Millihertz magnetic resonance spectroscopy of Cs atoms in body-centered-cubic 4He

We report the results of electron-spin-resonance studies of Cs atoms trapped in the body-centered-cubic phase of a solid 4He matrix. Free-induction-decay times longer than 100 ms have been measured in spin-transient experiments. In optical-rf double-resonance experiments we have obtained linewidths of less than 20 Hz (full width at half maximum). The magnetometric sensitivity of our sample is 26 nG/√Hz. We have also demonstrated that magnetic resonance in such samples can be detected with high sensitivity using forward-scattered optical radiation. copyright 1996 The American Physical Society