Influence of molecular relaxation dynamics on quartz-enhanced photoacoustic detection of CO2 at λ=2 μm

Carbon dioxide (CO2) trace gas detection based on quartz enhanced photoacoustic spectroscopy (QEPAS) using a distributed feedback diode laser operating at lambda=2 mu m is performed, with a primary purpose of studying vibrational relaxation processes in the CO2-N-2-H2O system. A simple model is developed and used to explain the experimentally observed dependence of amplitude and phase of the photoacoustic signal on pressure and gas humidity. A (1 sigma) sensitivity of 110 parts-per-million (with a 1 s lock-in time constant) was obtained for CO2 concentrations measured in humid gas samples.