Discharge sources, radical generation

The fundamental principle of PES is the photo-electric effect. A molecule M in the gas phase is irradiated with monochromatic UV light which is usually generated by a helium discharge source (Hel 21.22 eV, 58.43 nm Hell 40.81 eV, 30.38 nm). Electrons can be ejected when their binding energy is lower than the photon energy leaving behind a radical cation M+ in a certain electronic and vibrational state. 

There are several ways to generate radicals for use in spectroscopic studies. We discuss four of them controlled chemical reaction, discharge sources, pyrolysis, and photolysis. 

Various routes have been used to generate matrix-isolated phosphorus oxide radicals, for example, PO2 and PO3. These are (i) codeposition of phosphine with O atoms from a microwave discharge source (ii) the matrix reaction of P2 (generated by heating GaP) and O3 (iii) laser ablation of P4O10 (iv) laser ablation of red P in a stream of O2. Both... 

The gas-phase chemistry of Si-containing radicals forms one important sub-mechanism which is attracting considerable attention. The two most important Si radicals generated in either discharge, pyrolysis, or photochemical CVD sources are silylene, SiH2, and silyl, SiHs. 

The spectrum near 600 cm of the V2 band of the CH3 radical generated by a glow discharge in di-tert-butylperoxide (Yamada et. al. 1981) provides a second example of this selectivity. With Zeeman modulation all the observed lines arise from CH3, but with source frequency modulation many additional lines from diamagnetic species are detected in the same spectral region. 

It has been experimentally determined that a number of sources of radicals, including some generated by pyrolysis, may be used for this technique. However, the low-temperature glow-discharge is a convenient source of radicals for synthetic work. 

APCI. The column effluent is nebulised into an atmospheric-pressure ion source. Through a corona discharge, electrons initiate the reactant gas-mediated ionisation of the analytes. Proton transfers are typical reactions generating [M + H]+ or [M — H] ions, although radical ion formation is possible as in high vacuum chemical ionisation (Cl). The ions formed are injected into the high vacuum of the mass spectrometer. APCI typically accepts flow rates of up to 2 mL min-1. 

The reaction of CBr4 with potassium is reported to generate free C atoms and the rate constants for reaction with methane, ethylene, and benzene have been reported. The reaction of nitrogen atoms with CN radicals has also been used as a C atom source. Carbon atoms have also been produced by passing organics through a microwave discharge. ... 

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