Photoacoustic/FTIR spectroscopy with a step-scan interferometer

Limitations of rapid scan PAS measurements can be overcome with a step-scanning interferometer operated in a phase modulation mode. Phase modulation has the desirable effect of modulating all the wavelengths at the same Fourier frequency. This results in an even depth of penetration into a sample across the spectral range observed. Use of a phase demodulator allows for the detection of phase information from the photoacoustic signal rising from the sample. Collection of the in-phase (0 degree phase angle) and in-quadrature (90 degree phase angle) signals allows for the calculation of phase angle information at any chosen angle. This information can be used to produce spectra of a surface and a bulk component in a sample in a single experiment.