Chemistry Reference
In-Depth Information
Molecular plasmas are increasingly being used not only for basic research but
also, due to their favorable properties, for materials processing technology. These
fields of application have stimulated the development of infrared spectroscopic tech-
niques for industrial applications. In order to exploit the capabilities of infrared
TDLAS for effective and reliable online plasma diagnostics and process control in
research and industry, compact and transportable tunable infrared multicomponent
acquisition systems (IRMA, TOBI) have been developed [150,151]. These systems
are mainly focused on
1. High speed detection of stable and transient molecular species in plasmas
under non-stationary state excitation conditions
2. Sensitive (sub-ppb) trace gas detection with the aid of multi-pass absorption
cells
ThemaindisadvantageofTDLASsystemsusingleadsaltdiodelasersisthenecessary
cryogenic cooling of the lasers (and also of the detectors), since they operate at
temperatures below 100 K. Systems based upon lead salt diode lasers are therefore
typically large in size and require closed cycle refrigerators and/or cryogens like
liquid nitrogen. Compared to lead salt lasers, QCLs allow the realization of very
compact mid-infrared sources characterized by narrow line width combined with
single-frequency operation and considerably higher power values, i.e., of tens of
mW. The output power is sufficient to combine them with thermoelectrically cooled
infrared detectors, which permits a decrease of the apparatus size and presents a new
opportunity to design compact liquid nitrogen-free mid-IR spectroscopic systems.
These positive features of quantum cascade laser absorption spectroscopy (QCLAS)
can open up new fields of application in research and industry, including studies
of gases in atmospheric, environmental, and plasma chemistry but also for in-situ
control of industrial plasma processes.
Recently a compact quantum cascade laser measurement and control system
(Q-MACS) has been developed for time-resolved plasma diagnostics, process con-
trol, and trace gas monitoring. The Q-MACS system contains a tunable quantum
cascade laser which can be directed through a plasma or into a multipass cell for
exhaust gas detection. Rapid scan software with real-time line shape fitting provides
a time resolution of up to 100 ns for studying the kinetic processes of infrared active
compounds in plasmas or gases. The Q-MACS-Basic system has been designed as a
platform for various applications of QCLAS [135,152].
6.3.3 E
MISSION
S
PECTROSCOPY
6.3.3.1 General Considerations
In low-temperature molecular plasmas a variety of processes, such as electron col-
lisions, ion-molecule reactions, charge exchange and chemical reactions, lead to a
large number of atoms, molecules, radicals, and ions in many different excited states.
The generation of photons is mainly governed by the de-excitation of these species.
Provided the influence of self-absorption inside the plasma is relatively small or neg-
ligible (referred to as optically thin plasma conditions) the emitted photons can be
