Towards trapping of Rydberg Ps

Atoms and molecules in high Rydberg states can possess large electric dipole moments which can be exploited to control their motion using external electric fields. Ever since the demonstration of deflection of Rydberg Kr atoms using electric fields (, the field of atom optics has been widely investigated. Different techniques such as decelerators [EPJ Techniques and Instrumentation (2016) 3:2], mirrors [Phys. Rev. Lett. 97, 033002 (2006)], and traps [Phys. Rev. Lett. 100, 043001 (2008)] have been developed, however applications to positronium (Ps) are relatively recent. Only a handful of Ps atom optics experiments have been performed to date [Phys. Rev. Lett. 117, 073202 (2016) , Phys. Rev. A 95, 053409 (2017) , Phys. Rev. Lett. 119, 053201 (2017)]. Several Ps experiments can benefit from use of a cold Ps source but, due to the low mass of Ps, speeds are often on the order of 100 km/s when produced in targets like mesoporous silica. Also, using mesoporous silica, the Ps angular distribution during emission is broad and if a well collimated beam is required, significant loss in Ps number is unavoidable. The use of time-varying electric fields generated by a multi-ring structure can offer a solution to these limitations by capturing more emitted Ps and allowing manipulation via their electric dipole moments. Recently, we have developed a multi-ring structure which has been implemented to guide Rydberg Ps atoms using inhomogeneous electric fields, and, in principle, can also be used to decelerate and trap Ps.

The experimental set-up of the target and electrostatic guide including the excitation lasers and the gamma-ray detectors is shown in Figure 1. The Ps producing target is labelled T and the 11 electrodes of the guide are labelled 1, 2, 3,…, 11. Ground state Ps atoms, produced from the silica target, are excited to the n=13 Rydberg level in a two step process by the UV (1S –>2P) and IR (2P–>nD/nS) lasers in the region between T and electrode 1 (E1). Once excited to Rydberg states, Ps atoms have lifetimes on the order of μs, significantly longer than the triplet ground state lifetime of 142 ns. A 500 V/cm electric field, generated by biasing T and E1, Stark splits the Rydberg spectrum [Phys. Rev. Lett. 114, 173001 (2015)], enabling us to select specific states in the Stark manifold by tuning the IR laser wavelength. States selected with positive Stark shift(shorter wavelengths) are called low field seekers (LFS), because of their property to move toward regions of low field. States selected with negative Stark shift (longer wavelengths) are called high field seekers (HFS), because of their property to move toward regions of high field. Any atoms that are able to traverse the guide, LFS or HFS, can then be detected by detectors D2, D3, D4, and D5.

Figure 1: Schematic of the Ps production and excitation chamber. Inset shows the target electrode, T, and guide assembly comprising of 11 electrodes.
Figure 2: Simulated trajectories of atoms in HFS states (red solid lines) and LFS states (white solid lines) of the n=13 Stark manifold, with guide “off” (a) & (b) and “on” (c) & (d)

According to our simulations, see Figure 2, when the guide is not in operation (all electrodes grounded at 0V), the diverging cloud of Ps atoms from the target do not experience any external forces as expected, and therefore annihilate after colliding against the guide electrodes, with only a few forward collimated atoms reaching the end of the guide. If we apply a voltage to alternate electrodes (3,5,7 etc.), this turns the guide on. In this case the HFS states are deflected toward regions of high electric fields (i.e. away from the axis of the guide) while the LFS states are confined axially and transported along the length of the guide. These LFS atoms diverge after exiting the guide and some annihilate due to collisions with the surrounding vacuum chamber. Detectors D2, D3, D4, and D5, as shown in Figure 1, are able to record these annihilation events as time of flight (TOF) spectra, the results of which are shown in figure 3 below.

Figure 3: TOF spectra/annihilation events seen by detectors D2, D3, D4, and D5 for Ps atoms in (a) LFS and (b) HFS states with guide both on and off.

Neither the LFS states nor the HFS are detected at the end of the guide when the guide is off but an increased count rate is seen when both the guide is on and the IR laser is tuned to excite atoms to the LFS states.

To implement this device as a trap, we only have to reconfigure the voltages applied to the electrodes. For guiding, the odd numbered electrodes of the guide have a voltage applied (-4 kV) while the even numbered electrodes are kept at ground potential (0 V). If a positive voltage of order +1 kV is applied to E2 and E4 while E3 is at a potential of -4kV, then a region of electric field minimum is created in the centre of E3 (the voltages applied to the electrodes are based on the choice of n due to the field ionisation limit). Once the atoms have entered the trap, the LFS atoms should be confined in this region until the voltage applied to E4 is lowered,  opening the trap gate for the atoms to be guided towards the detectors. If atoms are trapped and guided out, then the TOF spectra, like in Figure 3(a), is expected to show a peak in events but occurring with a delay consistent with the length of trapping.