16 Why do some gases absorb infrared energy?
From a mechanical point of view, molecules in a gas could be compared to weights (the balls in the figures below), connected
together via springs. Depending on the number of atoms, their respective size and mass, the elastic constant of the springs,
molecules may move in given directions, vibrate along an axis, rotate, twist, stretch, rock, wag, etc.
The simplest gas molecules are single atoms, like helium, neon or krypton. They have no way to vibrate or rotate, so they
can only move by translation in one direction at a time.
Figure 16.1 Single atom
The next most complex category of molecules is homonuclear, made of two atoms such as hydrogen (H2), nitrogen (N2) and oxygen (O2). They have the ability to tumble around their axes in addition to translational motion.
Figure 16.2 Two atoms
Then there are complex diatomic molecules, such as carbon dioxide (CO2), methane (CH4), sulfur hexafluoride (SF6), and styrene (C6H5CH=CH2) (these are just a few examples).
Figure 16.3 Carbon dioxide (CO2), 3 atoms per molecule
This assumption is valid for multi-atomic molecules.
Figure 16.4 Methane (CH4), 5 atoms per molecule
Figure 16.5 Sulfur hexafluoride (SF6), 7 atoms per molecule
Figure 16.6 Styrene (C6H5CH=CH2), 16 atoms per molecule
Their increased degrees of mechanical freedom allow multiple rotational and vibrational transitions. Because they are built
from multiple atoms, they can absorb and emit heat more effectively than simple molecules. Depending on the frequency of
the transitions, some of them fall into energy ranges that are located in the infrared region where the infrared camera is
sensitive.
|
Transition type |
Frequency |
Spectral range |
|---|---|---|
|
Rotation of heavy molecules
|
109–1011 Hz
|
Microwaves, above 3 mm/0.118 in.
|
|
Rotation of light molecules and vibration of heavy molecules
|
1011–1013 Hz
|
Far infrared, between 30 μm and 3 mm/0.118 in.
|
|
Vibration of light molecules.
Rotation and vibration of the structure |
1013–1014 Hz
|
Infrared, between 3 μm and 30 μm
|
|
Electronic transitions
|
1014–1016 Hz
|
UV–visible
|
In order for a molecule to absorb a photon via a transition from one state to another, the molecule must have a dipole moment
capable of briefly oscillating at the same frequency as the incident photon. This quantum mechanical interaction allows the
electromagnetic field energy of the photon to be “transferred” or absorbed by the molecule.
FLIR GF3xx series cameras take advantage of the absorbing nature of certain molecules, to visualize them in their native environments.
FLIR GF3xx series focal plane arrays and optical systems are specifically tuned to very narrow spectral ranges, in the order of hundreds of
nanometers, and are therefore ultra selective. Only gases absorbent in the infrared region that is delimited by a narrow
band pass filter can be detected.
Since the energy from the gases is very weak, all camera components are optimized to emit as little energy as possible. This
is the only solution to provide a sufficient signal-to-noise ratio. Hence, the filter itself is maintained at a cryogenic
temperature: down to 60 K in the case of the FLIR GF3xx series LW camera that was released in the beginning of 2008.
Below, are the transmittance spectra of two gases:
- Benzene (C6H6)—absorbent in the MW region
- Sulfur hexafluoride (SF6)—absorbent in the LW region.
Figure 16.7 Benzene (C6H6). Strong absorption around 3.2/3.3 μm
