Controlling Two-Electron Threshold Dynamics in Double Photoionization of Lithium by Initial-State Preparation

Double photoionization (DPI) and ionization-excitation (IE) of $\mathrm{Li}(2s)$ and $\mathrm{Li}(2p)$, state-prepared and aligned in a magneto-optical trap, were explored in a reaction microscope at the free-electron laser in Hamburg (FLASH). From 6 to 12 eV above threshold ($\hslash \omega =85$, 91 eV), total as well as differential DPI cross sections were observed to critically depend on the initial state and, in particular, on the alignment of the $2p$ orbital with respect to the VUV-light polarization, whereas no effect is seen for IE. The alignment sensitivity is traced back to dynamical electron correlation at threshold.

Figure 1

Intuitive scheme for Li DPI by VUV light linearly polarized along the $z$ axis for different populations (a) $m_p=0$ and (b) $m_p=\pm 1$ of the magnetic ($m_p$) sublevels of the excited $2p$ orbital. For details see text.

Figure 2

Two-dimensional recoil ion momentum distributions for single ionization by 85 eV FEL light of (a) $\mathrm{Li}(2s)$ and (b) laser excited $\mathrm{Li}(2p)$. Here the laser and VUV-light polarizations are parallel as indicated by the arrows.

Figure 3

Same as for Fig. 2, but for double ionization. Projections of the momentum distributions onto the vertical axis are shown in the diagrams to the right.

Figure 4

Double ionization recoil ion momentum distributions as a function of total ${\mathrm{Li}}^{2+}$ momentum by (a) 85 eV and (b) 91 eV photons for the three different initial states indicated. The lines are to guide the eye.