  The dynamics triggered by the impact of an ion onto a solid surface has been
explored mainly by theoretical modeling or computer simulation to date. Results
indicate that the microscopic non-equilibrium relaxation processes triggered by
the interaction of the ion with the solid occur on (sub-)picosecond time
scales. A suitable experimental approach to these dynamics therefore requires a
pump-probe method with an appropriate time resolution. Recent experiments have
successfully used laser photoionization of noble gas atoms in combination with
a Wiley-MacLaren ion buncher to obtain arrival time distributions as narrow as
$t_{ion} = 180$ ps. Here we show that this setup can be significantly improved
by replacing the gas at a temperature of $T_{atoms}$ = 300 K with a supersonic
beam of cooled noble gas atoms at $T_{atoms}$ = 4 K. The detailed analysis of
measured arrival times of individual Ne$^+$ ions with a kinetic energy of 4 keV
reveals that the arrival time jitter can be reduced by this technique down to
(18$\pm$4) ps. This opens the door to pump-probe experiments with keV ions with
a time-resolution in the picosecond range.
