Molecular-rotation-induced splitting of the binary ridge in the velocity map of sub-eV H+ ions ejected from H2 molecules by ion impact
Résumé
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 H$^+$ fragment emission in collisions of 10-keV O$^+$ ions with 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 H$^+$ knockout is split into two parts arising from the rotational motion of the 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 H$_2$ dissociation in the atmospheres or exospheres of planets and moons.
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