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The production of molecular positronium


It has been known for many years that an electron and its antiparticle, the positron, may together form a metastable hydrogen-like atom, known as positronium or Ps (ref. 1). In 1946, Wheeler speculated2 that two Ps atoms may combine to form the di-positronium molecule (Ps2), with a binding energy3 of 0.4 eV. More recently, this molecule has been studied theoretically4; however, because Ps has a short lifetime and it is difficult to obtain low-energy positrons in large numbers, Ps2 has not previously been observed unambiguously5. Here we show that when intense positron bursts are implanted into a thin film of porous silica, Ps2 is created on the internal pore surfaces. We found that molecule formation occurs much more efficiently than the competing process of spin exchange quenching, which appears to be suppressed in the confined pore geometry. This result experimentally confirms the existence of the Ps2 molecule and paves the way for further multi-positronium work. Using similar techniques, but with a more intense positron source, we expect to increase the Ps density to the point where many thousands of atoms interact and can undergo a phase transition to form a Bose–Einstein condensate6. As a purely leptonic, macroscopic quantum matter–antimatter system this would be of interest in its own right, but it would also represent a milestone on the path to produce an annihilation gamma-ray laser7.

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Figure 1: Density dependence of the amount of long-lived Ps.
Figure 2: Temperature dependence of fd and Q.

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  1. Charlton, M. & Humberston, J. W. Positron Physics (Cambridge Univ. Press, Cambridge, 2001)

    Google Scholar 

  2. Wheeler, J. A. Polyelectrons. Ann. NY Acad. Sci. 48, 219–238 (1946)

    Article  ADS  Google Scholar 

  3. Hylleraas, E. A. & Ore, A. Binding energy of the positronium molecule. Phys. Rev. 71, 493–496 (1947)

    Article  ADS  CAS  Google Scholar 

  4. Schrader, D. M. Symmetry of dipositronium Ps2 . Phys. Rev. Lett. 92, 043401 (2004)

    Article  ADS  CAS  Google Scholar 

  5. Cassidy, D. B. et al. Experiments with a high-density positronium gas. Phys. Rev. Lett. 95, 195006 (2005)

    Article  ADS  CAS  Google Scholar 

  6. Platzman, P. M. & Mills, A. P. Possibilities for Bose condensation of positronium. Phys. Rev. B 49, 454–458 (1994)

    Article  ADS  CAS  Google Scholar 

  7. Mills, A. P., Cassidy, D. B. & Greaves, R. G. Prospects for making a Bose-Einstein condensed positronium annihilation gamma ray laser. Mater. Sci. Forum 445, 424–429 (2004)

    Article  Google Scholar 

  8. Greaves, R. G. & Surko, C. M. Emerging physics and technology of antimatter plasmas and trap-based beams. Phys. Plasmas 11, 2333–2348 (2004)

    Article  ADS  Google Scholar 

  9. Paulin, R. & Ambrosino, G. Annihilation libre de l'ortho-positronium formé dans certaines poudres de grande surface specifique. J. Phys. (Paris) 29, 263–270 (1968)

    Article  CAS  Google Scholar 

  10. Gidley, D. W. et al. Positronium annihilation in mesoporous thin films. Phys. Rev. B 60, R5157–R5160 (1999)

    Article  ADS  CAS  Google Scholar 

  11. Brandt, W., Berko, S. & Walker, W. W. Positronium decay in molecular substances. Phys. Rev. 120, 1289–1295 (1960)

    Article  ADS  CAS  Google Scholar 

  12. Deutsch, M. Evidence for the formation of positronium in gases. Phys. Rev. 82, 455–456 (1951)

    Article  ADS  CAS  Google Scholar 

  13. Cassidy, D. B., Deng, S. H. M., Greaves, R. G. & Mills, A. P. Accumulator for the production of intense positron pulses. Rev. Sci. Instrum. 77, 073106 (2006)

    Article  ADS  Google Scholar 

  14. Tanaka, H. K. M., Kurihara, T. & Mills, A. P. Evaluation of the diffusion barrier continuity on porous low-k films using positronium time of flight spectroscopy. Phys. Rev. B 72, 193408 (2005)

    Article  ADS  Google Scholar 

  15. Saito, H. & Hyodo, T. Direct measurement of the parapositronium lifetime in α-SiO2 . Phys. Rev. Lett. 90, 193401 (2003)

    Article  ADS  Google Scholar 

  16. Cassidy, D. B., Deng, S. H. M., Tanaka, H. K. M. & Mills, A. P. Single shot positron annihilation lifetime spectroscopy. Appl. Phys. Lett. 88, 194105 (2006)

    Article  ADS  Google Scholar 

  17. Cassidy, D. B. & Mills, A. P. Radiation damage in a-SiO2 exposed to intense positron pulses. Nucl. Instrum. Methods B 262, 59–64 (2007)

    Article  ADS  CAS  Google Scholar 

  18. Mills, A. P. Thermal activation measurement of positron binding energies at surfaces. Solid State Commun. 31, 623–626 (1979)

    Article  ADS  CAS  Google Scholar 

  19. Chu, S., Mills, A. P. & Murray, C. A. Thermodynamics of positronium thermal desorption from surfaces. Phys. Rev. B 23, 2060–2064 (1981)

    Article  ADS  CAS  Google Scholar 

  20. Martin, Bruinsma, R. & Platzman, P. M. Adsorption of positronium on metal surfaces: theory. Phys. Rev. B 43, 6466–6473 (1991)

    Article  ADS  CAS  Google Scholar 

  21. Sferlazzo, P., Berko, S. & Canter, K. F. Experimental support for physisorbed positronium at the surface of quartz. Phys. Rev. B 32, 6067–6070 (1985)

    Article  ADS  CAS  Google Scholar 

  22. Saniz, R., Barbiellini, B., Platzman, P. M. & Freeman, A. J. Physisorption of positronium on quartz surfaces. Preprint at 〈〉 (2007)

  23. Kim, S. M. & Buyers, W. J. L. Positronium-surface interaction in the pores of the vycor glass. J. Phys. C 11, 101–109 (1978)

    Article  ADS  CAS  Google Scholar 

  24. He, C. et al. Evidence for pore surface dependent positronium thermalization in mesoporous silica/hybrid silica films. Phys. Rev. B 75, 195404 (2007)

    Article  ADS  Google Scholar 

  25. Mogensen, O. E. Spur reaction model of positronium formation. J. Chem. Phys. 60, 998–1004 (1974)

    Article  ADS  CAS  Google Scholar 

  26. Mills, A. P. in Positron Spectroscopy of Solids (eds Dupasquier, A. & Mills, A. P. Jr) 209–258 (IOS Press, Amsterdam, 1995)

    Google Scholar 

  27. Mills, A. P. Chemistry and physics with many positrons. Rad. Phys. Chem. 76, 76–83 (2007)

    Article  ADS  CAS  Google Scholar 

  28. Varga, K., Usukura, J. & Suzuki, Y. Second bound state of the positronium molecule and biexcitons. Phys. Rev. Lett. 80, 1876–1879 (1998)

    Article  ADS  CAS  Google Scholar 

  29. Langmuir, I. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361–1403 (1918)

    Article  CAS  Google Scholar 

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We gratefully acknowledge R. G. Greaves for discussions and H. K. M. Tanaka for providing the porous silica film. This work was supported in part by the National Science Foundation.

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Cassidy, D., Mills, A. The production of molecular positronium. Nature 449, 195–197 (2007).

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