Perturbed-Angular-Correlation Spectroscopy: Renaissance of a Nuclear Technique
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MRS BULLETIN/JULY 1995
quadrupole interactions associated with some NMR-active nuclei are not only difficult to measure but they often obscure much of the chemical-shift information by producing spectral-line broadening. PAC spectroscopy can be used to accurately measure the nuclear electric-quadrupole interactions at the sites of nuclei of atoms that are trace dopants in crystals. But the PAC technique is insensitive to the effects of local charge distributions that produce the NMR chemical shifts. ME spectroscopy can be used to measure both of these effects as well as nuclear magnetic-dipole interactions. However, ME measurements are often only practical on crystals that have one of several elements as a major constituent, that is, either Fe or Sn. In addition, the ME sensitivity depends on temperature, and the NMR and EPR sensitivities also depend on temperature. However, the PAC measurement is independent of temperature, which can be a great advantage for studying phenomena such as phase transitions. Although each of these techniques involves measuring the interactions of nuclei, each also requires a different number of nuclear probes. For example, ME spectroscopy requires approximately 1014 probe atoms for absorber experiments, NQR spectroscopy typically 18 requires about 10 atoms, and PAC specn 12 troscopy requires only about 10 -10 probe atoms.1 Thus, techniques based on hyperfine interactions can be viewed as being complementary to each other as well as to other types of spectroscopy.
The Past In 1953, A. Abragam and R.V. Pound published a seminal paper that presents a thorough treatment of the theory of PACs.2 The immediate relevance of this work was to the field of nuclear structure because •γ-y angular correlations* were and are a powerful technique with which to determine the properties of nuclear levels, for example, spins and information about the angular momentum carried away by the γ-rays. The presence of extranuclear fields and the concomitant perturbations produced by these fields interfere with the accurate measurement of γ-y angular correlations and the determination of the associated aspects of nuclear structure. The use of a PAC as a probe of crystal and molecular structure remained an idea awaiting its own genesis. The first PAC experiments were performed during the 1950s, but the scope of these measurements was limited by the lack of sophisticated instrumentation as well as by the lack of envisioned applications. During the mid-1960s and the early 1970s, interest in performing PAC experiments grew both in the United States and in Europe. This growth paralleled the growth of interest in performing other measurements of hyperfine interactions, especially following the discovery of γ-ray recoilless nuclear-resonance absorption by R. Mossbauer in 1958.3 The analysis of the most noteworthy PAC experiments performed in the United States during this era served as contributions to the physics of metals4 and magnetism5 and to biophysics.6 In the United States, the practitioners were primarily physi
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