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) ; Phys. Status Solidi C 6 (2009) 2291) Emission of Ps－ from Mo and Ta surfaces (Michishio et al., J. Phys. Conf. Ser. 199 (2010) ) Durable emission of Ps－ from Na coated tungsten surface (Terabe et al., J. Phys. Conf. Ser. 262 (2011) , also in preparation) Ps－ photodetachment experiment (Michishio et al., Phys. Rev. Lett. 106 (2011) ) 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＋)

e－ e+ e－ positronium (Ps) positronium negative ion (Ps－)
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.

e－ e+ e－ positronium (Ps) positronium negative ion (Ps－)
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 eV = 7.13eV Mean distance　 e+ － e－ : 5.5a0 Only one state Lifetime in vacuum : 479ps Self-annihilates into 2γ.

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

（Igarashi et al, New J. Phys. 2 (2000) 17）
Examples of theoretical investigations on Ps－ : 1964 Calculation of the binding energy by Frost, Inokuti and Lowe 1979 Calculation of the binding energy by Ho 1987 Calculation of the photodetachment cross sections by Ward, Humberston and McDowell 2000 Calculation of the photodetachment cross sections by Igarashi, Shimamura and Toshima 2005 Calculation of the binding energy by Drake and Grigorescu EB＝　 au ~ eV （Igarashi et al, New J. Phys. 2 (2000) 17）

Examples of theoretical investigations on Ps－ :
1960 Calculation of the Ps- binding energy by Kolos, 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, Humberston and 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＝　 au ~ eV 2007 Calculation of the Ps- decay rate by Puchalski and Czanecki. Γ= (12) ns-1.

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

Ps－ formation was confirmed.
First observation of Ps－ (Mills, 1981) Slow positrons (470 eV) were guided to a thin carbon target. Ps－ emitted were accelerated by the electric field and detected by their Doppler-shifted annihilation lines. θ Ps－ formation was confirmed. Ps－ formation efficiency = 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)

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

　　　　1946 J.A. Wheeler predicted the existence. 　　　　1981 A. 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 ~ 100nm e+ lifetime in metals ~ 100ps
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+ are emitted from 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.

from the surface spontaneously.
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)
Element Φ+ (eV) Φ- (eV) ΦPs- (eV) Al (polycrystalline) -0.2 4.25 1.2 Al (1 0 0) -0.16 4.20 1.11 Al (1 1 1) 0.065 4.26 1.46 Cr (1 0 0) -1.76 4.46 0.03 Fe (polycrystalline) -1.2 4.4 0.5 Co (polycrystalline) -0.8 5.0 2.1 Ni (polycrystalline) 5.15 2.0 Ni (1 0 0) -1.0 5.22 2.3 Ni (1 1 0) -1.4 5.04 1.6 Cu (1 0 0) -0.3 5.10 2.8 Cu (1 1 0) 4.48 Cu (1 1 1) -0.4 4.94 2.4 Mo (polycrystalline) -2.2 4.6 -0.1 Mo (1 0 0) -1.7 4.53 0.2 Ta (polycrystalline) W (polycrystalline) -2.75 4.55 -0.78 W (1 0 0) -3.0 4.63 -0.9 W (1 1 0) 0.3 W (1 1 1) -2.59 4.45 -0.82 Pt (polycrystalline) -1.8 5.64 Au (polycrystalline) 0.9 5.2 4.2 Pb (polycrystalline) polycrystalline molybdenum tungsten (1 0 0) tungsten (1 1 1) polycrystalline tungsten Ps－ ions may be emitted.

Slow positron beam apparatus of Tokyo Univ. of Science
magnetically guided slow positron beam positron source ：22Na（740MBq, 20mCi） positron energy : 100 eV beam intensity : 1 x 105 e+/s beam diameter : 5 mm vacuum : 7 x 10-8 Pa (5 x torr) attained using a turbo-molecular pump and a getter pump.

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

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

e－and e+ near metal surface e－ energy level e+ energy lebel
(Achcroft and 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－formation efficiency decreases.
Ps－emission from polycrystalline tungsten surface D becomes larger by the effect of oxygen, H, H2O, ..... Ps－formation efficiency decreases.

