A theorem of fundamental importance in spectroscopy and angular momentum theory which provides both (1) an explicit form for the dependence of all matrix elements of irreducible tensors on the projection quantum numbers and (2) a formal expression of the conservation laws of angular momentum (Rose 1995).
The theorem states that the dependence of the matrix element
on the projection quantum numbers is entirely
contained in the Wigner 3j-Symbol (or, equivalently, the Clebsch-Gordan
Coefficient), given by
See also Clebsch-Gordan Coefficient, Wigner 3j-Symbol
References
Cohen-Tannoudji, C.; Diu, B.; and Laloë, F. ``Vector Operators: The WIgner-Eckart Theorem.'' Complement in
Quantum Mechanics, Vol. 2. New York: Wiley, pp. 1048-1058, 1977.
Edmonds, A. R. Angular Momentum in Quantum Mechanics, 2nd ed., rev. printing.
Princeton, NJ: Princeton University Press, 1968.
Gordy, W. and Cook, R. L. Microwave Molecular Spectra, 3rd ed. New York: Wiley, p. 807, 1984.
Messiah, A. ``Representation of Irreducible Tensor Operators: Wigner-Eckart Theorem.'' §32 in Quantum Mechanics, Vol. 2.
Amsterdam, Netherlands: North-Holland, pp. 573-575, 1962.
Rose, M. E. ``The Wigner-Eckart Theorem.'' §19 in Elementary Theory of Angular Momentum.
New York: Dover, pp. 85-94, 1995.
Shore, B. W. and Menzel, D. H. ``Tensor Operators and the Wigner-Eckart Theorem.'' §6.4 in Principles of Atomic Spectra.
New York: Wiley, pp. 285-294, 1968.
Wigner, E. P. Group Theory and Its Application to the Quantum Mechanics of Atomic Spectra, expanded and improved ed.
New York: Academic Press, 1959.
Wybourne, B. G. Symmetry Principles and Atomic Spectroscopy. New York: Wiley, pp. 89 and 93-96, 1970.