# Absorption Spectroscopy

Overview

As you learned in the quantum mechanics tutorial, light can interact with molecules. Here we will briefly examine this effect for the case of directing light (such as from a lamp) through a medium that can absorb the light (such as the gas in an engine).

Details

Beer’s Law: I/Io = exp(-kλL)*,

where Io [Watts] is the intensity entering the absorber, I [Watts] is the intensity transmitted through the absorber, kλ [cm-1] is the spectral absorption coefficient at a specific wavelength (color), and L is the length of the absorber (the thickness of the sunglass eyepiece in our example).

Plots of kλ versus wavelength λ are known as absorption spectra. The absorption spectrum for the sunglasses described here could be measured using a lamp, a prism and a linear camera as shown in the figure above. The absorption spectra of gases are much richer in information than the sunglasses in the example. Absorption spectra for some representative combustion gases are shown in the figure at the top. These are simulated using databases called HITRAN and HITEMP. There are several things to note, such as:

1. H2O (water vapor, in black) is the dominant absorber in this infrared wavelength range (for reference 1 µm wavelength corresponds to twice a wavelength we would call bluish-green).
2. H2O has ~ 5 features of similar shape in this wavelength range, getting generally stronger at longer wavelengths (these features are known as bands associated with both rotation and vibration of H2O, rather than rotation alone as described for OH in the quantum mechanics tutorial)
3. The spectral width of these bands increases as temperature increases. This property forms a basis for gas thermometry by absorption spectroscopy.
4. There are some spectral windows where species other than H2O have absorption stronger than H2O. Thus, with appropriate sources for measuring the spectra shown above, inside the engine cylinder, one can measure the gas composition as well as the temperature.
5. A disclaimer accompanies these spectra: they are simulated from databases known to contain errors. Thus, actual spectra acquired in engines using wavelength-agile sources often reveal discrepancies.

Actual engine measurements demonstrate that absorption spectroscopy is a simple yet powerful technique for studying combustion. Note that gas temperature and/or composition measurements made by absorption spectroscopy generally represent line-of-sight-averaged quantities, unlike fluorescence spectroscopy.

* click here for details on this law and a nice illustration of light attenuation in a cuvette; note that several forms are used (sometimes base e, sometimes base 10, often kλ is replaced by something similar but defined, for example, on a molar concentration basis).