Differential Cross Sections for Ionization of Laser-Aligned Atoms by Electron Impact

The first experimental data are given for ($e,2e$) ionization from laser-aligned atoms. A linearly polarized laser excited Mg atoms to the $3{}^{1}P_1$ state prior to ionization by low energy electrons. The scattered and ejected electrons were detected in coincidence and the differential cross section determined for a range of alignment angles. An asymmetric coplanar geometry was used, with one electron fixed and the other detected at different angles. The data are compared to that from the spherically symmetric $3{}^{1}S_0$ state. Significant differences are found, in both magnitude and angular distribution.

Figure 1

Superelastic and inelastic scattering from Mg (closed circles), compared to when the laser is detuned (open squares). The elastic peak defines zero energy. The superelastic peak is clearly resolved at $-4.35\mathrm{eV}$. A new peak appears at $\sim 1.75\mathrm{eV}$ due to electron scattering from laser-excited targets.

Figure 2

TDCS from the spherically symmetric $3S$-ground state of Mg for outgoing energies $E_1=E_2=20\mathrm{eV}$, with ${\theta }_1=30°$. The TDCS is normalized to unity at the peak, which occurs at ${\theta }_2\sim 55°$. The direction of the momentum transfer $\mathbf{q}={\mathbf{k}}_{\mathbf{0}}-{\mathbf{k}}_{\mathbf{1}}$ is shown.

Figure 3

QDCS for alignment angles $\epsilon =0°$, 45°, and 90° of the $3P$ state, normalized to the TDCS in Fig. 2. The uncertainty in the relative normalization compared to the ground state is $\pm 9\%$ for $\epsilon =0°$, $\pm 7\%$ for $\epsilon =45°$, and $\pm 22\%$ for $\epsilon =90°$.