Racah's W-coefficients were introduced by Giulio Racah in 1942.[1] These coefficients have a purely mathematical definition. In physics they are used in calculations involving the quantum mechanical description of angular momentum, for example in atomic theory.

The coefficients appear when there are three sources of angular momentum in the problem. For example, consider an atom with one electron in an s orbital and one electron in a p orbital. Each electron has electron spin angular momentum and in addition the p orbital has orbital angular momentum (an s orbital has zero orbital angular momentum). The atom may be described by LS coupling or by jj coupling as explained in the article on angular momentum coupling. The transformation between the wave functions that correspond to these two couplings involves a Racah W-coefficient.

Apart from a phase factor, Racah's W-coefficients are equal to Wigner's 6-j symbols, so any equation involving Racah's W-coefficients may be rewritten using 6-j symbols. This is often advantageous because the symmetry properties of 6-j symbols are easier to remember.

Angular momenta in the Racah W coefficients. The top is a 2d plane projection as a quadrilateral, the bottom is a 3d tetrahedral arrangement.

Racah coefficients are related to recoupling coefficients by

Recoupling coefficients are elements of a unitary transformation and their definition is given in the next section. Racah coefficients have more convenient symmetry properties than the recoupling coefficients (but less convenient than the 6-j symbols).[2]

Recoupling coefficients

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Coupling of two angular momenta   and   is the construction of simultaneous eigenfunctions of   and  , where  , as explained in the article on Clebsch–Gordan coefficients. The result is

 

where   and  .

Coupling of three angular momenta  ,  , and  , may be done by first coupling   and   to   and next coupling   and   to total angular momentum  :

 

Alternatively, one may first couple   and   to   and next couple   and   to  :

 

Both coupling schemes result in complete orthonormal bases for the   dimensional space spanned by

 

Hence, the two total angular momentum bases are related by a unitary transformation. The matrix elements of this unitary transformation are given by a scalar product and are known as recoupling coefficients. The coefficients are independent of   and so we have

 

The independence of   follows readily by writing this equation for   and applying the lowering operator   to both sides of the equation. The definition of Racah W-coefficients lets us write this final expression as

 

Algebra

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Let

 

be the usual triangular factor, then the Racah coefficient is a product of four of these by a sum over factorials,

 

where

 

and

 
 
 
 

The sum over   is finite over the range[3]

 

Relation to Wigner's 6-j symbol

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Racah's W-coefficients are related to Wigner's 6-j symbols, which have even more convenient symmetry properties

 

Cf.[4] or

 

See also

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Notes

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  1. ^ Racah, G. (1942). "Theory of Complex Spectra II". Physical Review. 62 (9–10): 438–462. Bibcode:1942PhRv...62..438R. doi:10.1103/PhysRev.62.438.
  2. ^ Rose, M. E. (1957). Elementary theory of angular momentum (Dover).
  3. ^ Cowan, R D (1981). The theory of atomic structure and spectra (Univ of California Press), p. 148.
  4. ^ Brink, D M & Satchler, G R (1968). Angular Momentum (Oxford University Press) 3  ed., p. 142. online

Further reading

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