Kinetic and Diagnostic Studies of Molecular Plasmas Using Laser Absorption Techniques

Abstract

Within the last decade mid infrared absorption spectroscopy between 3 and 20 µm, known as Infrared Laser Absorption Spectroscopy (IRLAS) and based on tuneable semiconductor lasers, namely lead salt diode lasers, often called tuneable diode lasers (TDL), and quantum cascade lasers (QCL) has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, organosilicon and boron compounds has lead to further applications of IRLAS because most of these compounds and their decomposition products are infrared active. IRLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species, which is of particular importance for the investigation of reaction kinetics. Information about gas temperature and population densities can also be derived from IRLAS measurements. A variety of free radicals and molecular ions have been detected, especially using TDLs. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of infrared spectroscopic techniques to industrial requirements. The recent development of QCLs offers an attractive new option for the monitoring and control of industrial plasma processes as well as for highly time-resolved studies on the kinetics of plasma processes. The aim of the present article is threefold: (i) to review recent achievements in our understanding of molecular phenomena in plasmas, (ii) to report on selected studies of the spectroscopic properties and kinetic behaviour of radicals, and (iii) to describe the current status of advanced instrumentation for TDLAS in the mid infrared.

Time–space resolved distribution of oxygen metastable atoms in an axially symmetrical atmospheric pressure barrier discharge

“…Moreover, the temporal resolution is rather poor, since in order to obtain a useful signal, this is integrated over a small period of time, which would be a drawback for pulsed discharges. A more appropriate diagnosis technique for such discharges is tunable diode laser absorption spectroscopy (TDLAS), applied across the spectrum from the vacuum ultraviolet (VUV) to the far infrared (FIR), allowing one to non-invasively monitor one specific species, with a temporal resolution limited only by the detectors and oscilloscopes (typically nanoseconds), at spatial resolution down to micrometers [13,14].…”

DBD Surface Modification of Polymers in Relation to the Spatial Distribution of Reactive Oxygen Species

“…Unlike emission spectrometry, TDLAS allows good spatial and temporal resolution even for small and transient discharges [18,19].…”

Is plasma unique? The presence of electrons and the importance of charge

“…IR absorption techniques have been employed for, e.g. CH 3 [32] and OH [33], absorption and cavity ring down have been used to measure CH [34], SiH, SiH 2 , SiH 3 [35], NH and NH 2 [36], to cite a few examples. Energy selective mass spectrometry has been used [37] with success to measure even more types of molecular radicals.…”