Chemistry Reference
In-Depth Information
long-standing recommendation for the choice of specific laser systems can barely be
given but the reader is referred to respective commercial sections of general physics
journals.
The outline of the observation optics is a typical issue in optical engineering. In
LIF, spectral transmission characteristics should be considered also in conjunction
with supplementary measurements like calibrations. Generally, a spectral filter such
as an interference filter can be employed to suppress plasma radiation and the intense
background of laser stray light. The choice of the detector is determined by the
required temporal resolution and spectral response. Photomultiplier tubes, avalanche
diodes, or even intensified charge coupled devices (ICCD) are used to detect the
fluorescence light with different response times and spatial resolution up to 2D
imaging.
Absolute measurements of fluorescence signal require calibration measurements
[245]. For this purpose, well-defined scattering processes such as Rayleigh scattering,
or titration techniques are employed. A different option is the combination detection
of LIF along the plasma with absorption measurements or even the consideration
of radiation transport [246]. The calibration procedure gives the sensitivity of the
detection (measure, e.g., in photons per mV). Calibration measurements should also
be used to determine the measurement uncertainties; a particular caveat has to be
raised since many quantities may cancel in a formal treatment of calibration, but the
errors do not.
6.4.3 F
URTHER
R
EADING
The overview given in the previous sections gives a survey of potentials and limits of
LIF as a diagnostic technique. For further background in general laser spectroscopy,
the monograph [235] is an outstanding entry. Specific to plasma chemistry, a compi-
lation by Amorim et al. [236] serves as an excellent overview. Table 6.2 reviews and
partly updates this compilation. This table may serve as a more specific entry to the
field to those readers who consider an application of laser-induced fluorescence.
6.5 MASS SPECTROMETRY
Gas discharge physics is connected with mass spectrometry since the beginning of
this measuring technique. Thomson [277] constructed the first apparatus, the parabola
spectrograph, with a dc discharge as ion source. The position of the parabola on the
detector plate is related to the ion mass (
m
z
,ionmass
m
, charge
z
), its length depends
on the ion energy distribution. For the first time, isotopes of a nonradioactive element
20
Ne and
22
Ne were detected in the canal rays by the two parabolas. The parabola
spectrograph with its possibility of ion identification by the mass/charge relation and
of the ion energy distribution can be denoted as the first plasma monitor.
/
6.5.1 M
ASS
S
PECTROMETER
A complete mass spectrometer consists of the gas inlet, an ion source, the mass
analyzer and ion detector. For more details, see textbooks on mass spectrometry like
[278-280]. Commercial plasma monitors, constructed also for ion and for neutral gas
