Strong Molecular Alignment Dependence of ${\mathbf{H}}_2$ Electron Impact Ionization Dynamics

Low-energy ($E_0=54\mathrm{eV}$) electron impact single ionization of molecular hydrogen (${\mathrm{H}}_2$) has been investigated as a function of molecular alignment in order to benchmark recent theoretical predictions [Colgan et al., Phys. Rev. Lett. 101, 233201 (2008) and Al-Hagan et al., Nature Phys. 5, 59 (2009)]. In contrast to any previous work, we observe distinct alignment dependence of the $(e,2e)$ cross sections in the perpendicular plane in good overall agreement with results from time-dependent close-coupling calculations. The cross section behavior can be consistently explained by a rescattering of the ejected electron in the molecular potential resulting in an effective focusing along the molecular axis.

Figure 1

Differential cross sections for the ionization of ${\mathrm{H}}_2$ by 54 eV electron impact as a function of the emission angle of one electron with the other electron fixed to ${\theta }_{e1}=-50°$, equal energy sharing ($E_1=E_2$), and averaged ${\mathrm{H}}_2$ alignment. (a) Experimental 3D cross section for nondissociative ionization and (b) the zoomed 3D cross section in the perpendicular region in (a). Panels (c) and (d) are the electron emission patterns in the perpendicular plane for the dissociative ionization. Panel (c) displays the results in a polar plot while (d) shows the same data in a Cartesian representation.

Figure 2

Fully differential cross sections for the ionization of aligned ${\mathrm{H}}_2$ molecules and equal energy sharing ($E_1=E_2=18\mathrm{eV}$) with one electron emission angle fixed to ${\theta }_{e1}=-50°$ [${\overrightarrow{p}}_1$ indicated in (b)] as a function of the emission angle of the second electron in the perpendicular ($x\mathrm{\text{−}}y$) plane. The ${\mathrm{H}}_2$ molecule is aligned as indicated by the (blue) spheres in the left column along the $z$ axis (a), the $x$ axis (b), and the $y$ axis (c). While the left column displays the FDCS in a polar plot, Cartesian plots are shown in the right column.

Figure 3

As in Fig. 2 with the $y$-axis alignment (${\theta }_{\mathrm{Mol}}=90°$, ${\mathit{\varphi }}_{\mathrm{Mol}}=90°$) and variable energy sharing. From the top row to the bottom row, the electron energies are $E_1=E_2=18\mathrm{eV}$ (a), $E_1=26\mathrm{eV}/E_2=10\mathrm{eV}$ (b), and $E_1=32\mathrm{eV}/E_2=4\mathrm{eV}$ (c).

Figure 4

As in Fig. 2 with ${\mathrm{H}}_2$ alignment angles for which the reflection symmetry with respect to the $x\mathrm{\text{−}}z$-plane is broken: (a) ${\theta }_{\mathrm{Mol}}=90°$, ${\mathit{\varphi }}_{\mathrm{Mol}}=45°$; (b) ${\theta }_{\mathrm{Mol}}=45°$, ${\mathit{\varphi }}_{\mathrm{Mol}}=45°$; (c) ${\theta }_{\mathrm{Mol}}=45°$, ${\mathit{\varphi }}_{\mathrm{Mol}}=90°$; and (d) ${\theta }_{\mathrm{Mol}}=45°$, ${\mathit{\varphi }}_{\mathrm{Mol}}=135°$.