Overtone spectroscopy and thermal lens detection limit of methane in cryo-solutions

Abstract

The overtone spectra of methane solutions from 5000 cm−1 to 17000 cm−1 in liquid nitrogen and liquid argon are presented. The region between 5000 and 15000 cm−1 has been measured using a Fourier transform spectrophotometer with methane concentrations between 0.41% and 37% in mole fraction. The thermal lens technique (TLS) was used to measure the fifth overtone, not observed with the Fourier transform spectrophotometer. For a fixed concentration, maximization of the TLS signal with low modulation frequency and high laser power of excitation is shown. Time-dependent signal profiles of single-beam thermal lens experiments are analysed with the parabolic and aberrant lens models. Fitting parameters are obtained from the time-dependent signals. The absorption coefficient of methane was calculated from the fitting parameters and thermo-optical properties of the solvent. The calculated absorption coefficient of the Δυ = 6 transition of methane is 1.7 × 10−3 cm−1, which is in good agreement with literature values. A limit of detection is calculated with measurements of the fifth overtone absorption of methane in liquid nitrogen for solutions of concentration from 0.012% to 0.36% in mole fraction. Calculated limits of detection are 54 ppm and 15 ppm according to two different definitions of limit of detection for the thermal lens technique.