Laser-Induced Fluorescence (LIF) is the emission from atoms or molecules that have been excited to higher energy levels by absorption of laser radiation. Fluorescence detection is advantageous compared to absorption due to the higher sensitivity of the method. Analytical applications of LIF  in gases include monitoring atmospheric pollution as well flame and plasma diagnostics. LIF  is a very sensitive, in-situ, non-intrusive spectroscopic method for sampling neutral or charged atomic or molecular species at their fundamental or excited states. The method can give qualitative and quantitative information, also spatially and temporally resolved. The detection limits of the method are below 10-10 cm-3.

lif jpg 1

The above scheme shows the simplest, yet efficient, experimental setup for detecting LIF signals from electric discharges. The laser beam has a path perpendicular to the symmetry axis of the chamber and parallel to the electrodes. Beam alignment and shape and quality of optical windows are of primary importance in order to maximize the signal and to minimize laser reflections. In this direction one uses two axis tilt mirrors and Brewster angle windows, at least at the beam exit from the chamber.

Fluorescence light is collected at right angles, using a suitable collimating and focusing lens assembly, starting a few nanoseconds after the excitation with a laser pulse having a typical duration of a few nanoseconds. The collected light is focused at the entrance slit of a monochromator (alternatively a PMT equipped with a suitable interference filter) that has a light detection device on its exit slit (PMT or CCD). A small part (or a reflection) of the Laser Beam is used to excite a fast photodiode (with a response time in the order of a few ns) for producing an electric trigger pulse used for synchronizing the experiment.

The electrical signal obtained from the detection device can then be transferred via a 50 Ω coaxial line, to a signal recording and manipulation unit. The traditional way is to use a so-called Boxcar Gated Integrator & Averager. This device uses the detector signal and the trigger pulse to recover and enhance the signal from a noisy background.

The gated integrator integrates the signal that is present during the time the gate is open, ignoring noise and interference that is present at all times (i.e. spontaneous emission). Boxcar Averaging refers to the practice of averaging the output of the gated integrator over many shots of a repetitive experiment. Since any signal present during the gate will add linearly, while noise will add in random way as the square root of the number of shots, averaging N shots will improve the signal-to-noise ratio by a factor of sqrt(N).  An adjustable time-delay (from the trigger) and time-width integration gate (in the order of ns to ms) is used to position the gate relative to the signal.

Further signal normalization, baseline subtraction and enhancement techniques can be used.

Another way of doing the same thing is by using a Digital Storage Oscilloscope with math and averaging capabilities. In this case one can acquire hundreds of entire waveforms that can be averaged, stored integrated in time etc, like the ones shown in the next figure:


lifsignal1This is is a very simple and educative method because you can actually see the fluorescence evolution in time and the averaging S/N enhancement as the waveform samples add up.

The figure shows the real-time average of a few hundreds LIF  waveforms from the 415.8 nm transition of Ar metastables in an Ar glow discharge (red line). The blue line shows the signal detected in the discharge off condition due to laser reflections (again the average of many waveforms). The true Fluorescence intensity in time (green curve) can be obtained by subtracting the two curves. You can easily obtain a figure representative of the LIF intensity (in nVs) by integrating this curve in time or the Fluorescence Lifetime from the intensity decay and if you calibrate the detection system you can even have absolute density measurements.

Finally, if you can use a gated CCD with a suitable computer software,  you can obtain entire parts of the spectrum (with a monochromator) or spatial and temporal mapping of the fluorescence light at once.

A complete description of the method is presented in the following PTLUP publications:

    1. "Spatial profiles of reactive intermediates in rf silane discharges"
         D. Mataras, S. Cavadias, D. Rapakoulias
         J. Appl. Phys. 66, 119 (1989) ©

    2. "Nitrogen ion dynamics in low-pressure nitrogen plasma and plasma sheath"
         D. E. Gerassimou, S. Cavadias, D Mataras and D. E. Rapakoulias
         J. Appl. Phys. 67, 146 (1990) ©

    3. "Power dissipation mechanisms in rf driven silane discharges, the influence of discharge geometry"
         D. Mataras, S. Cavadias, D. Rapakoulias
         J. Vac. Sci. Technol. 11,664 (1993) ©

    4. "Dilution enhanced radical generation in Silane Glow Discharges"
         D. Mataras, F. Coutelieris, P. Kounavis, D.E. Rapakoulias
         J. Phys. D: Appl. Phys. 29, 2452 (1996) ©

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