The generalized hyperfine sublevel coherence transfer experiment in one and two dimensions

Abstract

The generalized hyperfine sublevel coherence transfer pulsed ESR experiment is described that consists of a nuclear coherence generator, a first evolution period, a nonselective microwave π pulse, a second evolution period, and a nuclear coherence detector. Several incrementation schemes to perform one-dimensional experiments are discussed, namely, a four-pulse electron spin echo envelope modulation scheme for measuring combination frequencies, two types of nuclear coherence transfer echo experiments (DEFENCE) to record dead time free electron spin echo envelope modulation spectra of disordered systems, and a new hyperfine spectroscopy experiment for the direct measurement of hyperfine frequencies. The one-dimensional experiments can be combined to four two-dimensional schemes. In addition to hyperfine sublevel correlation spectroscopy (HYSCORE) where two nuclear frequency dimensions are correlated, three new two-dimensional experiments are introduced that correlate the hyperfine with the nuclear frequency, the combination frequency with the nuclear frequency, and the combination frequency with the hyperfine frequency. With a properly selected two-dimensional incrementation scheme the digital resolution and signal-to-noise ratio of the spectra can be improved and artifacts can be suppressed. The predicted features of the one- and two-dimensional experiments are verified experimentally for both ordered and disordered systems using echo and coherent Raman beat detection.

Properties of the HYSCORE Spin Echo Signal

“…Based on our analysis, we propose the processing of the experimental data that allows one to obtain the pure HYSCORE ESEEM. The results of this work are also directly applicable to other one-dimensional (1D) and 2D experiments (3,5,6,8) based on the same four-pulse sequence and differing in the incrementation schemes for the interpulse delays and in the types of spectral peaks obtained (fundamental, sum, or difference combination peaks and their various correlations).…”

“…HYSCORE is essentially the nuclear coherence transfer experiment (3,5,6,8) where ESEEM reflects the nuclear spin evolution between the mw pulses. However, the ESEEM is carried by the electron spin echo signal, whose amplitude is also determined by factors unrelated to the nuclei responsible for the ESEEM.…”

“…2, if the relative duration of the third mw pulse (with 3 ϭ ) decreases, the unwanted V HS signal also decreases. This represents the most direct way to suppress the contribution of this signal to V H (6).…”

“…In HYSCORE, this leads to the incomplete inversion of the electron spin magnetization by the third pulse. As a result, the ESE signals of two types are simultaneously formed in the HYSCORE experiment (2)(3)(4)(5)(6). One of the signals is contributed by those spins reoriented by the third mw pulse.…”

“…The HYSCORE and stimulated ESE signals cannot be separated by phase cycling (4,6), and they both contribute to the resulting ESEEM. In the 2D spectrum, the stimulated ESEEM produces peaks on the main diagonal of the (ϩϩ) quadrant, which may obscure the observation of weak HYSCORE lines located close to this diagonal.…”