Environnement AF22e, Technical Manual, Page: 23 / 118

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AF22e
Environnement
S.A
APRIL 2016
2–3
2 PRINCIPLE OF OPERATION
2.1 THEORETICAL BASIS
Refer to Figure 2–1.
Energy diagram:
Many developments in quantum mechanics were performed in the early twentieth century. They allowed
theoretical physicists to conceptualize processes involved in energy exchanges between a gaseous
molecule such as sulfur dioxide and its environment.
The diagram shown in Figure 2–1 helps to easily understand the various phenomena involved in
radiation absorption by the molecule.
Two essential comments clarify this diagram:
−
Energy levels are quantized and distributed according to a structure which differs depending on the
electronical, vibrational and rotational levels of the molecule.
−
At the considered wavelengths, the "electron" energy scale includes only the energy levels of
molecule valence electrons.
Likewise, transition from an electronical level to another by one-photon absorption always involves
vibrational and rotational transitions of lower energy levels.
The mathematical formalism of molecular physics associates a time and spatial function named « wave
function » with each molecule status. Then, a transition results in a new spatial « wave function »
distribution.
Absorption and emission:
In its fundamental state E
e
0
, the SO
2
molecule can absorb only photons having sufficient energy to
access the first excited state on the E
e
1
electronical scale.
The photon energy E is given by Einstein's equation:
c
E hv h
λ
= =
Where :
− λ
is the radiation wavelength output from a low-pressure zinc vapor lamp, given
λ
= 213.9 nm,
−
h is Plank's constant,
−
c is vacuum light speed.
The transition may be written as:
SO2 + h v SO2*
⎯⎯→
The molecule always reaches a vibrational and rotational sub-level of E
e1
(which is higher than E
e1
level).
From this E
e1
sub-level, the molecule quickly scatters its vibration and rotation energy to return to E
e1
level.