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...

The n =2-• n = 1 lines of hydrogenlike and heliumlike uranium have been observed and are being reported for the first time. The spectrum consists of all allowed and forbidden lines. We have done a preliminary energy measurement of these lines with a precision in the (4-12) x 10 ~4 range.

Traditional multilayer reflective optics that have been used in the past for imaging at x-ray photon energies as high as 200 keV are governed by classical wave phenomena. However, their behavior at higher energies is unknown, because of the increasing effect of incoherent scattering and the disagreement between experimental and theoretical optical properties of materials in the hard x-ray and gamma-ray regimes. Here, we demonstrate that multilayer reflective optics can operate efficiently and according to classical wave physics up to photon energies of at least 384 keV. We also use particle transport simulations to quantitatively determine that incoherent scattering takes place in the mirrors but it does not affect the performance at the Bragg angles of operation. Our results open up new possibilities of reflective optical designs in a spectral range where only diffractive optics (crystals and lenses) and crystal monochromators have been available until now.

We report optical saturation of the 13S-2'P transition in positronium using ultraviolet light from a frequency-doubled, pulsed dye laser. The increase in ground-state annihilation radiation due to magnetic triplet-singlet mixing in the excited state is used to monitor the transition. From the variation of the annihilation rate as a function of magnetic field, laser power, wavelength and polarisation, we verify that the transition is optically saturated, in agreement with model calculations of the system.

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