Influence of molecular relaxation dynamics on quartz - enhanced photoacoustic detection of CO 2 at λ = 2 μ

Carbon dioxide (CO 2) trace gas detection based on quartz enhanced photoacoustic spectroscopy (QEPAS) using a distributed feedback diode laser operating at λ = 2 µm is performed, with a primary purpose of studying vibrational relaxation processes in the CO 2-N 2-H 2 O 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 (1σ) sensitivity of 110 parts-per-million (with a 1 s lock-in time constant) was obtained for CO 2 concentrations measured in humid gas samples. 1 Introduction Photoacoustic spectroscopy (PAS) has proved to be a sensitive and reliable technique for detection of trace gases in various applications ([1–3] and references therein). A new approach to the photoacoustic (PA) signal detection was introduced in 2002 by Kosterev et al. [4] In this technique called quartz enhanced photoacoustic spectroscopy (QEPAS) a high Q-factor quartz resonator is used as a resonant sound transducer , which allows sensitive detection of a laser-generated photoacoustic wave. It has been already demonstrated that the application of quartz tuning forks (TFs) as QEPAS based trace gas transducers results in comparable detection sensitivities to traditional PAS offering in addition to all the characteristic features of PAS (e.g. intrinsically zero background and wavelength independence) such advantages as: immunity to ambient acoustic noise emitted from distant sources or measurement of extremely small volume samples (< 1 mm 3) [5]. Application of commercially available TFs resonant at 32 768 Hz makes the overall sensor very cost effective and allows miniaturization of the sensor. Such a relatively high operational modulation frequency introduces an important additional functionality to QEPAS based sensors. In contrast to classical PAS-based sensors, where in order to maximize photoacoustic signal these instruments operate typically at a modulation frequency much smaller than molecular relaxation time of the target species f 1/τ r (typically < 1 kHz), the QEPAS based sensor detecting the PA signal at ∼ 32 kHz preserves the capability to distinguish between different molecular relaxation times. This can be used as an additional spectroscopic parameter for discrimination between PA signals originating from molecular species with overlapping absorption spectra [6]. The same technique can be also applied for measurement of molecular relaxation times associated with different vibration–translation (V–T) and vibration– vibration (V–V) energy transfer mechanisms characteristic for a particular molecular gas mixture under investigation. The development of laser spectroscopy strongly …