Broadening power saturation

In the conventional ESR method using continuous microwave.radiation (cw ESR), the identification and quantification of radical species are made from the spectral shape and the spectral intensity, respectively, under the condition of a low enough level of microwave power incident to the sample cavity. If the power level is too high, the structure of ESR spectra becomes broadened and obscure and the intensity of the spectra is no longer proportional to the radical concentration (power saturation effect). Care is usually taken to avoid these effects in cw ESR measurements. 

Variable power measurements elucidate the saturation behavior of the nuclei. At low powers the NMR signal amplitude increases, and the line width remains unchanged with increasing rf power. At high powers saturation occurs, the resonant line broadens, and finally it decreases in amplitude with increasing rf power. Relaxation times may be eom-puted from saturation data (12). 

The result of this power broadening or saturation is to reduce the absorption in the line centre compared with the absorption in the wings of the line. This in turn leads to loss of analytical signal intensity and an apparent broadening of the absorption line profile. The resulting effect on the line shape function can be described by an equation due to Karplus and Schwinger, for low powers and incomplete saturation (ref 3, p. 50) ... 

The effect of saturation works out to a contribution of —43 kHz. On a line of width Av — 200 kHz, typical for those operating conditions, that would be about 5% rms sum addition to the linewidth. This example serves to illustrate the tradeoff between Q, source power, pressure and hence linewidth, in the choice of operating conditions. In the example given, this contribution to line broadening would hardly be noticeable. For quantitative work where spectral peak area is measured, the effect of power saturation will be less apparent than if height were the measurand. 

With the availability of more powerful sources at lower MMW fi equencies however, Pi typically —tens mW, even allowing for lower spectrometer 0 — 10, power broadening or saturation would have a more significant effect on the linewidth and consequently on the shape of quantitative response curves. This would become particularly important if signals from species of very different values were to be compared. 

In general, the intensity of an ESR spectrum increases with an increase in the microwave power R When the applied power level is sufficiently low, thermal relaxation processes can, to a good approximation, maintain the Boltzmann equilibrium between spin levels. When the power level exceeds that amount, the ESR spectrum broadens and its intensity begins to decrease and eventually disappears. This phenomenon is called the power saturation or saturation broadening effect and depends... 

It can be seen that the spectrum in the frequency domain can be obtained without power saturation in either the pressure or Doppler broadened limit. 

The spin-lattice relaxation, with a characteristic time Ti, is responsible for maintaining the population difference between levels, N and N+. The spin-spin relaxation time T2 reflects the lifetime of the excited state and its effect on the line width. If the electron-spin relaxation rate is too rapid, the lifetime of the excited state is short and the EPR spectrum becomes broadened. At high temperatures the spectrum may become too broad for detection, hence the use of cryogenic temperatures for some transition ions. However, if the spin-lattice relaxation is too slow, the population difference N - N+ cannot be maintained, and the amplitude of the signal is attenuated, a situation known as microwave power saturation. Electron-spin relaxation times may be estimated by measuring the amplitude of the signal as a function of applied microwave power. 

One effect of saturation, and the dependence of e on /, is to decrease the maximum absorption intensity of a spectral line. The central part of the line is flattened and the intensity of the wings is increased. The result is that the line is broadened, and the effect is known as power, or saturation, broadening. Typically, microwave power of the order of 1 mW cm may produce such broadening. Minimizing the power of the source and reducing the absorption path length t can limit the effects of power broadening. 

The more powerful the solvent-solute interaction, the more pronounced will solvent broadening be for this reason, saturated hydrocarbons are preferred as solvents for spectroscopy, and such strongly interacting media as methylene chloride and chloroform are to be avoided. It is obvious that the requirements of spectroscopy and those of solubility are in direct conflict. Carbon tetrachloride and carbon disulphide are often used as compromise solvents ) (although both of these react thermally or photochemically with many carbonyl complexes) but are generally inferior spectroscopically to alkanes. 

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