The detection of the methyl radical CH3, one of the most important compounds of the hydrocarbon plasma, has proven to be a particular challenge in measurement technology; in fact, the designation of absolute CH3 concentrations has been the subject of research worldwide for many years because this short-lived radical is accredited with playing a key role in the deposition of diamond layers. Synthetic diamond layers have various applications such as heat sinks in the semiconductor industry. Lead salt laser diodes have proven their worth many times over in the concentration measurement of reactive CH3 radicals, which usually only appear in very small concentrations of 1010-1011 particles/cm3.
The exact measurement of absolute particle concentration has just recently become possible. This vital step was achieved by reliably determining the line width of the n2 fundamental band at 606 cm-1 at the Institute for Low-Temperature Plasma Physics (INP), Greifswald, Germany, in cooperation with Cambridge University, United Kingdom, using lead salt laser diodes. The transition dipole moment μ2 was taken from the analysis of nine individually resolved rovibrational absorption lines (see Figure 1).
For this purpose, the methyl radical was made from butyl peroxide in a microwave argon plasma (f = 2.45 GHz). The molecule concentration was calculated by the recombination of CH3, which occurs within milliseconds and leads to the creation of ethane.
The experimental data of the dipole moment concurs precisely with theoretical predictions. [1]
[1] G. D. Stancu, J. Röpcke and P. B. Davies, Journal of Chemical Physics 122, 014306 (2005).
A number of physical units become measurable when using tunable diode laser absorption spectroscopy, including not only the absolute line position and width of optical transitions, but also the absolute particle concentration and how it subsides for unstable species as well. This also includes kinetic temperature, the temperature of rotation and vibration, and basic molecule parameters such as band centers, dipole moments, and magnetic parameters of line splitting in an electric or magnetic field.
