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Physical Review Letters

Cavity Induced Shift and Narrowing of the Positronium Lyman- α Transition

D. B. Cassidy, M. W. J. Bromley, L. C. Cota, T. H. Hisakado, Harry W. K. Tom, Allen Paine Mills

Published Jan 10, 2011• 37 citations
Publisher

Abstract

We report experiments in which the line shape of the Lyman-alpha ($1S\mathrm{\text{\ensuremath{-}}}2P$) transition was measured for positronium (Ps) atoms both inside and outside a porous silica target. The energy interval $\ensuremath{\Delta}E$ for confined atoms was observed to be larger than that of free Ps by $1.26\ifmmode\pm\else\textpm\fi{}0.06\text{ }\text{ }\mathrm{meV}$. A configuration interaction calculation yields results that are consistent with our $\ensuremath{\sim}5\text{ }\text{ }\mathrm{nm}$ sample, and suggests that $\ensuremath{\Delta}E$ decreases dramatically for larger cavity diameters. The linewidth of the transition, $(0.066\ifmmode\pm\else\textpm\fi{}0.004)\text{ }\text{ }\mathrm{nm}$ (FWHM), is about half of what one would expect for free Ps at room temperature due to the Dicke line narrowing effect of confinement. Such measurements can be used to determine void sizes in porous films and Ps dynamics therein, and elimination of the Doppler spread of atoms in a porous film could be useful for the efficient excitation of a Ps gas.

