Nuclear coherence-transfer echoes in pulsed EPR

Abstract

Nuclear coherence-transfer echoes created in disordered electron–nuclear spin systems by microwave pulses are described. Once nuclear coherence is generated, a nonselective microwave π pulse inverts the electron spins thereby interchanging nuclear coherence between the two electron spin manifolds. This exchange refocuses the inhomogeneity of the secular part of the hyperfine interaction. The mechanisms of echo formation for weak and strong hyperfine couplings are explained by using simple vector diagrams. Various methods to create and detect nuclear coherence by microwave pulses are discussed with special attention directed to the sequence π/2–τ–π/2–t1–π–t2–π/2–τ. In this four-pulse experiment the nuclear coherence-transfer echo is observed as an amplitude modulation of the electron spin echo intensity. The nuclear coherence-transfer echo can be shifted to a time interval fully separated from the one covered by the instrumental deadtime. The time evolution of an S=1/2, I=1/2 model spin system during the four-pulse sequence is described using the density operator formalism. Several one- and two-dimensional four-pulse experiments are discussed on this basis and their spectra are analyzed in terms of nuclear coherence-transfer pathways. A new one-dimensional experiment based on nuclear coherence-transfer echoes, deadtime free ESEEM by nuclear coherence-transfer echoes (DEFENCE) is proposed to exploit undistorted electron spin echo envelope modulation (ESEEM) spectra. DEFENCE is a fast pulsed EPR technique that is unpretentious with respect to the required microwave equipment. The predicted features of the nuclear coherence-transfer echoes and of the DEFENCE approach are verified experimentally.