Power, or saturation, broadening

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. 

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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... 

A high-resolution spectrum of the clock transition is shown in Fig. 2. The clock-laser power was reduced to 30 nW to avoid saturation broadening. The fit with a lorentzian curve results in a linewidth of 170 Hz (FWHM), corresponding to a fractional resolution bv/v of 1.3 10-13. A spectral window of 200 Hz width contains 50% of all excitations. According to our present experimental control of the ion temperature, electromagnetic fields and vacuum conditions, no significant Doppler, Zeeman, Stark or collisional broadening of the absorption spectrum of the ion is expected beyond the level of 1 Hz. The linewidth is determined by the frequency instability of the laser and the lineshape is not exactly lorentzian... 

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) ... 

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. 

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. 

Lamb dip stabilization onto very narrow saturation peaks allows one to achieve a very good frequency stability, especially when pressure or power broadening or transit broadening can be minimized, as in the case of the 3.39 ym He-Ne laser stabilized onto a CH transition. 

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. 

Saturation occurs if the relaxation rate is too slow (t is long) to dissipate the energy provided by the electromagnetic fleld to the spin system. This can be observed experimentally when the absorption does not increase and/or a line broadening occurs at increased microwave power. This unwanted situation can be avoided by decreasing the power, P, normally until the absorption becomes proportional to yP. 

Line-width The amplitude of an ESR 1st derivative line is inversely proportional to the line-width squared at a fixed concentration of free radicals. A narrow line-width is therefore desirable. The anisotropy of the g-factor and/or the hyperfine coupling causes line broadenings in several of the commonly used dosimeter materials, e.g. alanine and Li-formate. The line-width also tends to increase at microwave saturation, which is an additional reason for not increasing the power excessively. In practice the signal of an ESR-dosimeter may be distributed over several hyperfine lines, causing loss of sensitivity. For alanine five lines are present (Fig. 9.3). 

Figure 6. Spectral diffusion and its role in ENDOR and ESEEM. Top left The inhomogene-ously broadened EMR line is represented by a superposition of (homogeneously broadened) spin packets that may be saturated so that a hole is burned in the line (Bottom Left). Spectral diffusion spreads the saturating power through portions of the spectrum and may collapse all or part of the EMR line (Top Right). In an ENDOR experiment, the populatimi of a secmid spin packet is transferred to the portion of the spectrum where a hole has been burnt, and the whole is filled (recovery of saturated EMR signal). Cross-relaxati

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