FLIR GFx320, User Manual

Page: 121 / 164

Why do some gases absorb
infrared energy?
38
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,
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 38.1 Single atom
The next most complex category of molecules is diatomic, made of two atoms such as
hydrogen (H
2
), nitrogen (N
2
) and oxygen (O
2
). They have the ability to tumble around
their axes in addition to translational motion.
Figure 38.2 Two atoms
Then there are complex diatomic molecules, such as carbon dioxide (CO
2
), methane
(CH
4
), sulfur hexafluoride (SF
6
), and styrene (C
6
H
5
CH=CH
2
) (these are just a few
examples).
Figure 38.3 Simple mechanical model of carbon dioxide (CO
2
), 3 atoms per molecule
This assumption is valid for multi-atomic molecules.
Figure 38.4 Methane (CH
4
), 5 atoms per molecule
Figure 38.5 Sulfur hexafluoride (SF
6
), 7 atoms per molecule
Figure 38.6 Molecular orbitals of Styrene (C
6
H
5
CH=CH
2
), 16 atoms per molecule
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