Heavy Element Photophysics and Photochemistry

Raman Heterodyne Detection of Nuclear Magnetic Resonance (RHDNMR)

Raman Heterodyne Detection of Nuclear Magnetic Resonance (RHDNMR) and related methods of optically detecting nuclear magnetic resonance provide unprecedented detail in our studies of the interaction of actinide elements ions with their local environment in solids. We are applying these techniques to fundamental studies on radiation damage. Radiation damage is a central limiting factor in the performance of nuclear waste storage materials.

The basis for RHDNMR, one method of optically detecting nuclear magnetic resonance, is depicted below as an animated graphic. A heavy element atom (gray sphere) is shown within a solid lattice, depicted in "ball and stick" fashion where the "balls" represent some of the surrounding atoms.

When a resonant radio frequency field or wave (w) and a resonant optical field (W) interact with the heavy element atom, two new frequencies are generated. These are the sum (W + w) and the difference (W - w) between the interacting applied fields. These new frequencies are generated in a Raman process (denoted by the resonant atom changing color from gray to green). This is a coherent interaction of the applied radio frequency and optical fields that is mediated by the heavy element ion. The new frequencies are symbolized as W ± w in the graphic.

Because w is typically 10⁶ to 10⁸ Hertz and W is usually 10¹⁴ to 10¹⁵ Hertz, there is little difference (a few parts per million to a few parts per billion) in the frequency of the applied optical field and the two new light fields. Note that 1 Hertz = 1 cycle per second. We use a highly precise and selective technique that is based on heterodyne detection to measure the intensity of W - w, one of the newly produced frequencies, in some of our optically detected nuclear magnetic resonance studies.

Typically, the applied radio frequency, w, is varied during which time the optical field frequency, W, is held at a fixed frequency that is resonant with a f-electron transition of the heavy element ion. When the value of w comes into resonance with a spin-spin transition of the heavy element, the amplitude of the detected signal, W - w, reaches a maximum. Because the frequency of W is stable to within 1 part per billion of its nominal value, we achieve a high degree of site selectivity. Site selectivity is the ability to determine the characteristics of ions that have a particular local environment. In addition, we can measure radio frequency resonances as narrow as a few thousands of a Hertz which is a few parts per billion of the value of w. The combination of these capabilities provides us with unprecedented detail in investigating the interaction of heavy element ions with surrounding ions in crystalline and amorphous solids.

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