Nonlinear thermal lens signal of the (Δυ = 6) C–H vibrational overtone of C6H6 in liquid solutions of n-C6H14 and CCl4

Abstract

The thermal lens technique is applied to vibrational overtone spectroscopy of solutions of benzene. The pump and probe thermal lens technique has been found to be very sensitive for detecting samples of low concentration in transparent solvents. The C–H fifth vibrational (Δυ = 6) overtone spectrum of benzene is detected at room temperature for compositions per volume in the range (1 to 1 × 10−4) using CCl4 and n-C6H14 as solvents. By detecting the absorption band in a 100-ppm solution, the peak absorption of the signal is approximately (2.2 ± 0.3) × 10−7 cm−1. The parameters that determine the magnitude of the thermal lens signal such as the pump laser power and the thermodynamic properties of the solvent and solute are discussed. A plot of normalized integrated intensity as a function of composition of benzene in solution reveals a nonlinear behavior. The nonlinearity cannot be explained assuming solvent enhancement at low concentrations. A two-color absorption model that includes the simultaneous absorption of the pump and probe lasers explains the enhanced magnitude and the nonlinear behavior of the thermal lens signal for solutions of composition below 0.01.