ZhiWei Sun, Bo Li, Johan Zetterberg, Zhongshan Li and Marcus Aldén
Contact person: Zhongshan Li
Infrared spectroscopy has always been of great interest to the combustion community. The principal components of natural gas, methane and ethane, are two common fuel molecules that can only be detected in the infrared spectral regime. Other molecules which are not accessible in the UV/visible spectral range are product gases such as the greenhouse gas CO2 and the toxic HCl gas. From a global warming perspective the need for CO2 detection is obvious but with increasing combustion of waste and biofuel, HCl emissions need to be well understood and controlled. These and several other important molecules and radicals requires new detection schemes to be able to be measured with high temporal and spatial resolution.
In different wavelength ranges, the molecular spectra reflect different molecular motions. For example, rotations of polarized molecules respond to microwave radiation while the bending and twisting of small molecules causes the absorption or emission of far-infrared radiation. The diagnostics of reactive gas flows can be divided into three different wavelength regions, corresponding to different molecular properties.
- Mid-infrared – fundamental molecular stretching vibrations
- Near-infrared – overtone and combination bands
- Ultraviolet/visible – electronic transitions
Some key properties related to the diagnostic applications for the different spectral regions are listed in Table 1.
Table 1. Properties related to the diagnostic applications for the different spectra.
Absorption based techniques such as tunable diode-laser absorption spectroscopy (TDAS) have been widely used to probe molecular overtone and combination bands in the near-infrared. These methods provide a high sensitivity but are at the same time limited to line-of-sight measurements. Another drawback is the relatively weak absorption and emission associated with these processes, making the use of spatially-resolved techniques like laser-induced fluorescence (LIF) or polarization spectroscopy (PS) very difficult, if at all possible. By probing the electronic transitions in the UV/visible spectral range spatially resolved measurements, even single-shot two-dimensional imaging, is possible for the detection of minor species. But, as indicated in Table 1, this sensitive approach is only applicable to a limited number of molecules and radicals with electronic transitions accessible using laser radiation longer than 200 nm.
Despite the fact that the absorption and emission of the fundamental stretching vibrations is relatively strong, laser-spectroscopic probing of these have been limited by the poor availability of proper tunable laser-sources and highly sensitive IR-detectors. Thus far, the conventional FTIR spectroscopy is the most commonly utilized technique to detect either emission from hot gases or absorption of cold gases. Limitations of FTIR are its line-of-sight nature and slow detection speed (a few seconds is needed). Advanced laser-spectroscopic techniques are needed in order to achieve sensitive detection with high spatial and temporal resolution.
Development of sensitive laser-spectroscopic techniques in the mid-IR has been part of our research activities since 2002. Our infrared lab is well equipped with a powerful infrared laser (tunable 2-4.5 µm), sensitive IR detectors (point and 2D InSb) and different IR optics. Different spectroscopic techniques have been developed for different applications, both for fundamental and more applied studies. Figure 1 shows examples of single-shot IR LIF images of CO2 in a gas jet.
Figure 1. Single-shot LIF images of carbon dioxide in free gas jets with about 1 mJ per pulse The 2.7 µm infrared laser in the excitation laser sheet and infrared fluorescence at around 4.5 µm were detected with an InSb camera. A) shows results obtained in a 0.26% CO2 mixed in Argon, the Signal-background being 69 counts, and b) shows results obtained in a mixture of 4.50% CO2 in Argon, the Signal-background being 810 counts.
In the mid-IR spectral range, different laser spectroscopic diagnostics techniques are under development for different applications. Polarisation spectroscopy, PS, are promising techniques for sensitive detection of trace molecular species in harsh environments.