Microwave saturation

It is relatively easy to decide which vibronic bands have a common origin. This is accomplished by observing the phosphorescence intensity change of each band upon microwave saturation at a frequency that corresponds to transitions between rz and tx. This is known as phosphorescence-microwave double resonance (PMDR) spectroscopy. These frequencies for 2,3-dichloroquinoxaline are given in Table 6.3. 

From Figure 6.1 it is easy to see that microwave saturation of the t -tz transition of pyrazine should decrease the v = i transition intensity and increase the v = j + k transition intensity. Microwave saturation of the rx—Ty transition should have no effect on v = i, but decrease the intensity of v =j + k. 

As illustrated in Fig. 41 (see p. 84), the number of observed Cu-ENDOR lines (63Cu enriched sample217 ) does not follow the usual ENDOR selection rules for EPR observers with resolved hfs (two transitions for EPR observers with mi = I, four transitions for EPR observers with - I < n < I Sect. 3.2). This may be due to transfer of microwave saturation to EPR transitions other than the one which is used as an observer. A corresponding effect has been observed in Co-ENDOR spectra of Co(acacen)61. ... 

The various maceral types exhibit distinctly different microwave saturation properties. 

If the microwaves saturate the - tj transition, then the intensity ratio of the emission from zf level t is given by... 

ESR spectra at X-band were measured with a RE-1006 spectrometer operating at 9,6 GHz (empty cavity at ambient temperature) and 50-lcHz magnetic field modulation. Spectra were recorded at a microwave power of 1 mW and a modulation amplitude 0.02 mT to avoid line shape distortions, which could arise from experimental conditions such as microwave saturation and overmodulation. Scan range was 150-420 mT, Calibration of g-values is based on diphenylpicrylhydracyl (DPPH, g=2.0036) and Cr (g= 1.9796) standards. The amounts of the paramagnetic species (PMS) were calculated by double integration of the resonance line areas. 

Star-branched heptaradical 37 and decaradical 38 possess the S — 1/2 and S — 5 ground states, respectively.47,95 The relatively well-resolved EPR spectrum (Ams = 1) of 10 3 M 37 in 2-MeTHF/THF matrix at 100 K was numerically fitted to a threefold symmetric S — 1/2 state ( E/hc 0).95 The much less-resolved spectrum for 10 3M 38 consisted of a symmetrical progression of shoulders, of which positions agree reasonably well with calculated resonance positions for an S — 5 state.95 At low temperatures, the EPR spectra for both polyradicals were highly susceptible to microwave saturation. Magnetic moment for 2 x 10 3 M decaradical... 

EPR spectra of 58 at 140-150 K were obtained on the identical samples, for which SQUID magnetic studies were carried out. A single resonance at g 2 in the Aim, — 1 region was observed. For samples with the increasing values of S, the width of the resonance decreased and its propensity to microwave saturation greatly increased. These changes were mirrored by decreased intensity of the Ams — 2 signal.117... 

The experiments described above produce accurate decay constants only when sir is negligible. The apparent kinetic parameters that are obtained when this is not the case involve all of the k and W values simultaneously. It is not possible under these circumstances to obtain the actual decay constants from these experiments. Experimental methods have been developed, however, that allow extraction of the individual k and W values in the presence of slr. One method applies to the regime where sir is dominant, that is, W > k, whereas the other is applicable when W < k. The latter method has been applied to biopolymer ODMR and involves deconvolution of phosphorescence decays measured during continuous microwave saturation of pairs of triplet sublevels. Microwave saturation creates a pseudo-two-level system whose decays are easily deconvoluted and are amenable to analysis. The analytical development of the microwave-saturated phosphorescence decay method is rather lengthy, so it is not discussed in this chapter. Detailed descriptions of the method may be found elsewhere. 

Corrected for sir. Values obtained by microwave-saturated phosphorescence decay analysis. 

Microwave saturation The amplitude of an unsaturated ESR line increases with the square root of the applied microwave power. For high sensitivity one therefore attempts to use as high power as possible without saturating the sample. High microwave power can be applied to samples with short spin-lattice relaxation for alanine a value of up to about 20 mW has been employed with standard X-band ESR equipment. 

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). 

The power dependence in the first factor applies for the saturation of a so-called inhomogeneously broadened ESR line (see Section 9.5.1) where Po is a measure of the microwave saturation. At an applied microwave power P = Pq the ESR signal is 1 / /2 = 0.707 of that of an unsaturated signal, while the signal strength becomes independent of the microwave power for P >> Pq, see Fig. 9.4. The signal may also... 

The separation of signals from different species by means of different microwave saturation has been applied in work on irradiated spices [76]. A typical spectrum may contain contributions from transition metal ions, particularly, Mn, and a radiation organic radical, sometimes overlapped with another naturally occurring free radical spectrum. It was recommended that the microwave saturation properties should be considered in future protocols to survey the radiation history of soft foodstuffs. 

The method to obtain relaxation times from CW microwave saturation measurements for an inhomogenously broadened line is based on assumptions given in the literature [77, 78], The ESR line shape is then expressed as a convolution of a Gauss and a Lorentz function [81] ... 

Previous experiments in this laboratory (Chen et ai, 1973) have shown that it is possible to resolve hyperfine structure in a hf ODMR spectrum providing the spectrometer is operated at low power in order to avoid the combined effects of microwave saturation and forbidden electron-nuclear transitions (as in the zf experiment). To illustrate this, we show in Fig. 9 first-derivative hf spectra of C-benzophenone-dio with 1 mW power. The lines shown are the Antg = 1 transitions with H x (low-field), H y (low-field), and H z (high-field). Each is split into a doublet by the first-order hyperfine interaction. Similar splittings are observed in the... 

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