Photothermal CH vibrational overtone detection by two‐color laser absorption

Abstract

AbstractIn this experiment, the thermal lens technique is used with a modified arrangement of three lasers to induce a two‐color absorption. Two‐pump lasers, a variable wavelength orange dye laser (588–617 nm), a single line blue laser (488 nm), and a single line probe yellow laser (568 nm) are employed. A comparison is made between the magnitude of the thermal lens signal obtained with a one pump laser versus two pump lasers. The absorbing molecules are benzene and naphthalene in liquid n‐Hexane. The CH vibrational overtone spectra are obtained at room temperature for several concentrations. The molecules are excited to a high vibrational state (Δυ = 6) with the first laser and to an electronic level with a second laser (two‐color absorption). Using two pump lasers, the limit of detection of the molecule is several orders of magnitude more sensitive than using one pump laser. A nonlinear behavior of the integrated signal versus concentration is shown for the two‐color laser process. Linear behavior is shown for the one pump laser experiment. A model of signal amplification for a nonlinear absorption is presented to explain the results. The separation and identification of CH overtone bands in molecules and the sensitivity of the technique is emphasized to convey the potential use of CH overtone spectroscopy for imaging in thermal lens microscopy.