Two-photon absorption of few-electron heavy ions

The two-photon absorption of few-electron ions has been studied by using second-order perturbation theory and Dirac's relativistic equation. Within this framework, the general expressions for the excitation cross sections and rates are derived including a full account of the higher-order multipole terms in the expansion of the electron-photon interaction. While these expressions can be applied to any ion, independent of its particular shell structure, detailed computations are carried out for the two-photon absorption of hydrogen-, helium-, and berylliumlike ions and are compared with the available theoretical and experimental data. The importance of relativistic and nondipole effects in the analysis and computation of induced two-photon transitions is pointed out. Moreover, we discuss the potential of these transitions for atomic parity-violation studies in the high-$Z$ domain.

Figure 1

(Color online) The zero-rank angular polarization tensor $T_{00}$ for the $2E1$ transition (left panel) and the $E1-M1$ transition (right panel) as a function of the opening angle $\theta$. The polarization vector of the first photon lies within the reaction plane ${\chi }_1=0$, while the second photon is polarized under the angle ${\chi }_2=0$ (solid line), $\pi /4$ (dashed line), and $\pi /2$ (dashed–dotted line) with respect to this plane.