Mid-IR absorption made easy: Optical Photothermal Spectroscopy

Understanding the sub-micron chemical composition of materials and surfaces of the Solar System is essential to unraveling their formation and evolution. While bulk composition properties are already invaluable data, only the determination of the mineral heterogeneity of the surface material of planetary bodies at the sub-micron level allows for the investigation of their formation processes and environment. In addition, the search and inventory of organic materials also requires high spatial resolutions during sample analysis, as the mineral-organic associations at these scales can reveal how these bodies and their organic reservoirs have evolved during the history of the Solar System.

With this in mind, we present the use of a technology that is new to planetary sciences: Optical Photothermal Spectroscopy (OPTIR). By using a dual laser system, combining an IR excitation laser and a visible probe laser, this technology allows for the collection of mid-IR absorption spectra with little to no sample preparation at sub-micron resolution. The IR wavelengths probed can be adjusted by choosing an appropriate IR source. Our current prototype uses a mid-IR source ranging from 1800 to 980 cm⁻¹ (5.6 to 10.2 μm), which allows us to detect and identify most rocks and minerals of planetary relevance, as well as organic components and the fingerprint region of the IR spectrum.

We show the performance of OPTIR on various samples, including granular regolith simulants and meteorites. We are able to map the mineral disparities at sub-micron scales, as well as detect a variety of organic compounds on Murchison and Allende samples, including the detection of amino acids and amide I and II bonds, all at the sub-micron scale. We also combine the OPTIR technology with Raman for an even more powerful mineral and organic detection and identification instrument.