THE HYPERFINE-STRUCTURE OF CS ATOMS IN THE BCC PHASE OF SOLID HE-4

We have detected the ground state F = 4 --> F = 3 hyperfine transition of cesium atoms implanted in a b.c.c. matrix of solid He-4 using a microwave-optical double-resonance technique. The transition frequency is blue-shifted by similar to 197 MHz (2.1%) with respect to the free atomic transition and grows with increasing pressure at a rate of similar to 1.4 MHz/bar.