Radiofrequency Perturbation of Selectively Excited Nuclear Hyperfine Levels
Many interesting experiments have been performed using optical resonance radiation. Similar types of experiments can, in some instances, be performed with nuclear resonance radiation. In the experiments described here, the Mössbauer effect has been used to selectively excite hyperfine sublevels, in direct analogy to the technique employed in optical pumping. Specifically, each of the four mj sublevels of the 14.4 keV state o/Fe57 have been individually and exclusively populated using a constant-velocity drive system, a single-line Co57 source, and a metallic iron scatterer arranged in 90° scattering geometry. The excited-state nuclear polarization was detected by analyzing the scattered radiation with a second spectrometer.
This procedure has been used to detect NMR transitions between the hyperfine sublevels of the 100-nsec state of Fe57. This result is encouraging, not only because it allows an accurate and velocity-independent determination of the Fe57 hyperfine parameters; but, more importantly, the Mössbauer-NMR technique provides an unusually clear method for studying many fundamental physical problems associated with excited-state NMR. Results thus far obtained provide little evidence to support the predictions of Hack and Hammermesh, or Gabriel regarding the NMR-produced distortion of the gamma-ray lineshape; however, the frequency dependence of the NMR transition probability is generally in accord with theory, although some anomalies result from the application of a static field.
During the investigation of the NMR process, it was discovered that subjecting an iron absorbing foil to an rf magnetic field produced additional distinct lines in the Mössbauer absorption spectrum of Fe57. These additional lines have been interpreted as frequency-modulated side bands caused by magnetostrictively produced vibrations. This sideband effect is the most likely explanation of the anomalous effects observed in earlier attempts to use the Mössbauer effect to observe excited-state NMR.
KeywordsNuclear Magnetic Resonance Metallic Iron Excited Nucleus Hyperfine Level Nuclear Magnetic Resonance Frequency
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