Near-infrared laser photoacoustic detection of methane: the impact of molecular relaxation

Abstract

A photoacoustic sensor has been developed for trace-gas monitoring using a near-infrared semiconductor laser emitting in the 2ν 3 band of methane at 1.65 µm. The apparatus was designed for on-line process control in the manufacturing of the novel low-water-peak fibres developed for optical telecommunications. The importance of collisional relaxation processes in the generation of the photoacoustic signal is reported in the particular case of CH 4 detection in dry O 2 and O 2 -N 2 mixtures. The negative influence of these effects results in a strongly reduced and phase-shifted photoacoustic signal, induced by a fast resonant coupling between the vibrational states of methane and oxygen, associated with the slow relaxation of the excited oxygen molecules. An unusual parabolic response of the sensor with respect to the methane concentration has been observed and is discussed. Finally, the beneficial effect of several species, including water vapour and helium, acting as a catalyst to hasten the relaxation of the CH 4 -O 2 system, is demonstrated.PACS 42.62.Fi; 33.20.Ea; 34.50.Ez IntroductionInfrared photoacoustic spectroscopy (PAS) is widely recognized for its high performances in trace-gas monitoring and is one of the most sensitive techniques to measure low gas concentrations at atmospheric pressure [1]. The high sensitivity of PAS mainly results from its zero-background nature, which means that no signal is produced in the absence of an absorbing species. Furthermore, one of the most outstanding features of this technique is its achromaticity: the response of a photoacoustic (PA) sensor, represented by the cell constant (i.e. the PA signal normalized by the laser power and absorption coefficient), is usually independent of the laser excitation wavelength (however, a few exceptions may occur in some particular situations, as will be discussed later). It means that the same PA detector can be used with any type of laser and at any wavelength, from ultraviolet to mid infrared (MIR), with identical performances in terms of cell constant (provided that a suitable window material is used in the PA cell). This is definitively not the case for other high u Fax: +41-21-693-26-14, E-mail: stephane.schilt@epfl.ch sensitivity laser spectroscopy techniques, such as wavelengthor frequency-modulation spectroscopy (WMS/FMS), cavity ring-down spectroscopy (CRDS) or intracavity laser absorption spectroscopy (ICLAS), which all make use of optical detection that has poorer performances in the MIR region. Indeed, not only is the sensitivity of MIR photodiodes strongly reduced in comparison to near-infrared (NIR) ones, but also MIR detectors require low-temperature operation and are much more expensive than NIR detectors.For several years, there has been much interest in the use of NIR semiconductor lasers in PAS, such as distributedfeedback (DFB) lasers mass produced for the optical telecommunications market, whereas, primarily, it has been predominantly implemented using MIR gas lasers (CO or CO 2 lasers). Extreme detectio...

Molecular relaxation effects in hydrogen chloride photoacoustic detection

“…In this process, only part of the energy (ν HCl −ν O 2 ) is transformed into kinetic energy contributing to the PA signal, the remaining being transferred into internal energy of O 2 , which has a long relaxation time in the case of collisions with O 2 . However, the presence of He plays the role of a catalyst for this relaxation (as already shown for the case of CH 4 in O 2 [4]), which drastically reduces the time decay of the O 2 excited state (see FIGURE 5 Variation of the PA signal amplitude corresponding to 5 ppm HCl as a function of O 2 fraction in He. The PA signal has been normalized by the PA signal recorded for H 2 O in the same buffer gas and normalized to the value obtained in pure helium reaction R 11 ).…”

“…The efficiency of water vapor acting as a catalyst was already demonstrated for the detection of methane in the presence of oxygen [4]. The same procedure was applied to the HCl-He-O 2 and HCl-He-N 2 systems.…”

“…Molecular relaxation effects have been described in the NIR for the detection of methane (CH 4 ) [4], carbon dioxide (CO 2 ) [5] and hydrogen cyanide (HCN) [6]. In this paper, we report the influence of molecular relaxation on hydrogen chloride (HCl) detection in different gas mixtures containing helium (He).…”

Sub-ppm multi-gas photoacoustic sensor

“…The reported tuning spectral range was obtained by varying the laser temperature from 5 to 40 • C. The temperature and current tuning coefficients were determined using a wavemeter and the average optical power in our experimental conditions has also been measured. Intensity modulation (IM) or wavelength modulation (WM) may be obtained by modulating the injection current of the laser, depending on the modulation conditions [7]. In our case WM was applied since it has produced a slightly higher signal than IM (25% in the case of CH 4 [8]).…”

Novel Helmholtz-based photoacoustic sensor for trace gas detection at ppm level using GaInAsSb/GaAlAsSb DFB lasers

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