Figure 4: Halo formation via fluorescence.

Variation in density of halo ions with excitation laser pulse temporal separation. (a) Solid lines (left) and dashed lines (right) show the blue ionization and red excitation laser pulse timing. The excitation laser rise time is limited by the acousto-optical modulator used to modulate the laser beam. (b) Measured ion bunch radial profiles for varying ionization delay, for free-propagation distance of 70 cm. Owing to the slow rise of the excitation laser intensity, the charge density of the beam ‘core’ was kept constant for different timings by adjusting the power of the excitation laser. With a constant core density the expansion rate is approximately constant, but the 5P–5S fluorescence increases with pre-ionization excitation delay, increasing the halo density.