Hyperfine sublevel correlation spectroscopy (HYSCORE ) of disordered solids

Abstract

In the study of small hyperfine interactions unresolved in the EPR spectrum, in diluted paramagnetic systems, electron spin-echo envelope modulation has proven to be a very useful technique ( 1). For example, in a three-pulse (stimulated-echo ) experiment (2, 3), the nuclear spin frequencies of the nuclei that are coupled to the unpaired electron will appear as a modulation on the electron spin-echo (ESE) intensity which is recorded as a function of the time T between the second and third microwave pulses. The first two microwave pulses, which are separated by time T, will create nuclear spin coherences in the two electron spin ms manifolds, while the last microwave pulse will transform these coherences into a detectable electron spin-echo signal. The stimulated-echo ESEEM experiment has also been demonstrated as a two-dimensional technique, i.e., recording the electron spin-echo amplitude as a function of both times T and T to show the correlation of the nuclear spin coherences of the two electron spin ms manifolds to each other (4). A serious drawback of this experiment is the limited time range available in the 7 direction due to the usually very short T2 relaxation time of the electron spin system. In the last decade many new ESEEM experiments have been introduced (5). In particular, the two-dimensional four-pulse stimulated-echo experiment introduced by P. Hofer et al. as HYSCORE (hyperline sublevel correlation spectroscopy) (6) provides a useful alternative to the classical two-dimensional three-pulse technique. The HYSCORE experiment is derived from the three-pulse experiment by the insertion during the second time interval (see Fig. 1) of a x pulse which exchanges the populations of the ms manifolds. In terms of two-dimensional correlation spectroscopy, this pulse serves as a mixing pulse which creates correlations between nuclear spin transitions associated with the two electron spin ms manifolds. The mixing pulse terminates the evolution interval t, and is followed by the detection interval t2. As in the three-pulse experiment, the nuclear coherences evolving during this time interval are sampled by the last microwave pulse creating an electron spin echo. Compared to the classical