Chemistry Reference
In-Depth Information
direct manipulation of digital data, simultaneous detection across a specific spectral
window, high sensitivity and a dynamic range of the order of 10
5
. A wide variety of
light sources, dispersive elements, detectors and data aquisition methods can be used
for absorption spectroscopy [130]. The classic dispersion experiment for measuring
the density of atomic or molecular states in a plasma by absorption spectroscopy uses
a continuous light source in combination with a narrow bandwidth frequency filter,
e.g., a spectrograph, with a detector suitable for the spectral range under investigation.
The Fourier Transform Infrared (FTIR) technique uses a Michelson interferometer
setup. The intensity depends on the (variable) optical path difference between the
mirrors in the two arms of the interferometer. While one mirror is fixed the other is
moved continuously between identical path length (maximum signal) and a shifted
path length of λ
2 (zero signal). The inverse transform yields an absorption spectrum
in the frequency domain. In contrast to dispersion techniques the FTIR spectrometer
records the whole spectrum simultaneously (multiplex or Fellgett advantage). In prin-
ciple the resolution of FTIR can be as high as 0.002 cm
−
1
, determined by the distance
scanned by the moveable mirror, but at the expense of time resolution. Fractional
absorptions as small as 10
−
4
can be measured.
The current development of tunable, narrow band light sources, such as tunable
dye lasers, infrared diode lasers, or quantum cascade lasers, has led to their substi-
tution as continuous light sources in AS experiments. These laser sources have the
advantageofhighspectralintensity,narrowbandwidth,andcontinuoustunabilityover
the absorption line profile. Figure 6.7 shows an example of an experimental arrange-
ment used to investigate the plasma chemistry and kinetics in a planar microwave
/
Microwave appliance
Module (2.45 GHz)
Plasma region
Field
mirror box
Objective
mirror box
Discharge vessel with long path cell
Microwave window
Monochromator
HgCdTe
detector
He closed cycle
refrigerator TDL's
TDL System
FIGURE 6.7
Experimental arrangement of a planar microwave plasma reactor (side view)
with White cell multiple pass optical arrangements, and tunable diode laser (TDL) infrared
source used for hydrocarbon (CH
4
,CH
3
OH) plasma diagnostics. The path of the diode laser
beam is indicated by dotted lines. (From Hempel, F. et al.,
Plasma Source. Sci. Technol.
, 12,
598, 2003.)
