Positronium Negative Ions

Positronium Negative Ions Yasuyuki NAGASHIMA Department of Physics Tokyo University of Science JAPAN

Our studies for Ps－ • We started Ps－experiments in Tokyo Univ. of Sci. in 2005. • Observation of Ps－ emission from tungsten surfaces (Nagashima et al., New J. Phys.8 (2006) 319; Mat. Sci. Forum 607 (2009) 161) • Efficient emission of Ps－ using Cs coated tungsten surface (Nagashima et al., New J. Phys. 10 (2008) 123029; Phys. Status Solidi C 6 (2009) 2291) • Emission of Ps－from Mo and Ta surfaces (Michishio et al., J. Phys. Conf. Ser. 199 (2010) 012003) • Durable emission of Ps－from Na coated tungsten surface (Terabe et al., J. Phys. Conf. Ser. 262 (2011) 012058, also in preparation) • Ps－photodetachment experiment (Michishio et al., Phys. Rev. Lett. 106 (2011) 153401) • Detection of Ps formed by the Ps－photodetachment (Koji Michishio, O-19, on Wednesday)

OUTLINE OF THIS TALK • What is Ps－? • Efficient emission of Ps－ using Cs coated tungsten surface • Emission of Ps－from Mo and Ta surfaces • Durable emission of Ps－from Na coated tungsten surface • Ps－photodetachment experiment • Future plans • Conclusions

Bound states composed of e+ and e－ Existence has been confirmed. Existence has been reported. (Cassidy and Mills, Nature 449 (2007) 06094) Ps Ps Ps e+ e－＝ positronium molecule(Ps2) positronium(Ps) e－ e+ e－ positronium negative ion(Ps－) Theoretical studies have been started. (Frolov, Phys. Lett. A 372 (2008) 6721) Ps Ps e－ bi-positronium negative ion(Ps2e－) Nobody has produced. Ps Ps e+ e+ e－ e+ bi-positronium positive ion(Ps2e＋) positronium plus ion(Ps＋)

positronium (Ps) positronium negative ion(Ps－) e－e+e－ • H atom like state • The theoretical approach for H atoms can be applied. • The wave function can be obtained • without any approximations. • H－ion like state • All the constituents of Ps－ • have the same mass. • The theoretical approach for H－ • (Born-Oppenheimer approximation) • CANNOT be used for Ps－. • Many theoretical approaches using variational principles have been performed.

positronium (Ps) positronium negative ion(Ps－) e－e+e－ • Binding energy : 6.80eV • Mean distance e+－e－: 2a0 • Two eigenstates • ortho-Ps (S=1, triplet) • lifetime in vacuum : 142ns • Self-annihilates into 3γ . • para-Ps (S=0, singlet) • lifetime in vacuum : 125ps • Self-annihilates into 2γ . • e－binding energy to Ps = 0.33eV; • Total binding energy (the energy • required to break up Ps－ into 3 • isolated particles) : • 0.33eV + 6.80eV = 7.13eV • Mean distancee+－e－: 5.5a0 • Only one state • Lifetime in vacuum : 479ps • Self-annihilates into 2γ.

positronium negative ion(Ps－) e－e+e－ History of Ps－research 1946J.A. Wheeler predicted the existence. Many theoretical researches have been performed since the prediction of Wheeler.

Examples of theoretical investigations on Ps－ : 1960 Calculation of the Ps- binding energy byKolos, Rootrhaan and Sack. 1964 Calculation of the Ps- binding energy by Frost, Inokuti and Lowe. 1968 Calculation of the Ps- decay rate by Ferrante. 1979 Calculation of the Ps- binding energy by Ho. 1983 Calculation of the Ps- binding energy and decay rate by Bhatia andDrachman. 1985 Calculation of Ps- photodetachment cross sections by Bhatia and Drachman. 1987 Calculation of Ps- photodetachment cross sections by Ward, Humberstonand McDowell. 1990 Calculation of the Ps- decay rate by Ho. 1993 Calculation of Ps- binding energy by Ho. 2000 Calculation of the Ps- binding energy by Korobov. 2000 Calculation of Ps- photodetachment cross sections by Igarashi, Shimamura and Toshima. 2002 Calculation of Ps- binding energy by Drake, Grigorescu and Nistor. 2005 Calculation of the Ps- binding energy by Drake and Grigorescu. EB＝　0.261 998 108 122 au ~ 7.129 330 97 eV 2007 Calculation of the Ps- decay rate by Puchalski and Czanecki. Γ= 2.087 963 (12) ns-1.

positronium negative ion(Ps－) e－e+e－ History of Ps－research 1946J.A. Wheeler predicted the existence. 1981A. P. Mills, Jr. succeeded in the production. (formation efficiency=0.028%) Ps－formation efficiency＝number of formed Ps－/number of incident slow e+

First observation of Ps－(Mills, 1981) • Slow positrons (470 eV) were guided to a thin carbon target. • Ps－emittedwere accelerated by the electric field and detected by their Doppler-shifted annihilation lines. θ Ps－formation was confirmed. Ps－formationefficiency = 0.028% (A.P. Mills, Jr., Phys. Rev. Lett. 46 (1981) 717)

Measurement of the Ps－decay rate Γ＝2.09±0.09ns-1 A. P. Mills, Phys. Rev. Lett. 50 (1983) 671

Measurement of the Ps－decay rate Tandem acceleration method of detecting Ps－ A. P. Mills, Jr., P. G. Freeman and D. M. Zuckerman, NASA Conference Publication (1989)

Measurement of the Ps－decay rate Ps- fraction = 1 x 10-4 （30eV） Stripping-based detection technique (Fleischer et al., Phys. Rev. Lett. 96 (2006) 063401)

positronium negative ion(Ps－) e－e+e－ History of Ps－research 1946J.A. Wheeler predicted the existence. 1981A. P. Mills, Jr. succeeded in the production. (formation efficiency=0.028%) Many theoretical researches have been performed. Only a few experiments have been performed. Ps－formation efficiency is too low.

e+ near metal surface When e+ are incident onto metal surfaces, ... Diffusion length of e+ in defect free metals ~ 100nm annihilation cross section << collision cross section e+ lifetime in metals ~ 100ps A significant fraction of e+ incident onto surface with a few keV energy diffuse back to the surface.

