Dual beam photoacoustic infrared spectroscopy of solids using an external cavity quantum cascade laser

Quantum cascade laser-based instrumentation for dual beam photoacoustic (PA) spectroscopy is described in this article. Experimental equipment includes a 4.55 μm (2141–2265 cm⁻¹) continuous wave external cavity quantum cascade laser (EC-QCL), two gas-microphone PA cells, and two lock-in amplifiers. Correction for the time and wavenumber dependence of the laser output is effected through real-time division of the PA signals derived from the sample and reference channels. Source-compensated mid-infrared absorption spectra of carbon black powder and aromatic hydrocarbon solids were obtained to confirm the reliability of the method. Absorption maxima in the EC-QCL PA spectra of hydrocarbons are better defined than those in Fourier transform infrared spectra acquired under similar conditions, enabling the detection of several previously unknown bands.