Laser Absorption Spectrometry - Laser Absorption Spectrometry Using Fundamental Vibrational or Electronic Transitions

Laser Absorption Spectrometry Using Fundamental Vibrational or Electronic Transitions

The second way of improving the detection limit of LAS is to employ transitions with larger line strength, either in the fundamental vibrational band or electronic transitions. The former, which normally reside at ~5 μm, have linestrengths that are ~2–3 orders of magnitude higher than those of typical overtone transition. As an example, the figure (to appear soon) shows the linestrengths for the second and first overtones in NO (at ~1.8 and 2.65 µm) as well as those for the fundamental vibrational transitions in the MIR region (around 5.3 µm). On the other hand, electronic transitions have often yet another 1–2 orders of magnitude larger line strengths. The transitions strengths for the electronic transitions of NO, which are located in the UV range (at ~227 nm, not included in the figure) are ~2 orders of magnitude larger than those in the MIR region! If transitions with such large linestrengths can be used efficiently, a significant increase in sensitivity would result.

The recent development of quantum cascade lasers (QC) lasers working in the MIR region has opened up new possibilities for sensitive detection of molecular species on their fundamental vibrational bands. It is more difficult (although not impossible) to generate stable cw light addressing electronic transitions, since these often lie in the UV region. Despite the fact that transitions with larger transition strength can be reached by either MIR or UV emitting lasers, the limits of detection (LODs) of AS techniques using these transitions have not yet been improved as much as has been anticipated. The reason is that these types of laser have a number of unique properties that limit their practical applicability. Their full potential can only be used whence these limitations have been circumvented. This is a rapidly developing but still only partly explored field of science that can help overcoming some of the present limitations of the AS technique.