Microwave discharges

Unstable species such as O, FI and N atoms, molecular radicals and vibrationally excited diatomics can be injected by passmg the appropriate gas tluough a microwave discharge. In a SIFT, the chemistry is usually straightforward since there is only one reactant ion and one neutral present in the flow tube. 

Gorry P A and Grice R 1979 Microwave discharge source for the production of supersonic atom and free radical beams J. Rhys. E Sc/. 12 857-60... 

Microwave discharges at pressures below 1 Pa witli low collision frequencies can be generated in tlie presence of a magnetic field B where tlie electrons rotate witli tlie electron cyclotron frequency. In a magnetic field of 875 G tlie rotational motion of tlie electrons is in resonance witli tlie microwaves of 2.45 GHz. In such low-pressure electron cyclotron resonance plasma sources collisions between tlie atoms, molecules and ions are reduced and the fonnation of unwanted particles in tlie plasma volume ( dusty plasma ) is largely avoided. 

Ferreira C M and Moisan M (ed) 1993 Microwave discharges, fundamentais and appiications NATO ASI Series, Series B Physics voi 302 (New York Pienum)... 

Ohi A 1998 Fundamentais and iimitations of iarge area pianar microwave discharges using siotted waveguides J. Physique iV 8 Pr7 83-98... 

Far-ultraviolet Microwave discharge in noble gases Lyman discharge LiF (or no windows) Grating Photomultiplier photodiode photographic plate... 

SiF, cychc (SiOF2), (BOF), B20F3, F2BOS1F2 microwave discharge ... 

Sulfur bromides are but poorly characterized and there are few reliable data on them. SBr2 probably does not exist at room temperature but has been claimed as a matrix-isolated product when a mixture of S2Cl2/SCl2 Br2 Ar in the ratio 1 1 150 is passed through an 80-W microwave discharge and the product condensed on a Csl... 

XeCl2 has also been trapped in a matrix of solid Xe after Xe/Cl2 mixtures had been passed through a microwave discharge, but these halides are too unstable to be chemically characterized. 

Trace amounts of disulfane have been obtained in the gas phase by recombination of HS radicals generated by oxidation of hydrogen sulfide by oxygen atoms which in turn are produced from molecular oxygen in a microwave discharge [25] ... 

A number of hydrocarbon radicals having multiple bonds at the radical centre have also been trapped in inert matrices and studied by IR spectroscopy. Thus, ethynyl radical was obtained by vacuum UV photolysis (9) of matrix-isolated acetylene (Shepherd and Graham, 1987) as well as when acetylene and argon atoms excited in a microwave discharge were codeposited at 12 K (Jacox and Olson, 1987). An appearance of diacetylene bands was observed when the matrices were warmed up, while the absorptions of the radical C2H disappeared. Detailed isotopic studies of D-and C-labelled ethynyl radicals showed a surprisingly low frequency of the C=C bond stretching vibration at 1846 cm instead of c.2100cm for a true C=C triple bond (the band at 2104 cm was attributed to the... 

For stndying elementary reactions of importance in combnstion, a more controlled environment may be reqnired. Radicals or atoms may be generated by photolysis with UV radiation or with a microwave discharge on a snitable molecniar precursor. 

For example, a microwave discharge on H2 in an inert diluent, such as argon gas, is an efficient method for producing H atoms as reactants. Subsequent reaction of these H atoms with NO2 will yield OH and NO, and can serve as a useful source of hydroxyl radicals. These methods of reactant formation are well suited for experiments involving either static or flow reactor systems. 

In a typical MIP-MS instrument, the ICP portion is replaced with one of a variety of microwave discharge sources, usually a fairly standardised (modified) Beenakker cavity connected to a microwave generator. The analytical MIP at intermediate power (<500 W) is a small and quiet plasma source compared with the ICP. The mass spectrometer needs no major modifications for it to be interfaced with the MIP. With MIP used as a spectroscopic radiation source, typically consisting of a capillary (1mm i.d.), a power of 30-50 W and a gas flow below 1 L min 1, multi-element determinations are possible. By applying electrodeposition on graphite electrodes, ultratrace element determinations are within reach, e.g. pg amounts of Hg. 

Non-thermal plasmas can be produced in a number of ways, including a variety of electrical corona discharges, radio frequency discharges, microwave discharges and electron beams. The most common NTP technologies for emission reduction in engine exhaust streams are the following. 

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