Selective excitation in electron spin-echo modulation experiments

Abstract

Electron spin-echo spectroscopy (1-6) has become a powerful method for the study of hyperfine and quadrupole interactions in inhomogeneously broadened systems. Both twoand three-pulse experiments have been used to measure the echo modulation envelope as a function of pulse separation. Modulation effects are caused by interactions which do not change sign under a 7r pulse and necessarily involve a violation of the high-field assumption for nuclear resonance under the particular measurement condition (1, 2). It is now widely accepted that weak selective microwave pulses cause the echo envelope modulation to disappear (4, 5). Reduction of modulation depth due to partial excitation has recently been analyzed in detail by Barkhuijsen et al. (7). On the other hand we have recently found that weak extended-time excitation reproduces the entire echo envelope modulation in a single experiment, provided that the weak irradiation is followed by a short and strong refocusing pulse (8). In this communication we give further experimental evidence for the fact that a weak selective excitation pulse followed by a strong refocusing pulse produces the same envelope modulation as obtained with two strong pulses. We also present an explanation for this finding. The experimental results are summarized in Fig. 1. Figure la gives the echo envelope modulation for a Cu(II)-aquo complex at 4.2 K using short microwave pulses both for excitation (10 ns) and refocusing (20 ns) with nominal flip angles of (a/2) and K, respectively. The Fourier transform of this pattern consists of two peaks at 14 and 28 MHz corresponding to the proton Zeeman frequency and its second harmonic. When the short excitation pulse is replaced by a long (210 ns) weak pulse, no change in the modulation pattern is observed (Fig. lb). However, a loss of signal amplitude occurs caused by the reduced bandwidth of excitation, not visible in Fig. lb due to normalized amplitudes. In Fig. lc, the short refocusing x pulse is also replaced by a long (2 10 ns) and weak pulse. In contrast to the first two experiments the echo modulation pattern disappears completely. By increasing the amplitude of the two pulses part of the echo modulation may be recalled (Fig. Id). The pattern observed, however, does not fully correspond to