Photoacoustic Spectroscopy of Strongly Absorbing Solids: Bismuth Tri-Iodide

We found a need to optically characterize bismuth tri-iodide, BiI₃, which has been used in antireflective thin film coatings on potassium chloride laser windows. We received a 7mm thick, 18mm diameter disc shaped sample of BiI₃ which exhibited a dark gray, glassy, metallic appearance. Several commercial spectrophotometers in the laboratory showed it to be opaque from 0.3μm to 50μm. We therefore decided to see if relevant optical data could be obtained on the sample using an in-house built photoacoustic spectrometer.