SPECIAL ISSUE ‐ RESEARCH ARTICLE Photothermal C―H vibrational overtone detection by two‐ color laser absorption Marlon Diaz | Parashu R. Nyaupane | Ashley Diaz | Carlos E. Manzanares Department of Chemistry & Biochemistry, Baylor University, Waco, Texas, USA Correspondence Carlos E. Manzanares, Department of Chemistry & Biochemistry, Baylor University, Waco, Texas 76798. Email: carlos_manzanares@baylor.edu Abstract In this experiment, the thermal lens technique is used with a modified arrange- ment of three lasers to induce a two‐color absorption. Two‐pump lasers, a var- iable 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 com- parison 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 mol- ecules 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 inte- grated 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. KEYWORDS Benzene, C―H overtone, Naphthalene, Nonlinear absorption, Photothermal 1 | INTRODUCTION A nonfluorescent molecule that is excited with one laser (pump), subsequently releases the energy to return to the ground state. The heat produced during the release of energy increases the temperature along the excitation region. The temperature gradient in solution produces a localized nonuniform distribution of the refractive index and acts like a concave lens. Discovery of the thermal lens effect was made by Gordon and collaborators when attempting to enhance the intensity of laser Raman scat- tering from benzene. [1] In the thermal lens technique, a second laser (probe) is dispersed divergently when it passes through the excitation region. The decrease of the probe beam intensity is proportional to the absorption process. [2–4] Two‐color enhancement of thermal lens sig- nals have been reported using two single line lasers to excite electronic levels of molecules and a probe to observe the magnitude of the absorption. [5,6] We have reported two‐color absorption for vibrational overtones of benzene [7] and naphthalene [8] using two lasers (pump and probe). Vibrational overtone spectra using the ther- mal lens technique were initially reported by Albrecht, Swofford, and others. [9–14] The selectivity of overtone spectroscopy, in combina- tion with the local mode model, [15–17] have been used Received: 18 July 2018 Revised: 12 September 2018 Accepted: 9 October 2018 DOI: 10.1002/jrs.5510 J Raman Spectrosc. 2018;1–7. © 2018 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jrs 1