：e+ work function (The energy required to emit e+) e+are emitted from the surface.

The energy required to emit Ps : : e+work function : e－ work function Ps atoms are emitted from the surface.

The energy required for Ps－emission : e－work function e+ work function Ps－binding energy (The energy required to break up Ps－into three isolated particles) Ps－might be emitted from the surface spontaneously.

polycrystalline molybdenum • tungsten (1 0 0) • tungsten (1 1 1) • polycrystalline tungsten Ps－ions may be emitted.

Ps－emission from polycrystalline tungsten surface (Nagashima and Sakai, New J. Phys. 8 (2006) 319) vacuum : 7 x 10-8 Pa (5 x 10-10torr) Ps－formation efficiency was only 0.007%. (1/4 of that of beam-foil method)

Ps－emission from polycrystalline tungsten surface (Nagashima and Sakai, New J. Phys. 8 (2006) 319) Ps－formation efficiency decreases. Change of the surface condition

e－and e+ near metal surface D e－energy levele+energy lebel (Achcroftand Mermin) (Schultz and Lynn, Rev. Mod. Phys. 60 (1988) 701) e－ work function : e+work function : ：e－ chemical potential ：e+ chemical potential ：effect of surface dipole

Ps－emission from polycrystalline tungsten surface D becomes larger by the effect of oxygen, H, H2O, ..... Ps－formation efficiency decreases.

Ps－emission from polycrystalline tungsten surface Time dependence of Ps- formation efficiency 7 x 10-8 Pa 3 x 10-8 Pa When the vacuum was improved, the fraction became stable.

Ps－emission from polycrystalline tungsten surface Time dependence of Ps- formation efficiency 7 x 10-8 Pa 3 x 10-8 Pa When the vacuum was improved, the fraction became stable. The dependence was due to the adsorbate coverage of the target surface by residual molecules in the target chamber. (Nagashima and Sakai, New J. Phys. 8 (2006) 319, Nagashima, Hakodate and Sakai, Appl. Surf. Sci. 255 (2008) 217)

Ddecreases by Cs coating. Effect of Cs coating for the Ps－emission Change of for tungsten by Cs coating D decreases. decreases. Ps－ formation efficiency might increase. Kiejna and Wojciechowski, Prog. in Surf. Sci. 11 (1981) 293

Effect of Cs coating for the Ps－ emission e+transport energy ：100eV e+ incident energy onto the target : 100eV + eW TargetW(100) vacuum：2×10-8Pa （1.5×10-10torr） Target was annealed at 1500℃ for 30 min. target potential ：-3kV (Nagashima et al. New J. Phys. 10 (2008) 123029)

Effect of Cs coating for the Ps－ emission (Nagashima et al. New J. Phys. 10 (2008) 123029)

Effect of Cs coating for the Ps－ emission Ps－intensity is the highest at 2.2×1014cm-2（0.8ML). Change of for tungsten by Cs coating Kiejna and Wojciechowski, Prog. in Surf. Sci. 11 (1981) 293

Effect of Cs coating for the Ps－ emission The highest efficiency was 1.25%, which is two orders of magnitude higher than that obtained for uncoated surface, and 45 times greater than the beam-foil method.

Ps－emission from Cs coated Mo surfaces 2 x 1014 atoms/cm2 Ps－ions were detected for uncoated Mo and Cs coated Mo.

Ps－emission from Cs coated Mo surfaces 2 x 1014 atoms/cm2 Ps－ions were detected for uncoated Mo and Cs coated Mo. First experimental evaluation for EB

Ps－emission from Cs coated Mo surfaces 2 x 1014 atoms/cm2 Ps－ions were detected for uncoated Mo and Cs coated Mo. for the uncoated Mo surface. ：Ps- binding energy

Ps－emission from Cs deposited Ta surfaces 2 x 1014 atoms/cm2 Ps－ions were not detected for uncoated Ta, but Ps－ions were detected for the Cs coated Ta. The efficiency was 1.5%, which is higher than that for the Cs coated W(100).

Ps－emission from Cs deposited Ta surfaces 2 x 1014 atoms/cm2 Ps－ions were not detected for uncoated Ta. for the uncoated Ta surface. ：Ps－binding energy

Effect of Cs coating for the Ps－ emission carbon film Time dependence of f The decrease might be due to the accumulation of residual molecules.

Effect of K and Na coating for the Ps－ emission K and Na are less reactive chemically than Cs. Ps－emission might stay longer. Cs K Na The effect might be smaller. Kiejna and Wojciechowski, Prog. in Surf. Sci. 11 (1981) 293

Effect of K and Na coating for the Ps－ emission

Effect of K and Na coating for the Ps－ emission Na coating is as effective and the effect remains LONGER!

Positronium Negative Ions