Nuclear Polarizability Contribution to P-Levels

This energy shift is negative because when the electron polarizes the nucleus this leads to an additional attraction of the induced dipole to the electron. 

The contribution induced by the magnetic susceptibility may also be easily calculated [23], but it is even of higher order in Za (order (Za) ) since the magnetic field strength behaves as 1/r. This additional suppression of the magnetic effect is quite reasonable, since the magnetic field itself is a relativistic effect. 

Consideration of the P-state polarizability contribution provides us with a new perspective on the 5-state polarizability contribution. One could try to consider the matrix element in (6.30) between the 5 states. Due to nonvanishing of the 5-state wave functions at the origin this matrix element is 

The nuclear size and polarizability corrections of order Zaf obtained in Sect. 

2 are very small. As was explained there, the suppression of this contribution is due to the large magnitude of the characteristic momenta responsible for this correction. The nature of the suppression is especially clear in the case of the Zemach radius contribution in (6.13), which contains the small factor m) (r )(2). The nuclear size correction of order [Zotf m contains an extra factor Za. in comparison with the nuclear size and polarizability corrections of order [Za f but its main part is proportional to the proton charge radius squared. Hence, we should expect that despite an extra power of Za this correction is numerically larger than the nuclear size and polarizability contributions of the previous order in Za. As we will see below, calculations confirm this expectation and, moreover, the contribution of order Za) m is additionally logarithmically enhanced. 

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