First observation of resonant excitation of high-nstates in positronium

Abstract

We have performed the first excitation of high-rt states of positronium using resonant, two-photon excitation applied to the «*1 to 2 and 2 to n transitions. Absolute values for the line positions were quantitatively determined for n ~14 and 15 and compared well with calculated predictions. The prediction of the n ~3 scaling of the relative transition rates was observed for n = 13 to 19.PACS numbers: 36.10.Dr, 32.80.Pj Since the discovery of positronium (Ps), spectroscopic studies have been performed on sublevels of the ground and n =2 energy levels of the system. These include the singlet-triplet splitting in the ground state, 1 the twophoton \S-2S transition, 2 and the 2S-2P splitting in the first excited state. 3 These measurements represent a sensitive test of quantum electrodynamics due to the precision of the experimental results, lack of importance of strong forces, and high calculational accuracy available from the theory. In this Letter, we report the first observation in Ps of the excitation of high-H states, « -13-15, a preliminary measurement of the energy of two of these levels, ^ = 14 and 15, and preliminary observations on the relative transition probabilities for n = 13-19. Excitation of high-rt states was performed by a resonant, twophoton excitation n = 1 to 2 and n sss 2to 13-19 and using an extension of our previous investigations into the production of an optically saturated population of n =2 Ps. 4 The basic quantum electrodynamic interactions in Ps, particularly energy shifts induced by perturbative external fields, may be tested in new ways by measurements of the energies and transition properties of high-/z states. Excitation to high-/i states results in a population of long-lived, neutral Ps because the deexcitation and annihilation lifetimes scale as n~3. Such excited populations of Ps can be used to study areas such as antihydrogen production or exotic many-body states of Ps. 5 We can also tune through the resonant excitation profile using narrow laser linewidths allowing us to map the velocity distribution of the ground-and (n =2)-state populations. Since these transitions leave the Ps system in a bound state, they can be of particular use in laser-cooling experiments. 5 In this experiment we excite high-rt states in Ps through a two-step excitation process using the n = 2 state as an intermediate state. The experiment was conducted using extensions of the techniques previously employed to observe the population of the n = 2 levels of Ps. 4 Ps and laser light interacted in the ultrahigh-vacuum, experimental chamber used with the intense, low-energy positron beam at the 100-MeV electron linac at Lawrence Livermore National Laboratory. Thermal Ps was formed by guiding a 1-keV pulsed positron beam with a 200-G magnetic field onto a hot, 1000-K, clean copper single crystal cut along the 100 face. The target was biased with respect to a grid to attract any reemitted positrons. Pulses of 15-ns duration typically contained 10 5 positrons which were converted into Ps with an overall effici...

Experimental progress in positronium laser physics

“…Using a second dye laser these researchers were also able to produce highly excited (Rydberg) Ps states for the first time [105] using a two-step 1 3 S → 2 3 P → n 3 S/n 3 D excitation process (see Sect. 3.2).…”

“…Accelerator-based beams of this type are generally large and expensive, and are not widely available. Thus, the LLNL experiments [104,105] remain the only ones in which the intrinsically pulsed output of a linac was utilized for Ps excitation experiments with pulsed lasers (although Ps ion photodetachment has been performed in this way [116]). …”

Development and applications of time-bunched and velocity-selected positron beams

“…Positronium spectroscopy 17,18 is an underdeveloped field and studies of positronium states with principal quantum numbers greater than two appear to be worthwhile, including further work on so-called Rydberg states. 31 This could lead to advances in laser cooling of positronium. 32 Mach-Zehnder-type experiments on positronium also seem feasible 33,34 as do studies of above threshold ionization of this unique species.…”

Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms

“…Rydberg Ps was first generated by Ziock et al using a linac-based positron beam [34], but it was only possible to demonstrate the production of a few high-lying states with principal quantum numbers n = 13-15. Modern positrontrapping [35] and detection [36] techniques have facilitated much more efficient production of Rydberg Ps [37].…”

“…Modern positrontrapping [35] and detection [36] techniques have facilitated much more efficient production of Rydberg Ps [37]. In particular, it has been possible to selectively populate individual Rydberg-Stark states [38] through a two-step excitation scheme Ps(1s) → Ps(2p) → Ps(ns, nd) [34,37,39]. These devel-opments make further experimentation feasible, with a view to creating a focused Ps beam suitable for gravity measurements [40,41].…”