Chemistry Reference
In-Depth Information
reactor [132], in which a tunable diode laser (TDL) spectrometer [133] is combined
with a White cell multiple pass optical arrangement to increase the absorption length
and the sensitivity of the AS spectrometer [134,135]. The discharge configuration in
planar microwave plasmas has the advantage of being well suited for end-on observa-
tions because considerable homogeneity can be achieved over relatively long plasma
path lengths. The combination of the plasma source with the multiple pass optics
of the White design provides the opportunity to use more than two passes through
the chamber. The White geometry with two objective mirrors and one field mirror,
allows different path lengths from 4 to 40 passes, (or more), through the plasma.
The diode laser light beam enters and leaves the chamber through a KBr window
and after passing through the plasma reactor is sent to a monochromator, which is
necessary for mode selection and to block the broad band infrared emission from the
discharge, before detection with a mercury cadmium telluride (HgCdTe) detector.
Further improvements could be achieved by using long path cells with astigmatic
optics instead of the White cell configuration.
Alternatively tunable infrared radiation can be generated using difference fre-
quency mixing or optical parametric oscillators (OPO). Although in the past these
systems had the disadvantage of rather low radiation power and were restricted
to specific wavelength regions [136,137] new technical developments have led to
solutions which provide up to a mW of single mode power and tunability of up to
100 cm
−
1
[138,139].
Cavity ring-down spectroscopy (CRDS) is another high-sensitivity laser absorp-
tion technique [140]. This method is based on the measurement of the intensity decay
rate of a short laser pulse injected into an optical cavity formed by two very highly
reflective mirrors which may also enclose the plasma. For each reflection of the
laser pulse, a small fraction (depending on the reflectivity of the mirrors forming
the cavity) leaks through one of the mirrors. Therefore, the light intensity detected
outside the cavity decays incrementally with time to zero. Since the laser pulse is
trapped in the cavity for many thousands of round trips, absorption path lengths in the
kilometers range can be easily achieved, and absorptions as low as 10
−
9
can be mea-
sured with an acceptable signal-to-noise ratio [141,142]. In the visible spectral range
CRD spectroscopy has been applied for density measurements of the SiH
2
radical
and of nanometer-sized dust particles in silane plasmas [143] as well as for detecting
N
2
ions in nitrogen discharges [144]. Near- and mid-infrared tunable diode lasers
have been used as the light sources for CRD spectroscopy [145-148]. The recent
development and commercial availability of quantum cascade lasers (QCL) offers an
attractive new option for infrared absorption spectroscopy in the mid-infrared range
(MIR). Recently, the potential application of pulsed and cw QCLs, as easily tunable
MIR light sources, combined with cavity enhanced methods, like pulsed CRDS and
cavity-enhanced absorption spectroscopy (CEAS), has been established [149].
As well as the implementation of innovative techniques like CRDS, the perfor-
mance of the spectrometers used in plasma diagnostics have been greatly enhanced
by improved technology. The introduction of CCD cameras, holographic gratings,
tunable sources, and microprocessor instrument control and data collection are a
few examples of the experimental innovations, which have become commercially
available within the last decade.
