Transverse momentum of ionized atoms and diatomic molecules acquired in collisions with fast highly charged heavy ions

Abstract

Using a recoil-ion momentum spectrometer (RIMS), transverse momenta (${q}_{\ensuremath{\perp}}$) of recoil atomic and molecular ions emerging from collisions between 2.5-MeV/u Xe${}^{34+}$ projectiles and neutral Ne, Ar, CO, N${}_{2}$, and O${}_{2}$ gases were measured as a function of the degree of target ionization. For the molecular targets the resulting distributions of ${q}_{\ensuremath{\perp}}$ corresponding to different dissociation channels were separated. Measurements with all the targets were performed under virtually identical conditions so that the results could be directly compared in detail. It was found that the shapes of the ${q}_{\ensuremath{\perp}}$ distributions are characterized by single asymmetric peaks and that, at least in the vicinity of their maximum values, they are well described by Weibull functions. Except for the Ar target, the velocities corresponding to the maxima of the ${q}_{\ensuremath{\perp}}$ distributions were found to follow (within the uncertainties of the results) a universal function of the target charge-to-mass number ratio for values up to 0.33. For larger charge-to-mass number ratios the results for the molecular targets appear to remain universal, but are slightly lower than the results for the Ne target. A more elaborate scaling was required to obtain a universal function that describes the low-charge-state results for Ne and Ar targets in a similar way. For different dissociation channels of the same parent molecular ion with charge states exceeding 6+, significant differences were found between the measured ${q}_{\ensuremath{\perp}}$ distributions. These differences were ascribed to increases in the ionization potential and to more prominent contributions from target-to-projectile electron transfer for the more asymmetric dissociation channels. For symmetric and nearly symmetric molecular dissociation channels the fragments with combined charge number $Q$ exceeding 9 for N${}_{2}$ and 10 for O${}_{2}$ were found to be distributed with a reduced probability at angles close to 90\ifmmode^\circ\else\textdegree\fi{} relative to the beam direction. The magnitude of this effect was found to depend on the number of electrons removed in addition to one half of the number of available electrons ($Q$--$Z$). On the other hand, for highly asymmetric dissociation channels the angular distributions seem to indicate that the fragments of highly charged molecular ions may be distributed with an enhanced probability at angles close to 90\ifmmode^\circ\else\textdegree\fi{} relative to the beam direction. A slight dependence of the ${q}_{\ensuremath{\perp}}$ distribution on molecular orientation was found to be present for the symmetric and nearly symmetric dissociation channels having $Q--Z\ensuremath{\geqslant}0$. The magnitude of this effect also seems to be determined by the value of $Q$--$Z$.