Infrared characterization of environmental samples by pulsed photothermal spectroscopy

The low concentration of toxic radioactive metals in environmental samples often limits the interpretation of results of infrared studies investigating the interaction processes between the metal ions and environmental compartments. For the first time, we could show that photothermal infrared spectroscopy performed with a pulsed free-electron laser can provide reliable infrared spectra throughout a distinct spectral range of interest. In this model investigation, we provide vibrational absorption spectra of a rare earth metal salt dissolved in a KBr matrix and a natural calcite sample obtained by thermal beam deflection technique and FT-IR spectroscopy, respectively. General agreement was found between all spectra of the different recording techniques. Spectral deviations were observed with samples containing low concentration of the rare earth metal salt indicating a lower detection limit of the photothermal method as compared to conventional FT-IR spectroscopy. Furthermore, the photothermal method provide spatial information of a sample surface. This may result in a microspectrometric technique for determining the distribution of metal species on mineral surfaces. First experiments exploring the spatial resolution of photothermal spectroscopy were carried out by scanning the surface of a germanium substrate showing a localized region where O-ions were implanted. The border range of this region was investigated by recording time curves of the deflection signal at distinct positions of the substrate surface with a constant free-electron laser wavelength of 11.6 micrometer.