Chemistry Reference
In-Depth Information
achieving this. The need for a better scientific understanding of plasma physics and
chemistry has stimulated the application of TDLAS, which has proven to be one of
the most versatile techniques for studying molecular plasmas. Based on the recent
development of quantum cascade lasers the further application of this method of
high-resolution mid-infrared spectroscopy to industrial applications has become a
reality.
6.4 LASER INDUCED FLUORESCENCE SPECTROSCOPY
Atoms and ions both in their ground states and excited levels play a key role in reactive
plasmas. The spatial and temporal density distributions of these particles are central
parameters to understand the complicated chemical kinetics of heavy particles. In
order to reveal details for modeling, state-selective densities for the reactive species
are required for the refinement and validation of physical models. Laser-induced
fluorescence measurements address this issue experimentally.
Laser-induced fluorescence (LIF) is a diagnostic technique for measuring prop-
erties of excitable atomic species (atoms, molecules, ions) [235]. Usually, tunable
laser light is employed to excite atoms or ions. The subsequent re-emission, the fluo-
rescence, is then detected, mainly to infer densities and temperatures of the emitting
particles. Different from laser absorption techniques (see Section 6.3), laser-induced
fluorescence gives spatially resolved information on the excited state. However, LIF
results are relative measurements, i.e., a calibration is required for a quantitative
interpretation of experimental results. The spatial resolution is given both by the
exciting laser beam and the observation arrangement. An attractive option of LIF
is its potential to provide two-dimensional measurements by observing a laser sheet
with appropriate camera systems. High time resolution may be attained by short-pulse
excitation with pulsed lasers.
LIF is a long established technique in laser spectroscopy; for a review see [236].
Compared to sophisticated techniques like coherent anti-Stokes Raman scattering
(CARS), the experimental effort is less complicated. LIF is applied in many engi-
neering and natural science contexts; an example is the investigation of combustion
systems [237].
Since LIF is a state-selective measurement, the laser light is to be tuned to an
atomic transition. If small-band, tunable laser light sources are employed, any particle
property reflected in absorption line shape can be inferred. Hence, in addition to the
density, also the temperature of excitable atoms or ions can be determined [238]. More
specifically, tuning techniques allow one to access all parameters determining the
absorptionlineshape, i.e., drift velocities, collisional de-excitationrates bycollisional
quenching, and even details of ionic distribution functions [239]. And scanning
the laser over split levels gives access to electromagnetic fields. for example, the
combination with absorption to Stark-states may be employed to derive small electric
field strengths as occurring in low temperature plasmas [240].
The remainder of this section introduces basic physics background for LIF
measurements. A brief overview on tools is part of a practically oriented section.
A selection of applications relevant to plasma chemistry will finish the section.
