Rapid field sweeps

In order to combat this problem, Wenckebach and co-workers devised an approach whereby the external magnetic field is rapidly swept through the entire ESR line during the application of mw fields for DNP. °° This rapid field sweep is typically performed... 

Answer The spectrometer was sweeping the magnetic field to find a lock signal, so the resonant frequencies were changing rapidly during the collection of the FID. One observes the same ringing pattern when continuous wave spectra are swept too fast. If one tries to collect several transients under these conditions, one obtains many copies of the spectrum superimposed at different frequencies. The field sweep can be turned off if a lock is not going to be used. 

Fig. 8. Adiabatic rapid passage signal observed by monitoring the phosphorescence intensity at 456 nm of [Rh (bpy)3] (0104)3 after a rapid microwave sweep through the zero-field I> -1 resonance, at 1.4 K...

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