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Emission Spectroscopy

emission_spectroscopy

Overview

As you learned in the quantum mechanics tutorial, molecules can store energy, for example by rotating rapidly. Furthermore, they can release this energy in the form of photons (light). We are familiar with hot objects emitting light. Here we will see that, as in the case of absorption spectroscopy, hot gases will have information-rich spectra relative to dense matter such as molten metal. We will also see that, in addition to heating a gas, a gas can be made to give off light by exciting it in other ways (chemically or optically). The optical excitation case is treated in the laser-induced fluorescence tutorial.

Details

Heat transfer textbooks generally treat the subject matter of radiation, and hot objects such as the broiler in a household oven are considered. We know that these objects emit visible light, and the textbooks provide Planck’s blackbody distribution (the bold blue curve above) as an idealized version of their emission over all wavelengths (colors). They teach Wien’s law, which explains why hot objects (like the stove element) appear reddish while really hot objects (like the sun) appear whitish. Radiation explains why you can feel warm standing 10 feet from a campfire even though the air between you and the campfire is cold. Radiation heat transfer is generally explained as infrared rays, because infrared wavelengths often dominate heat transfer problems; however, a 10W green laser beam will ignite a black piece of paper every bit as well as 10W of sunlight focused onto the same paper (again, both transmitted through cold air).

Using HITRAN and HITEMP, thermal emission spectra for hot gases can be calculated as for absorption spectra. Results are shown above for representative combustion gas. In addition to similar observations made in the absorption spectra, the following new observations arise in emission:

  1. The thermal emission from gases cannot exceed the emission from a blackbody; the gas spectra have features that saturate, at least approximately, at Planck’s blackbody curve. (Disclaimer: they should saturate exactly there, so there is an error somewhere in these simulations that has not yet been identified – as stated before, the HITRAN and HITEMP databases are known to contain errors and these could be responsible)
  2. Because of (1), the wavelength of maximum emission power tends to track that of the blackbody distribution
  3. Across the spectrum, the emission increases dramatically as temperature increases, just like the blackbody curve.
  4. CO2 gas approximates a blackbody around the 4.3 µm range.

Again, these spectra can form the basis for sensors used to determine gas temperature and composition. However, such sensors are generally biased toward high temperatures because of observation (3) above, and are often plagued by re-absorption of emitted light in cold boundary layers. Also, spectrally-resolved thermal emission is weak, relative to laser powers, and window fouling effects are more problematic. Therefore, laser sensors by absorption spectroscopy are generally favored over sensors based on thermal emission.

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There are forms of emission spectroscopy other than thermal emission spectroscopy. A common form is chemiluminescence spectroscopy. In combustion, chemiluminescence is the term given to light emitted from hot gases that would not be expected from the thermal emission depicted in the animated spectra above. An example is the blue light emitted from the flame of a propane torch. This light is due to excitation of CH radicals in the flame as part of the chemistry in the flame rather than by the heating of the CH molecules.

Chemiluminescence can be used to identify the presence of certain species in combustion, for example, in measured chemiluminescence spectra recorded in an engine, but it is often difficult to quantify such measurements, even within an order of magnitude in species mole fraction. Still, such sensors can be extremely useful for identifying the start or existence of combustion.