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.

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)

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

Suggestion for Ps－ formation using Cs coated surface :
W(110) e+ Ps－ “at grazing angles, good yields of monoenergetic Ps－ might be obtained at positron incident energies of a few eV and might result in a well-collimated tunable Ps beam with a small energy spread.” (G. Laricchia, “Positron Spectroscopy of Solids”, (1995) 401)

e+ transport energy ：100eV e+ incident energy onto the target : 100eV + eW Target　W(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) )

(Nagashima et al. New J. Phys. 10 (2008) )

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

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.

Element Φ+ (eV) Φ- (eV) ΦPs- (eV) Al (polycrystalline) -0.2 4.25 1.2 Al (1 0 0) -0.16 4.20 1.11 Al (1 1 1) 0.065 4.26 1.46 Cr (1 0 0) -1.76 4.46 0.03 Fe (polycrystalline) -1.2 4.4 0.5 Co (polycrystalline) -0.8 5.0 2.1 Ni (polycrystalline) 5.15 2.0 Ni (1 0 0) -1.0 5.22 2.3 Ni (1 1 0) -1.4 5.04 1.6 Cu (1 0 0) -0.3 5.10 2.8 Cu (1 1 0) 4.48 Cu (1 1 1) -0.4 4.94 2.4 Mo (polycrystalline) -2.2 4.6 -0.1 Mo (1 0 0) -1.7 4.53 0.2 Ta (polycrystalline) W (polycrystalline) -2.75 4.55 -0.78 W (1 0 0) -3.0 4.63 -0.9 W (1 1 0) 0.3 W (1 1 1) -2.59 4.45 -0.82 Pt (polycrystalline) -1.8 5.64 Au (polycrystalline) 0.9 5.2 4.2 Pb (polycrystalline)

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

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

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

Element Φ+ (eV) Φ- (eV) ΦPs- (eV) Al (polycrystalline) -0.2 4.25 1.2 Al (1 0 0) -0.16 4.20 1.11 Al (1 1 1) 0.065 4.26 1.46 Cr (1 0 0) -1.76 4.46 0.03 Fe (polycrystalline) -1.2 4.4 0.5 Co (polycrystalline) -0.8 5.0 2.1 Ni (polycrystalline) 5.15 2.0 Ni (1 0 0) -1.0 5.22 2.3 Ni (1 1 0) -1.4 5.04 1.6 Cu (1 0 0) -0.3 5.10 2.8 Cu (1 1 0) 4.48 Cu (1 1 1) -0.4 4.94 2.4 Mo (polycrystalline) -2.2 4.6 -0.1 Mo (1 0 0) -1.7 4.53 0.2 Ta (polycrystalline) W (polycrystalline) -2.75 4.55 -0.78 W (1 0 0) -3.0 4.63 -0.9 W (1 1 0) 0.3 W (1 1 1) -2.59 4.45 -0.82 Pt (polycrystalline) -1.8 5.64 Au (polycrystalline) 0.9 5.2 4.2 Pb (polycrystalline)

Ps－ emission from Cs deposited Ta surfaces
2 x 1014 atoms/cm2

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).

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

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

Na coating is as effective and the effect remains LONGER!

（Igarashi et al, New J. Phys. 2 (2000) 17）
Theoretical investigations for the photodetachment of Ps－ : Calculation of the photodetachment cross sections by Bhatia and Drachman 1987 Calculation of the photodetachment cross sections by Ward, Humberston and McDowell 2000 Calculation of the photodetachment cross sections by Igarashi, Shimamura and Toshima （Igarashi et al, New J. Phys. 2 (2000) 17）

Ps－ lifetime is 479ps. Ps－photodetachment experiment Pulsed e+ beam
High intensity pulsed laser Pulsed e+ beam synchronized to the laser

Pulsed e+ beam in KEK

Ps－photodetachment experiment
e+ beam : 　　 (from KEK Linac) 　　pulse width 12ns 　　repetition 50Hz Laser : 　　Pulsed Nd: YAG 　　(Spectra Physics GCR290) 　　wave length 1064nm 　　repetition 25Hz 　　power 10W (Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima Phys. Rev. Lett. 106 (2011) )

