Interference effects in double ionization of spatially aligned hydrogen molecules by fast highly charged ions

Cross sections differential in target orientation angle were measured for $19\mathrm{MeV}$ ${\mathrm{F}}^{8+}+{\mathrm{D}}_2$ collisions. Multihit position-sensitive detectors were used to isolate the double-ionization channel and determine a posteriori the full momentum vectors of both ejected ${\mathrm{D}}^{+}$ fragments. A strong dependence of the double ionization cross section on the angle between the incident ion direction and the target molecular axis is observed with a $\approx 3.5:1$ enhancement for molecules aligned perpendicular to the projectile axis. This clear asymmetry is attributed to interference effects, analogous to Young’s two-slit experiment, arising from coherent contributions to the ionization from both atomic centers. The data are compared to a simple scattering model based on two center interference.

Figure 1

Fragment momentum distributions in the laboratory frame. The ion beam is propagating in the $z$ direction. Slices of the momentum distributions are presented along three planes: (a) $P_x$ vs $P_y$, corresponding to the extraction field vs gas jet directions, (b) $P_z$ vs $P_y$, corresponding to the ion beam vs gas jet directions, and (c) $P_z$ vs $P_x$, corresponding to the ion beam vs extraction field directions. The long arrow through each ring represents the ion-beam direction, making clear the fragment distributions peaked perpendicular to the beam.

Figure 2

(Color online) Cross section for double ionization as a function of $\cos \left(\theta \right)$, where $\theta$ is the angle between the molecular axis and the ion beam direction. The solid curve is the fit to the data as described in the text.