Near-infrared laser photoacoustic detection of methane: The impact of molecular relaxation
A photoacoustic sensor has been developed for trace-gas monitoring using a near-infrared semiconductor laser emitting in the 2ν3 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 CH4 detection in dry O2 and O2-N2 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-O2 system, is demonstrated.
Keywords: Methane ; Photoacoustic effect ; Optical sensors ; Optical communication ; Acoustic signal processing ; Molecules ; Vapors ; Helium ; Near-infrared semiconductor laser ; Near-infrared laser photoacoustic ; Molecular relaxation ; Photoacoustic sensor
Nanophotonics and Metrology Laboratory, Ecole Polytechnique Federale de Lausanne (EPFL), 1015 Lausanne, Switzerland, Compilation and indexing terms, Copyright 2008 Elsevier Inc., 06039649694, 0946-2171, Near-infrared semiconductor laser, Near-infrared laser photoacoustic, Molecular relaxation, Photoacoustic sensor
Record created on 2008-09-29, modified on 2016-08-08