Evidence for positronium molecule formation at a metal surface

Physical Review A - Atomic, Molecular, and Optical Physics

Volumn 76, Issue 6, 2007, Pages

  Cassidy, D B ^a   Deng, S H M ^a   Mills, A P ^a  

^a UNIVERSITY OF CALIFORNIA   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ALUMINUM; DATA REDUCTION; SPECTROSCOPIC ANALYSIS; THERMODYNAMICS;

METAL SURFACE; POSITRON BEAM DENSITY; POSITRONIUM MOLECULE FORMATION;

POSITRONS;

EID: 37649010360     PISSN: 10502947     EISSN: 10941622     Source Type: Journal    
DOI: 10.1103/PhysRevA.76.062511     Document Type: Article

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34. The total energy deposited by a fully compressed positron pulse is U=1.5× 107 ×1500 eV×1.6× 10-19 J eV=3.6× 10-9 J. The total volume into which this energy is deposited is V=π× (0.02)2 ×2.5× 10-6 cm=3× 10-9 cm3. The heat capacity of Al at 300 K is Cp =5.82 cal K mole=24.4 J K 27 g=2.44 J K-1 cm-3. The temperature rise neglecting heat conduction is then given by ΔT=U V Cp =3.6× 10-9 J (2.44 J K-1 ×3× 10-9 cm3) =0.5 K. This is the maximum temperature variation. In reality the heat will diffuse during the 1 ns implantation time and the actual surface temperature would be much less than this. The Al heat capacity was taken from the Handbook of Chemistry and Physics, 66th ed., edited by, R. C. Weast, (CSC Press, Boca Raton, 1985).
35. The maximum slope of a fd vs T curve is 0.03 200 °C=1.5× 10-4 [A. P. Mills, Jr., Phys. Rev. Lett. PRLTAO 0031-9007 10.1103/PhysRevLett.41.1828 41, 1828 (1978)] which gives a Ps delayed fraction thermally induced by beam impact of ∼7.5× 10-5 which is an upper limit and is more than 10 times smaller than the effect we observe.
36. Large shifts in the work function have been observed on Cu surfaces due to a partial coverage of Cs. [C. A. R. Koymen, K. H. Lee, D. Mehl, A. Weiss, and K. O. Jensen, Phys. Rev. Lett. PRLTAO 0031-9007 10.1103/PhysRevLett.68.2378 68, 2378 (1992).] The shifts do not simply imply a change in the surface dipole potential, but must involve changes in the width and/or depth of the potential well seen by the surface positrons.
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