Corrections to atomic ground state energy due to interaction between atomic electric quadrupole and optical field
Hu Jie1, †, Chen Yu1, Bai Yi-Xiu1, He Pei-Song2, Sun Qing1, Ji An-Chun1
       

(color online) Typical atomic energy level structure. Δ e1 and Δ e2 are the detuning of the electric dipole transition (red arrow) and the quadrupole transition (blue arrow), respectively. E e1 and E e2 are the energy of the excited P and D orbital states, respectively. ħ ω is the energy of the optical field.