Broadband cw-terahertz spectroscopy for characterizing reactive plasmas

Terahertz spectroscopy is seeing increasing use in analytical chemistry because of its ability to access ground state molecular rotational and vibrational transitions. Experiments reported on the use of THz spectroscopy to the studies of low pressure plasmas have focused on the use of pulsed sources, which have limited spectral resolution. These are suitable in the characterization of plasma parameters such as electron density, but somewhat inadequate in the study of molecular absorption spectra. The recent commercial availability of continuous THz sources with much higher spectral resolution has afforded the detailed probing of molecular absorption features but they have not yet been exploited in studies of low pressure plasma chemistry. One of the major benefits of continuous-wave THz spectroscopy (cw-TS) compared to optical and infrared methods is its applicability to the study of both plasma chemistry as well as parameters, i.e. electron density and collision frequency. In this article, we provide first insights into the use of cw-TS as a plasma diagnostic, and demonstrate the opportunities and challenges by example measurements in a low-pressure argon-methanol radio frequency inductively coupled plasma in which we estimate both the plasma-induced dissociation in the precursor methanol concentration, as well as the electron density and momentum scattering collision frequency.