Dead-time free measurement of dipole-dipole interactions between electron spins

A four-pulse version of the pulse double electron-electron resonance (DEER) experiment is presented, which is designed for the determination of interr adical distances on a nanoscopic length-scale. With the new pulse sequence electron-electron couplings can be studied without dead-time artifacts, so that even broad distributions of electron-electron distances can be charact erized. A version of the experiment that uses a pulse train in the detectio n period exhibits improved signal-to-noise ratio. Tests on two nitroxide bi radicals with known length indicate that the accessible range of distances extends from about 1.5 to 8 nm, The four-pulse DEER spectra of an ionic spi n probe in an ionomer exhibit features due to probe molecules situated both on the same and on different ion clusters. The former feature provides inf ormation on the cluster size and is inaccessible with previous methods, (C) 2000 Academic Press.