Chemistry Reference
In-Depth Information
calibration inherent in the OES methods. Absorption spectroscopy has been applied
across the spectrum from the VUV to the far-infrared (FIR).
Two principle cases need to be distinguished:
1. The measurement of the absorption of light emitted by the plasma itself (
self
absorption or reabsorption
)
2. Absorption of an external light source
In the case of self-absorption measurements the light from the plasma is mea-
sured with and without a retroreflector or compared with the direct light emitted
from an identical plasma. This ensures identical line profiles for the emitted and
the absorbed light. This method is particularly important for checking the optical
thickness of the plasma. Further discussions about self-absorption can be found
in [127,128]. The probing light intensity of an external light source in case 2 has
to be low to avoid saturation effects of the excited states of the species under
investigation.
In the case where an external light source has much higher intensity than that of
the plasma itself, the absorption of radiation can be described by the Beer-Lambert
law which is
I
ν
(
l
)
=
I
ν
(
0
)
exp
(
−
κ
(
ν
)
l
)
,
(6.22)
κ
i
(
ν
)
=
n
i
σ
i
(
ν
)
,
(6.23)
where
I
ν
(
are the fluxes of the radiation entering and leaving the plasma
l
is the length of the absorbing (homogeneous) plasma column
κ
0
)
and
I
ν
(
l
)
is the absorption coefficient
n
i
is the concentration under study
σ
i
the absorption cross section
(
ν
)
Figure 6.11 illustrates this situation [135].
Plasma
Exponential
decay
I
0
I
ν
Distance
FIGURE 6.11
Absorption of external radiation in a plasma according to the Beer-
Lambert Law.
