Elliptic and Circular Dichroism Effects in Two-Photon Double Ionization of Atoms

An ab initio parametrization of the two-photon double ionization amplitude from an $s^2$ subshell of an atom in a ${}^{1}S$ state is presented and used to predict two light polarization effects on photoelectron angular distributions that do not exist in single-photon double ionization: (i) elliptic dichroism and (ii) circular dichroism at equal energy sharing. Estimates for He show large magnitudes for these effects, which provide a means for polarization control of double ionization by vacuum ultraviolet light.

Figure 1

Schematic diagrams for the knockout contributions to the TPDI transition amplitude. The solid dots indicate summations over intermediate states. Diagrams with interchanged ${\mathbf{p}}_1$ and ${\mathbf{p}}_2$ must also be included.

Figure 2

TDCS for TPDI of He by linearly polarized photons of energy $\omega =45\mathrm{eV}$. Full (red) curves, present LOPT knockout results; dotted (blue) curves, TDCC results [3]; dashed (green) curves, CCC results [5] scaled to the TDCC results as in 5.

Figure 3

Geometries for measurement of the ED and equal energy sharing CD effects in TPDI. (a) Orthogonal geometry for measurement of ED at $p_1=p_2$. (b) Coplanar geometry for measurement of ED, with one electron (${\mathbf{p}}_1$) detected in the $xy$ plane and the other (${\mathbf{p}}_2$) detected in the plane spanned by $\mathbf{k}$ and ${\mathbf{p}}_1$. (c) Geometry for measurement of CD at $p_1=p_2$ with fixed ${\phi }_2$.

Figure 4

TDCS, $\sigma$, and the ED parameter, ${\delta }_{\mathrm{ED}}$, for TPDI of He for equal energy sharing ($E_1=E_2=5.5\mathrm{eV}$). (a–d) Orthogonal geometry [Fig. 3a]. (e),(f) Coplanar geometry [Fig. 3b]. Left column: full (red) curves, $\sigma (|\xi |)$; dashed (blue) curves, $\sigma (-|\xi |)$. Right column: full (red) curves, ${\delta }_{\mathrm{ED}}$.

Figure 5

Equal energy sharing CD effect for the geometry in Fig. 3c. Parameters and curves are as in Fig. 4.