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Optical Spectroscopy of Positronium Atoms

  • Author(s): Hisakado, Tomu
  • Advisor(s): Tom, Harry W.K.
  • et al.

Single shot Positronium annihilation lifetime spectroscopy (SSPALS) is a technique that records 1¡Á¡¼10¡½^7 Positronium(Ps) atoms annihilating into gamma rays in a single shot from a positron accumulator. Here, SSPALS is combined for the first time with ns tunable lasers to study the electronic structure and emission of Ps atoms from solid surfaces. In our experiment the injection of a high density positron pulse into a target surface produces Ps atoms in vacuum which interacts immediately with the pulsed dye laser at 0.16T at 243nm. In our investigation the Ps kinetic energy along the surface of the sample decreased with increasing implantation energies and a lower limit of Ex=42¡À3 meV was discovered. The lower limit of the Ps kinetic energies corresponds to the region in which the Ps de Broglie wavelength approaches the pore diameter¡¯s of the target sample. As a result a classical model can no longer be used to describe the energy loss mechanism. Instead we find the lower limit values in agreement with the quantum confinement model.

In additional studies, the excitation of the 1S-2P transition for Ps was explored in the Paschen Back Regime (high magnetic field) of 2T. This investigation has found that Zeeman mixing of the 2P states is reduced in high magnetic fields. In the event of reduced mixing in the 2P state, which reduces conversion of triplet state Ps atoms into singlet state Ps atoms, this may result in the possibility of laser cooling Ps atoms using multiple pulsed laser events. We also measured the Positronium speed distribution perpendicular to the sample using copper and silicon as our target with time of flight. We used a two step, two photon excitation to the Rydberg states to enable single Rydberg atom annihilation to be detected using a multi-channel plate located 1.7m away from the target surface. We observed the Ps atom¡¯s kinetic energy to have a thermal Maxwell-Boltzmann speed distribution centered around 350,000 m/s, which is non- monoenergetic. This constitutes the first measurement of the kinetic energies perpendicular to the sample and demonstrates the need for further studies of Ps kinetic energies perpendicular and parallel to the sample.

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