Laser Excitation of Positronium in the Paschen-Back Regime

Zeeman mixing of singlet and triplet 2P states of positronium (Ps) atoms, followed by decay back to the ground state, can effectively turn a long-lived triplet atom into a short-lived singlet state, which would seem to preclude laser cooling of Ps in a magnetic field. Here we report experiments which show that, in fact, because of the large splitting of the n=2 states in a high magnetic field (the Paschen-Back regime), the amount of such mixing diminishes approximately exponentially with an increasing magnetic field >0.01T and is essentially eliminated above ∼2T. Thus, laser cooling of Ps should be feasible at high fields, which will facilitate the production of a Ps Bose-Einstein condensate. © 2011 American Physical Society

Optical spectroscopy of molecular positronium

We report optical spectroscopic measurements of molecular positronium (Ps ), performed via a previously unobserved L=1 excited state. Ps molecules created in a porous silica film, and also in vacuum from an Al(111) crystal, were resonantly excited and then photoionized by pulsed lasers, providing conclusive evidence for the production of this molecular matter-antimatter system and its excited state. Future experiments making use of the photoionized vacuum L=1 Ps could provide a source of Ps ions, as well as other multipositronic systems, such as Ps H or Ps O. © 2012 American Physical Society

New mechanism for positronium formation on a silicon surface

We describe experiments in which positronium (Ps) is emitted from the surface of p-doped Si(100), following positron implantation. The observed emission rate is proportional to a Boltzmann factor exp{-E/kT}, which is dependent on the temperature T of the sample and a characteristic energy E=(0.253±0.004)eV. Surprisingly, however, the Ps emission energy has a constant value of ∼0.16eV, much greater than kT. This observation suggests the spontaneous emission of energetic Ps from a short-lived metastable state that becomes thermally accessible to available surface electrons once the positron is present. A likely candidate for this entity is an electron-positron state analogous to the surface exciton observed on p-Si(100) c(4×2) by Weinelt et al. © 2011 American Physical Society

Photoemission of positronium from Si

We have observed that the amount of positronium (Ps) emitted from the surface of p-Si(100) is substantially increased if the sample is irradiated with 532 nm laser light just prior to the implantation of positrons. The energy of the emitted Ps has a constant value of ∼0.16eV and is independent of the Si temperature and the applied laser fluence, while the photoemission yield depends on both of these parameters. These observations are consistent with Ps production via a previously observed excitonlike positron surface state that is populated in response to the production of electron-hole pairs in the Si. Possible applications of Ps photoemission include probing surface electron dynamics on Si, the generation of ultrashort Ps or positron pulses using ps lasers, and efficient production of Ps in cryogenic environments. © 2011 American Physical Society

Positronium hyperfine interval measured via saturated absorption spectroscopy

We report Doppler-free measurements of the positronium (Ps) Lyman-α transition using saturated absorption spectroscopy. In addition to a Lamb dip at wavelength λ =243.0218±0.0005nm, we also observed a crossover resonance at λ =243.0035±0.0005nm, arising from the excitation of 1S13 atoms to Zeeman mixed 2P states, followed by stimulated emission to the 1S01 ground state. Since (λ - λ ) is related to the Ps hyperfine interval E , this observation constitutes the first optical measurement of this quantity and yields E =198.4±4.2GHz. We describe improvements to the methodology that could lead to the ∼ppm level of precision required to address the long-standing discrepancy between QED calculations and precision experiments using microwave radiation to induce transitions between Zeeman shifted triplet Ps states. © 2012 American Physical Society

Reply: Cassidy et al.

A Reply to the Comment by D. G. Green and G. F. Gribakin

Cold ablation driven by localized forces in alkali halides

Laser ablation has been widely used for a variety of applications. Since the mechanisms for ablation are strongly dependent on the photoexcitation level, so called cold material processing has relied on the use of high-peak-power laser fluences for which nonthermal processes become dominant; often reaching the universal threshold for plasma formation of ∼1 J cm-2 in most solids. Here we show single-shot time-resolved femtosecond electron diffraction, femtosecond optical reflectivity and ion detection experiments to study the evolution of the ablation process that follows femtosecond 400 nm laser excitation in crystalline sodium chloride, caesium iodide and potassium iodide. The phenomenon in this class of materials occurs well below the threshold for plasma formation and even below the melting point. The results reveal fast electronic and localized structural changes that lead to the ejection of particulates and the formation of micron-deep craters, reflecting the very nature of the strong repulsive forces at play