Scattered light effects in photoacoustic spectroscopy.

ERDA Ames Laboratory, Iowa State University, Ames, Iowa 50011. Received 19 July 1976. Photoacoustic spectroscopy has experienced considerable interest recently because it allows spectra to be measured for samples with properties not compatible with conventional transmission and reflection techniques. Many of these samples are in the form of powders, filaments, or semisolids and are associated with studies where samples must be measured in an undisturbed state. The photoacoustic spectrum measured for a sample is due to thermal deexcitation of energy states excited by light absorption from an incident monochromatic beam of variable wavelength. Since the light beam is chopped, the sample and adjacent gas are intermittantly heated, causing pressure oscillations proportional to the absorption in the sample cell. These oscillations are sensed by a transducer, and the resulting signal is amplified and measured by a lock-in amplifier. The applicability of the photoacoustic technique to a wide variety of previously difficult sample configurations hs attracted the interest of workers in many fields. For this reason it is important to investigate the dependence of the photoacoustic signal on properties of both the sample and the cell. The signal dependences on sample reflectivity and the absorption coefficient have recently been discussed, and signal saturation has been predicted and observed at large values of the absorption coefficient. In this Letter observation of a scattered light contribution to the signal is reported, and a method for reducing this spurious effect is demonstrated. In conventional transmission measurements, scattered light causes errors in the measurement of absorbances. Likewise, scattered light can contribute significantly to the photoacoustic signal in regions of low optical density and/or high reflectivity, where the signal is small due to low absorption of optical power by the sample. In these regions a certain fraction of light scattered from the surface or interior of the sample reaches the cell walls, where a portion may be absorbed, thus producing a spurious scattered light background component to the signal with a wavelength dependence due to the optical properties of both the sample and cell walls. Measurements on aqueous solutions of Congo Red at varying concentrations provide a means of evaluating the photoacoustic signal as a function of the sample's absorption coefficient. These measurements have clearly shown loss of sensitivity to changes in the absorption coefficient in the low absorption coefficient region due to scattered light. The light source used in the experiment was a nominally 8-10 mW HeCd laser chopped at 50 Hz. In Fig. 1 curves a and c diverge from the theoretically calculatedsignal dependence (slope of one) for the region before the onset of signal saturation, which is seen to occur in the high absorption coefficient region for all curves. Curve b closely fits the expected slope and retains sensitivity for low values of the absorption coefficient. The data in Fig. 1 were obtained using the two cell designs shown schematically in Fig. 2. The signal for curve b was measured with a cell (Cell II) which has transparent UVT Plexiglas walls and quartz windows surrounding the sample, thereby allowing scattered light to exit the cell. Curve a was obtained by placing a thin flat black liner in this same cell to increase sensitivity to scattered light. A substantial fraction