In studies of ion-induced molecular fragmentation, the challenging measurement of the velocity distribution of fragments emitted below 1-eV kinetic energy is rarely achieved, although most fragments have an energy below this value. Here, we study ${\mathrm{H}}^{+}$ fragment emission in collisions of 10-keV ${\mathrm{O}}^{+}$ ions with ${\mathrm{H}}_2$ molecules using a field-free time-of-flight technique developed specifically to detect sub-eV fragments. We find that, in the velocity map, the binary ridge due to direct ${\mathrm{H}}^{+}$ knockout is split into two parts arising from the rotational motion of the ${\mathrm{H}}_2$ molecule, and that this split scales with rotational velocity. The velocity distribution of the nuclei in the original molecule is determined and the thermally populated $J=1$ rotational level is found to be the dominant contributor, although asymmetry in the split indicates projectile-induced rotational transitions between $M$ sublevels. These rotation effects influence fragment emission probabilities, thus carrying important consequences for the radiation-induced hydrogen loss and ${\mathrm{H}}_2$ dissociation in the atmospheres or exospheres of planets and moons.

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• Charge-transfer collisions
• Collimation
• Quantum coherence & coherence measures
• Ions
• Molecules