DOI
10.1103/physrevlett.106.023401

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We report experiments in which the line shape of the Lyman-alpha (1S\ensuremath-2P1S\mathrm{\text{\ensuremath{-}}}2P1S\ensuremath-2P) transition was measured for positronium (Ps) atoms both inside and outside a porous silica target. The energy interval \ensuremathΔE\ensuremath{\Delta}E\ensuremathΔE for confined atoms was observed to be larger than that of free Ps by 1.26\ifmmode±\else\textpm\fi0.06  meV1.26\ifmmode\pm\else\textpm\fi{}0.06\text{ }\text{ }\mathrm{meV}1.26\ifmmode±\else\textpm\fi0.06  meV. A configuration interaction calculation yields results that are consistent with our \ensuremath∼5  nm\ensuremath{\sim}5\text{ }\text{ }\mathrm{nm}\ensuremath∼5  nm sample, and suggests that \ensuremathΔE\ensuremath{\Delta}E\ensuremathΔE decreases dramatically for larger cavity diameters. The linewidth of the transition, (0.066\ifmmode±\else\textpm\fi0.004)  nm(0.066\ifmmode\pm\else\textpm\fi{}0.004)\text{ }\text{ }\mathrm{nm}(0.066\ifmmode±\else\textpm\fi0.004)  nm (FWHM), is about half of what one would expect for free Ps at room temperature due to the Dicke line narrowing effect of confinement. Such measurements can be used to determine void sizes in porous films and Ps dynamics therein, and elimination of the Doppler spread of atoms in a porous film could be useful for the efficient excitation of a Ps gas.
We report experiments in which the line shape of the Lyman-alpha (1S-2P) transition was measured for positronium (Ps) atoms both inside and outside a porous silica target.The energy interval ÁE for confined atoms was observed to be larger than that of free Ps by 1:26 Æ 0:06 meV.A configuration interaction calculation yields results that are consistent with our $5 nm sample, and suggests that ÁE decreases dramatically for larger cavity diameters.The linewidth of the transition, ð0:066 Æ 0:004Þ nm (FWHM), is about half of what one would expect for free Ps at room temperature due to the Dicke line narrowing effect of confinement.Such measurements can be used to determine void sizes in porous films and Ps dynamics therein, and elimination of the Doppler spread of atoms in a porous film could be useful for the efficient excitation of a Ps gas.
We have measured the kinetic energy of positronium (Ps) atoms emitted into a vacuum from a porous silica film subsequent to positron bombardment, via the Doppler spread of the linewidth of the Ps 13S{1}^{3}S13S-23P{2}^{3}P23P transition. We find that the deeper in the target film that positrons are implanted the colder is the emitted Ps, an effect we attribute to cooling via collisions in the pores as the atoms diffuse back to the film surface. We observed a lower limit to the mean Ps kinetic energy associated with motion in the direction of the laser, ${E}_{x}=42\ifmmode\pm\else\textpm\fi{}3$ meV, that is consistent with conversion of the confinement energy of Ps in the 2.7-nm-diameter pores to kinetic energy in vacuum. An implication is that a porous sample would need to be composed of pores greater than around 10 nm in diameter in order to produce thermal Ps in vacuum with temperatures of less than 100 K. By performing Doppler spectroscopy on intense pulses of Ps we have experimentally demonstrated the production of many excited-state Ps atoms simultaneously, which could have numerous applications, including laser cooling and fundamental spectroscopic studies of Ps and the production of antihydrogen.
Laser cooling of positronium (Ps) is an important technique to advance the frontier of science by, for instance, realizing the first Bose-Einstein condensation of antimatters. It was proposed that performing laser cooling on Ps trapped in a cryogenic porous material would be more efficient, but a recent study (Cooper, et al., Phys. Rev. B 97, 205302 (2018)) reported that Ps in the excited (2P) state inside nano pores of silica had a large decay rate into γ-rays, and the resonant spectrum of the transition was significantly broadened, both of which are critically problematic for the proposed cooling scheme. A possible cause of these unexplained results was proposed to be residual impurities which trapped Ps in the vicinity of the pore surface and then enhanced interactions between Ps and the material. This work reports fabrication and inspection of a new porous silica cavity without such impurities, and construction of an experimental system to induce the Lyman-α transition of Ps trapped in pores of the cavity.
Abstract The quest for making a triplet positronium (Ps) Bose–Einstein condensate confined in a micron-sized cavity in a material such as porous silica faces at least three interrelated problems: (1) About 10710^7107 10 7 spin polarized Ps atoms must be injected into a small cavity within a porous solid material without vaporizing it. (2) It is known that Ps atoms confined in 30–100 nm diameter cavities in porous silica do not remain in the gas phase, but become stuck to the cavity walls at room temperature (Cooper et al., Phys. Rev. B 97:205302, 2018). (3) Cooling a gas of Ps atoms to cryogenic temperatures by energy exchange with the walls would be a very slow process (Saito and Hyodo, in: Surko, Gianturco (eds) New Directions in Antimatter Chemistry and Physics, Springer Dordrecht, Netherlands, 2001) because of the relatively low collision rate with the walls and the large mismatch between the masses of the Ps and the wall atoms. A possible solution of these difficulties is presented, based on cooling the implanted positrons in an isotopically pure diamond single crystal target, subsequent saturating of the wall Ps coverage so that a substantial portion of the Ps will be in the gaseous state, and thermalizing the gas-phase Ps via collisions with the low effective mass wall Ps. A design process for the target material is outlined as well, including preliminary results in porous cavity fabrication using focused ion beam milling. Graphical abstract
We report measurements of shifts in the frequencies of $1\phantom{\rule{0.16em}{0ex}}^{3}S_{1}\ensuremath{\rightarrow}2\phantom{\rule{0.16em}{0ex}}^{3}P_{J}$ and $2\phantom{\rule{0.16em}{0ex}}^{3}P_{J}\ensuremath{\rightarrow}n\phantom{\rule{0.16em}{0ex}}^{3}D/n\phantom{\rule{0.16em}{0ex}}^{3}S$ transitions optically driven in positronium atoms while they are inside the open volumes of MgO smoke powder. The observed intervals are larger than the corresponding vacuum excitations, but, surprisingly, the transitions to Rydberg states are less strongly affected, and the energy shifts exhibit no dependence on the principal quantum number nnn of the final state. We attribute these shifts to resonant interactions between Ps atoms and MgO surfaces, mediated via spectrally overlapping MgO ultra violet (UV) photo-luminescent absorption bands. Since many insulating materials suitable for Ps confinement exhibit similar broadband UV absorption characteristics, the observed phenomena have implications for optical diagnostics and laser cooling schemes of relevance to studies of high-density Ps ensembles in insulating cavities, including the production of a Ps Bose-Einstein condensate.
Fine structure transitions in the first excited state of positronium (Ps) have been measured using 'Backscatter Ps' production on a Mo surface by observation of a change in the emitted Lyman- alpha intensity under resonant microwave irradiation. Production, fine structure transitions and Lyman- alpha decay of the Ps atoms took place inside a waveguide designed to transmit the microwave frequencies of 8.6, 13.0 and 18.5 GHz for the transitions from the 2 3 S 1 state to the 2 3 PJ, J=2, 1, 0, states, respectively. In the presence of a magnetic field, all transitions observed show a shift to higher frequencies, compared with earlier calculations and measurements in zero magnetic field. The deviations exceed the expected Zeeman shift significantly but may be explained by assuming a motional Stark effect as predicted by Lewis and Hughes for Ps with kinetic energies of several eV.