Sensitivity Enhancement by Matched Microwave Pulses in One- and Two-Dimensional Electron Spin Echo Envelope Modulation Spectroscopy

Abstract

The concept of microwave pulse matching is applied to three-pulse electron spin echo envelope modulation and sublevel correlation (HYSCORE) spectroscopy. Matched pulses enhance the efficiency of forbidden transfers and may drastically increase the signal intensity of basic frequency and combination frequency transitions in these conventional pulse EPR experiments. The theory of matched pulses is extended to the case of strong and largely isotropic hyperfine interactions, and numerical simulations are presented to gain a deeper insight into the inner working of the matched-pulse approach. It is shown that the enhancement of combination frequencies in matched HYSCORE can be used to determine the relative sign of hyperfine coupling constants as well as the number of equivalent nuclei. The enormous capacity of the approach is demonstrated on ordered and disordered systems. In particular, it is shown that in HYSCORE experiments the signal-to-noise ratio improvement for strongly coupled nitrogens and for proton combination peaks may be considerably larger than one order of magnitude, corresponding to a reduction in measuring time of more than a factor of 100.