Multiple Quantum Transitions in ENDOR

This is in agreement with the observed linewidth in DQT spectra which are approximately one half (or even less) of the corresponding linewidth of the SQT. 

DQT generated by a single rf field may be helpful, for example, to elucidate complicated energy level schemes or to assign ENDOR transitions to the corresponding nucleus4.  

Two rf fields411. The restriction E, = (Em + Ek)/2 for the intermediate energy level E may be dropped if photons of two different frequencies generate a DQT. The corresponding transition probability is then given by 

To observe a DQT with sufficient intensity V and v2, with h(v + v2) = Ero - Ek, may be chosen such that A is small for any value of E( (Fig. 20 b). With two pumping fields and by using appropriate modulation schemes, the SQT can be suppressed and selective detection of the DQT is possible41.  

6 ENDOR with Circularly Polarized rf Fields (CP-ENDOR) 

CP-ENDOR has been introduced by Schweiger and Gunthard to reduce the density of ENDOR lines of complicated paramagnetic systems with a large number of interacting nuclei. ENDOR spectra of solutions (liquid or frozen), polycrystalline powders and single crystals can often be simplified remarkably using this technique. 

In a spin system, each nuclear spin precesses around its individual effective static field Beff = Bo + Be(ms), (Sect. 3.3). Since the resonance frequency of a nuclear transition is proportional to B s, ENDOR lines for different types of nudei may be observed in the same frequency range. 

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Multiple quantum transitions (MQT) in ENDOR spectra may be observed for nuclei with I 1 if two or more (rf) photons of the same or of different frequencies combine to produce an ENDOR transition41,62,99"101). In a MQT the magnetic quantum number nij changes by An = n. The MQT should therefore be clearly distinguished from corresponding forbidden single quantum transitions (SQT) with Am] > 0 discussed in Sect. 3.3. 

During the last few years the versatility of ENDOR spectroscopy has been improved by a number of new techniques which make use either of special types of pumping fields (CP-ENDOR, PM-ENDOR), of more than one rf field (DOUBLE ENDOR, multiple quantum transitions, nuclear spin decoupling) or a different display of the spectrum (EI-EPR). In addition to these techniques, alternative methods have been developed (electron spin echo and electron spin echo ENDOR) which are able to supplement or to replace the ENDOR experiment under certain conditions. The utility of all these various advanced techniques, particularly in studies of transition metal compounds, has recently been demonstrated. 

These overlapping structures mask the intrinsic ENDOR lineshape due to the spin density distribution of the soliton, which can be observed in pristine cis-rich samples [105], The above discussion of multiple and fractional quantum transitions in C-enriched samples is further supported by the fact that the structures due to the interaction of C and H nuclear spins are also weakly observed in the pristine samples because of the non-zero natural abundance of C. Figure 6.25 shows the frequency-derivative ENDOR spectra at 12 K in a 230% stretched pristine cis-rich sample, which are the same spectra shown in Figure 6,20 for the v+ branch. The external field is parallel and perpendicular to the stretch direction of the sample in the upper and lower curves respectively. In the figure, small structures other than distinct turning points at V are resolved and it has been found that their frequencies exactly... 

In DOUBLE ENDOR (Sect. 4.3) the spin system is simultaneously irradiated with two rf fields. The frequencies of both fields have to be set independently from each other. Generation of rf fields by broadband DOUBLE ENDOR instruments are often strong enough for solid state studies at low temperature. In spin decoupling experiments (Sect. 4.4), however, the amplitude of one of the two fields (namely the decoupling field) should be as large as possible, whereas in studies of multiple quantum transitions ) (Sect. 4.5) two strong rf fields have to be applied. 

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