Improved optogalvanic detection with voltage biased Langmuir probes

Abstract

Optogalvanic detectors show great potential for infrared spectroscopy, especially in cavity enhanced techniques where they, in contrast to ordinary absorption detectors, can perform intracavity measurements. This enables them to utilize the signal-to-noise ratio improvement gained from the extended effective path length inside an optical cavity, without losing signal strength due to the limited amount of light exiting through the rear mirror.However, if optogalvanic detectors are to become truly competitive, their intrinsic sensitivity and stability has to be improved. This, in turn, requires a better understanding of the mechanisms behind the generation of the optogalvanic signal. The study presented here focuses on an optogalvanic detector based on a miniaturized stripline split-ring resonator plasma source equipped with Langmuir probes for detecting the optogalvanic signal. In particular, the effect of applying a constant bias voltage to one of the probes is investigated, both with respect to the sensitivity and stability, and to the mechanism behind the generation of the signal. Experiments with different bias voltages at different pressures and gas composition have been conducted. In particular, two different gas compositions (pure CO 2 and 0.25% CO 2 in 99.75% N 2 ) at six different pressures (100 Pa to 600 Pa) have been studied. It has been shown that probe biasing effectively improves the performance of the detector, by increasing the amplitude of the signal linearly over one order of magnitude, and the stability by about 40% compared with previous studies. Furthermore, it has been shown that relatively straightforward plasma theory can be applied to interpret the mechanism behind the generation of the signal, although additional mechanisms, such as rovibrational excitation from electron-molecule collisions, become apparent in CO 2 plasmas with electron energies in the 1-6 eV range. With the achieved performance improvement and the more solid theoretical framework presented here, stripline split-ring resonator optogalvanic detectors can evolve into a compact, inexpensive and easy-to-operate alternative for future infrared spectrometers.

Manufacturing miniature Langmuir probes by fusing platinum bond wires

“…The values for these fusings were not logged but rather extrapolated from other events given the voltage and current settings. Figure 12 shows a comparison between one of these probes and a cylindrical probe from [12] when evaluated as Langmuir probes in the SSRR plasma source. From Langmuir probe theory, it is evident that a properly functioning, spherical Langmuir probe should exhibit a higher saturation current at positive bias above the plasma potential compared with a cylindrical one [15].…”

“…The wire was stretched between the pads in the same manner as with the test array in figure 1, between the two probepads. The wire was fused with a voltage and current limit of 25 V and 5 A, respectively, to form two spherical Langmuir probes, replacing the cylindrical probes used in previous studies [10,11,12]. For a description of the positioning of the plasma probes inside the SSRR see [10].…”

“…The probes were used to study the I-V characteristics of the plasma, by applying a constant bias voltage, V B , to one of the probes using a Model 2400 source meter, Keithley Instruments Inc., USA, while connecting the other probe to ground. The bias voltage was varied between -60 V and 60 V, while recording the current, I B , through the probes with the source meter, in a way similar to that in [12].…”

“…Finally, a Parylene coated wire was integrated in a SSRR plasma source, resembling those described in [12], but with a 3 mm wide plasma gap. The wire was stretched between the pads in the same manner as with the test array in figure 1, between the two probepads.…”

“…In [10,11,12], a concept employing a microplasma source with integrated cylindrical Langmuir probes for carbon isotope measurements measuring an optogalvanic signal was reported on. The plasma source was fabricated on printed circuit board, and the Langmuir probes were made from 25 µm thick gold wires, by letting the tails of the bonded wires protrude into the plasma.…”