There is another type of splitting that takes place giving rise to what is known as the hyperfine structure of the rubidium atom. This splitting is a result of the interaction between the total nuclear angular momentum, I, and the total angular momentum produced by the electron, J. The total nuclear angular momentum is a quantized value. For odd-even nuclei, (nuclei with an odd number of protons and an even number of neutrons), the value of i = 1/2, 3/2, 5/2. . . In the case of ⁸⁵Rb and its isotope, ⁸⁷Rb, the values of I are 5/2 and 3/2 respectively. The total angular momentum for the atom, F, can be calculated using the equation,
|J - I| < F < J + I. The values of F for ⁸⁵Rb are, F = 2 and F = 3 for both the 5s_1/2 and 5p_1/2 states. The values of F for ⁸⁷Rb are F = 1 and F = 2 for both the 5s_1/2 and 5p_1/2 states. The result is a more complex energy level diagram than that shown previously for the fine structure of rubidium.

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