XI Laser Magnetic Resonance LMR

1 X 10 molecules cm , compared with limits of about 10 for e.s.r. and 3 X 10 molecules cm for u.v. resonance fluorescence employing a water-vapour discharge light source. Rate constants for the reaction of OH radicals with CO, NO, and NO2 at 296 K using LMR are in excellent agreementwith values obtained from a variety of other techniques. Positive identification of the HO2 radical is based partially on spectral analysis and partly on the use of a variety of different chemical methods to produce the radical. Extension of the work to study reactions of HCO and HO2 radicals should follow swiftly. 

The method may be illustrated by reference to the photolysis of azomethane, and for simplicity it willl be assumed that only reactions (IS) and (16) occur. 

At low flashing frequency, [CHs ] follows the photolysis lamps closely, and in a very short time compared with the photolysis period it reaches the steady-state concentration given by equation (19). As the absorption arising from the radical 

is measured experimentally, kie can be calculated if lo is known. Further, as diazomethane is removed soiely in reaction (IS), its concentration is proportional to exp( -/ot/2) if the lamps are on for 50% of the time, where t is the overall running time. From equation (19), [CH3]. oc exp( -Iot/4), and lo can be determined from measurements of the decay of the absorption with overall running time at very low photolysis frequencies. The analysis becomes more complicated if the CHa radical is removed simultaneously by a first-order process. 

Spectra have been assigned to CH3-, t-C4H9-, CHsOa-, CaHsO -, i-CaHTOa, and t-C4H90a radicals. The spectra have been established partly by spectral analysis and partly by the use of difierent chemical reactions to obtain the spectrum. Gas-chromatographic analysis of products is used to confirm mechanisms. The results are discussed in Section 5. 

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