Electron spin-echo envelope modulation induced by slow intramolecular motion

A new type of electron spin-echo envelope modulation (ESEEM) was observed in a three-pulse stimulated echo experiment on nitroxides in a molecular glass. With fixed interval T between the second and the third pulses and variable interval tau between the first and the second ones, it appears in the frequency domain as a broad peak around similar to5 MHz. It is attributed to a sudden change of the EPR resonance frequency due to reorientation of the methyl groups of the nitroxide during the T interval. The rate of the growth of this peak with increasing T reflects the methyl group reorientation rate, which was found to be strongly temperature dependent.