The Hadamard transform technique has been extended to include photoacoustic detection. The feasibility of Hadamard transform photoacoustic spectrometry in the visible spectral region has been demonstrated. Depth profiling has been illustrated in a layered sample consisting of a thin (20 μm) surface layer of neodymium oxide dispersed in a polystyrene matrix on a thick (2.5 mm) interior layer of holmium oxide dispersed in a polystyrene matrix. Depth profiling may be accomplished by varying the modulation frequency or by phase-sensitive detection at the appropriate modulation frequency.
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