Measurement of the M1 amplitude and hyperfine mixing between the 6S1/2-7S1/2 caesium states

We have measured the ratio M1/β of the M 1 amplitude to the vector polarizability β of the 6S1/2-7S 1/2 transition in Cs, by a method using Stark-induced interference effects, free from background difficulties. The amplitude varies from one hyperfine component to the other, according to M1 + (F' - F) Mhf1 with M1 = - (4.13 ± 0.18) x 10-5| μB |/c and Mhf 1 = - (0.763 ± 0.066) x 10-5 |μB|/c. Mhf 1 results from a mixing between 6S1/2 and 7S1/2 induced by the hyperfine interaction Hhf and yields a value of < 7S1/2 | Hhf | 6S1/2 > which agrees with a theoretical prediction involving the product of the 6S and 7S hfs splittings. The theoretical significance of this result is discussed.