applied to the entire spectral range. This is particularly useful for photoacoustic (PA) detection since, in the absence of saturation, the PA response at all wavelengths in step-scan FT-IR PA spectroscopy will correspond to the same depth in the sample. In a typical continuous-scan FT-IR PA spectrum each wavelength corresponds to a different sampling depth. This results in inefficient probing at shorter wavelengths.

A step-scan FT-IR PA spectral depth profile can be obtained by changing the modulation frequency for different scans. Alternatively, depth information can be obtained from the phase of the PA signal. The step-scan mode of data collection permits easier access to the PA phase than does the continuous-scan mode. The combined use of modulation-frequency variation and analysis of the phase allows a wider range of depths to be probed. This type of spectrally resolved depth information is particularly useful when applied to polymeric materials with special surface properties.

Depth Profiling

One method for depth profiling uses the phase of the PA signal to distinguish between depths of different absorptions. Theoretically, this can be done two different ways, by using the phase spectrum or by looking at different components of the signal by detecting at different phases with respect to the infrared light modulation. It has been shown theoretically that the signal from a weakly absorbing thermally thin layer should be separated by 45° from the signal of the weakly absorbing substrate.

In order to eliminate instrumental phase contributions, a reference phase must be established. This is done by placing a complete absorber into the PA cell and rotating the phase of the lock-in until all of the signal is in one channel of the lock-in, (e.g. in the quadrature channel, Q). By replacing the reference with the real sample and collecting the in-phase (I) and quadrature (Q) spectra, the bulk signal (B/Ö 2) should be in one channel (I), while information proportional to the surface signal (S+B/Ö 2) should be in the other channel (Q). As a consequence, the surface signal can be found by rotating the original interferograms by 45o and obtaining S/Ö 2. Figure 2 illustrates the relationship between the various components of the step-scan photoacoustic signal.

Figure 2. Photoacoustic phase relationships for a weakly absorbing thermally thin layer.

Examples of Depth Profiling

The sub-micron resolution capability of the technique has been demonstrated for a coated system having a sub-micron layer as the top layer. The system under study was a micron thick multi-layered structure of a mixture of acrylic polymers coated on a poly (ethylene terephthalate) (PET) base. The base is several tenths of microns thick, substantially thicker than that of the coated layer.

Figure 3 shows the evolution of the photoacoustic signal as a function of the degree of rotation for the experiment performed with 750 Hz phase modulation. The evolution of the overtone of the carbonyl band at 3428 cm-1 that belongs to the PET substrate can be clearly followed as we move to phase delays that correspond to deeper depths. In addition, it is also easy to see the appearance of the C-H aromatic stretching modes above 3000 cm-1, as the phase rotation angle increases. These bands were due to the PET base and, as expected, their intensity increased as the overall signal was dominated by the contribution of the polyester substrate. These results demonstrate the ability of technique to completely separate the IR signatures of the top layer from the bulk material.