1064nm Threshold : 0.33eV (Bhatia and Drachman, Phys. Rev. A 32 (1985) 3745, Ward, Humberston and McDowell, J. Phys. B 20 (1987) 127, Igarashi, Shimamura and Toshima, New J. Phys. 2 (2000) 17)

e+ beam : 　　 (from KEK Linac) 　　pulse width 12ns 　　repetition 50Hz Laser : 　　Pulsed Nd: YAG 　　(Spectra Physics GCR290) 　　wave length 1064nm 　　repetition 25Hz 　　power 10W (Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima Phys. Rev. Lett. 106 (2011) )

75% o-Ps, annihilates into 3γ. 25% p-Ps, annihilates into 2γ. Ps－ If Ps－ ions are photodetached, the peak intensity will decrease. (Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima Phys. Rev. Lett. 106 (2011) )

75% o-Ps, annihilates into 3γ. 25% p-Ps, annihilates into 2γ. Ps－ If Ps－ ions are photodetached, the peak intensity will decrease. Ps－ photodetachment has been observed for the first time! (Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima Phys. Rev. Lett. 106 (2011) )

(Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima Phys. Rev. Lett. 106 (2011) )

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 Measurement of the photodetachment cross sections Production of energy tunable Ps beam Precision measurement of the Ps－decay rate

Measurement of the Ps－ photodetachment cross sections
(Michishio, Tachibana, Terabe, Igarashi, Wada, Hyodo, Kuga, Yagishita, Hyodo and Nagashima Phys. Rev. Lett. 106 (2011) )

Measurement of the Ps－ photodetachment cross sections
resonances （Igarashi et al, New J. Phys. 2 (2000) 17）

“Production of an energy tunable positronium beam
Production of energy tunable Ps beam using Ps－ photodetachment technique and its applications “Production of an energy tunable positronium beam by the photodetachment of positronium negative ions” by Koji Michishio on Wednesday

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

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

Precision measurement of the Ps－ decay rate
Ne dispenser By changing the distance and voltage between the target and the earthed grid, we aim high precision measurement with the error ～ 0.1%

CONCLUSIONS (1) We have succeeded in the efficient formation Ps－. (2) We have succeeded in the first estimation of the binding energy of Ps－. (3) We have succeeded in first observation of the photodetachment of Ps－. (4) We have started the next experiments for Ps－.

Member of our group Tokyo University of Science Akira Yagishita (KEK)
Koji Michishio, Takayuki Tachibana Hiroki Terabe, Ryohei Suzuki Ayaka Miyamoto, Toshihide Hakodate Takahiko Sakai Akira Yagishita (KEK) Toshio Hyodo (KEK) Ken Wada (KEK) Akinori Igarashi (Miyazaki Univ.) Takahiro Kuga (Univ. of Tokyo)

Thank you very much for your attention !

Why Ps- fraction increased dramatically?
If we assume that the Ps- is formed from positrons in the bottom of the positron band, holds for the Ps- emission, where and are the energy levels of two electrons measured from the vacuum level. Ps- fraction : : electron density of states If is constant below the top of the conduction band, This is too low to explain the experimental results.

Why Ps- fraction increased dramatically?
The Ps- formation depends on the overlap of a e+ wave function and two e- wave functions just outside of the surface, which depend on their corresponding work functions. The Ps- formation mechanism should be affected by the Cs deposition. Cs deposited uncoated e+ and e- wave functions

Ps生成の微分断面積は前方にピークを持つ (Brown, Positron Annihilation (1985) 328)
e+ビームと気体分子との電荷交換反応 Ps生成の微分断面積は前方にピークを持つ , where (Brown, Positron Annihilation (1985) 328)

(Weber et al., Phys. Rev. Lett. 61 (1988) 2542)
エネルギー可変Psビーム応用例：LiF表面でのPs の鏡面反射 (Weber et al., Phys. Rev. Lett. 61 (1988) 2542)

Ps energy distribution: (Mills and Crane, Phys. Rev. A 31 (1985) 593)

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