Off-diagonal hyperfine interaction between the 6p_1∕2 and 6p_3∕2 levels in ¹³³Cs

The off-diagonal hyperfine interaction between the 6p_1∕2 and 6p_3∕2 states in ¹³³Cs is evaluated in third-order MBPT giving 37.3 and 48.3 Hz, respectively, for second-order energies of the 6p_3∕2 F=3 and F=4 levels. This result is a factor of 10 smaller than one obtained from an uncorrelated first-order Dirac–Hartree–Fock calculation and used in the analysis of a recent high-precision (⩽2 kHz) measurement of the 6p_3∕2 hyperfine structure [ Gerginov et al. Phys. Rev. Lett. 91 72301 (2003)]. The factor of 10 difference has negligible effect on the conclusions of the recent experiment but will become important for experiments carried out at a precision of better than 1 kHz.