Path to ion-based pump-probe experiments: Generation of 18 picosecond keV ${\mathrm{Ne}}^{+}$ ion pulses from a cooled supersonic gas beam

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 nonequilibrium 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 $t_{\mathrm{ion}}=180$ ps. Here, we show that this setup can be significantly improved by replacing the gas at a temperature of $T_{\mathrm{atoms}}\simeq 300$ K with a supersonic beam of cooled noble gas atoms at $T_{\mathrm{atoms}}\simeq 4$ K. The detailed analysis of measured arrival times of individual ${\mathrm{Ne}}^{+}$ 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.

Figure 1

Three-dimensional rendering of the chamber system for the generation of a geometrically cooled supersonic beam and picosecond ion pulses. An ultrasonic nozzle generates a supersonic beam (green) in the expansion chamber on the left which is led through three skimmers, as seen in the inset on the top. Gas atoms with a nonzero velocity component perpendicular to the beam axis (green arrows) will be cut off before ionization when the supersonic beam is crossed with the red marked laser beam within the ion buncher (see also Fig. 2), as seen on the bottom right inset.

Figure 2

Schematic of the ion buncher. The laser (red) is focused in the center of the Ne supersonic beam (green) between two electrodes $E_1$ and $E_2$ which are at a distance of $d_1$. An electric field between those plates accelerates ions towards the pin hole in $E_2$, which therefore travel the distance $d_2$ towards the ultrafast MCP detector. Every segment of the buncher can be moved independently in height and rotated by ${\varphi }_x$ and ${\varphi }_y$ with respect to the Ne supersonic beam for an optimal alignment [34].

Figure 3

${\mathrm{Ne}}^{+}$ signal of the backfilled Ne (black) to $1.0\times {10}^{-7}$ mbar and the Ne beam (red) with variation of laser focus position. (a) Laser scan in $z$ axis between $E_1$ and $E_2$. The supersonic beam can be measured with a diameter of 1.55 mm at the point of ionization, which is not seen in the backfilled gas signal due to homogeneous distribution of the gas atoms. (b) Variation of the $x$ coordinate and the 80-$\mu \mathrm{m}$ pin hole in $E_2$. A shift of the maximum signal in the beam signal of 20 $\mu \mathrm{m}$ compared to the backfilled signal is given through the parabolic trajectories after ionization.

Figure 4

Time-of-flight measurement of ionized gas atoms with the supersonic beam active with 2 keV kinetic energy. Components in the supersonic beam consist of ${}^{20}{\mathrm{Ne}}^{+}$, ${}^{22}{\mathrm{Ne}}^{+}$, whereas ${\mathrm{O}}^{+}$, ${\mathrm{H}}_2{\mathrm{O}}^{+}$ residual gas is seen even without the supersonic beam active. The inset shows the ${}^{20}{\mathrm{Ne}}^{+}$ signal magnified and the effective pulse width FWHM of 135 ps.

Figure 5

Pulse width dependency of the ${\mathrm{Ne}}^{+}$ ions over the extraction potential between the two buncher electrodes. Black dots represent data taken with the chamber backfilled with Ne gas and the red dots represent data taken with ions produced from the supersonic beam. In (a) the dashed line guides the eye and shows at lower extraction potentials a strong dependency of the pulse width, which reaches the resolution limit at different potentials depending on the source of the ion. In (b) the extraction potential is inversed to show the linear dependency of pulse width with $\sqrt{\mathrm{T}}$, confirmed by the linear fit.