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Figure 13: A sweep in frequency over the absorption line at v0 • I states the
recorded intensity at v0 and ! 0 the intensity if no absorption would take place.
Spectroscopy. The diode laser has to be operated with a narrow, single-mode
profile to be able to detect the absorption lines.
The sample
In GAS MAS the free gas of molecular oxygen inside scattering materials is
studied. Scattering occurs when an incident beam interacts with a particle
and the reemission of the energy, or parts of it, is in many directions. This
effect is a result of the emission from the oscillating electric charges forced
by the alternating electrical fields. This scattering process results in the
path length, the distance the photons have traveled, being greater than the
thickness of the sample, see figure 14.
Figure 14: The distance the light travels zn a scattering sample is highly
dependent on the scattering of the sample.
This scattering process complicates the use of Beer-Lambert law, since the
actual trajectory is more difficult to correlate with the sample thickness. A
unit called equivalent mean path length, Leq, can be used to describe the
signal. This quantity is obtained by the so called standard-addition method
discussed further ahead.
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