Why Do Some Gases Absorb Infrared Energy - FLIR GF300 User Manual

28 Why do some gases absorb
infrared energy?
From a simplistic 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, mole-
cules 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 28.1 Single atom
The next most complex category of molecules is diatomic, made of two atoms such as hy-
drogen (H ), nitrogen (N ) and oxygen (O ). They have the ability to tumble around their
2 2 2
axes in addition to translational motion.
Figure 28.2 Two atoms
Then there are complex diatomic molecules, such as carbon dioxide (CO ), methane
2
(CH ), sulfur hexafluoride (SF ), and styrene (C H CH=CH ) (these are just a few
4 6 6 5 2
examples).
Figure 28.3 Simple mechanical model of carbon dioxide (CO ), 3 atoms per molecule
2
This assumption is valid for multi-atomic molecules.
Figure 28.4 Methane (CH ), 5 atoms per molecule
4
Figure 28.5 Sulfur hexafluoride (SF ), 7 atoms per molecule
6
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