Abstract
This paper shows that, when samples of methane in dry air are irradiated with an OPO tuned at the ν 3 mode at 3019 cm−1, the photoacoustic signal is lower than in mixtures in pure nitrogen. In order to explain this change, we developed a rate-equation-based model which describes the methane excitation and relaxation to heat by collisions with the buffer. In particular, we reduced the problem to a system of six energy levels taking into account the fast energy exchange between methane and oxygen resonant states and the slow relaxation rate of oxygen. Thus, the photoacoustic signal is calculated for methane–nitrogen and methane–air systems and a very good agreement is found with the experimental results.
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