Exploiting the energetic interaction of intense femtosecond laser pulses with deuterium clusters, it is possible to create conditions in which nuclear fusion results from explosions of these clusters. We have conducted high-resolution neutron time-of-flight spectroscopy on these plasmas and show that they yield fast bursts of nearly monochromatic fusion neutrons with temporal duration as short as a few hundred picoseconds. Such a short, nearly pointlike source now opens up the unique possibility of using these bright neutron pulses, either as a pump or a probe, to conduct ultrafast studies with neutrons.

The free volume of metallic glasses has a significant effect on atomic relaxation processes, although a detailed understanding of the nature and distribution of free volume sites is currently lacking. Positron annihilation spectroscopy was employed to study free volume in a Zr-Ti-Ni-Cu-Be bulk metallic glass following plastic straining and cathodic charging with atomic hydrogen. Multiple techniques were used to show that strained samples had more open volume, while moderate hydrogen charging resulted in a free volume decrease. It was also shown that the free volume is associated with zirconium and titanium at the expense of nickel, copper, and beryllium. Plastic straining led to a slight chemical reordering.

Recent experiments on the interaction of intense, ultrafast pulses with large van der Waals bonded clusters have shown that these clusters can explode with sufficient kinetic energy to drive nuclear fusion. Irradiating deuterium clusters with a 35 fs laser pulse, it is found that the fusion neutron yield is strongly dependent on such factors as cluster size, laser focal geometry, and deuterium gas jet parameters. Neutron yield is shown to be limited by laser propagation effects as the pulse traverses the gas plume. From the experiments it is possible to get a detailed understanding of how the laser deposits its energy and heats the deuterium cluster plasma. The experiments are compared with simulations.

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 cleaved surfaces of untwinned, single-crystal YBa2Cu3069 have been probed with synchrotronradiation photoemission, utilizing both high energy and angular resolution. Acute spectral structure was observed, both at the Fermi energy and at higher binding energies, particularly near the high-symmetry points of the two-dimensional Brillouin zone, I, X, Y, and S. Many band crossings of the Fermi energy were seen, with obvious and important differences between the bands near X and those near Y. A large superconducting gap was not observed: The data are consistent with a gap of less than 10 meV. The assignment of bands and Fermi-level crossings to chain and plane states will be discussed, including comparisons to the predictions of theory, particularly local-density-approximation calculations.