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Physical Review A

Populations and lifetimes in the v=n−ℓ−1=2 and 3 metastable cascades of p¯He+ measured by pulsed and continuous antiproton beams

M. Hori, J. A. Eades, E. Widmann, T. Yamazaki, R. Hayano, Takashi Ishikawa, H. A. Torii, Till von Egidy, F. J. Hartmann, B. Ketzer, C. Maierl, Randolf Pohl, M. Kumakura, N. Morita, D. Horváth, I. Sugai

Published Jul 26, 2004• 38 citations
Publisher

Abstract

The time evolution of the state populations in the $v\ensuremath{\equiv}n\ensuremath{-}\ensuremath{\ell}\ensuremath{-}1=2$ and 3 metastable cascades of antiprotonic helium (${\overline{p}}^{4}{\mathrm{He}}^{+}$) atoms were studied using laser spectroscopy. The antiprotonic states $(n,\ensuremath{\ell})=(39,35)$, (40,36), and (41,37) in the $v=3$ cascade were estimated to initially contain, respectively, $(0.28\ifmmode\pm\else\textpm\fi{}0.04)%$, $(0.06\ifmmode\pm\else\textpm\fi{}0.02)%$, and $(0.02\ifmmode\pm\else\textpm\fi{}0.01)%$ of the antiprotons stopped in the helium target, while the corresponding values for states $(37,34)$, $(38,35)$, and $(39,36)$ in the $v=2$ cascade were $(0.21\ifmmode\pm\else\textpm\fi{}0.04)%$, $(0.49\ifmmode\pm\else\textpm\fi{}0.07)%$, and $(0.19\ifmmode\pm\else\textpm\fi{}0.07)%$. The shortening of the state lifetimes due to collisions between $\overline{p}{\mathrm{He}}^{+}$ and helium atoms was studied. As the atomic density of the target was increased from $\ensuremath{\rho}=2\ifmmode\times\else\texttimes\fi{}{10}^{20}$ to $2\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, the lifetime of state $(37,34)$ decreased by an order of magnitude, whereas the higher states $(38,35)$ and $(39,35)$ were relatively unaffected. Between densities $\ensuremath{\rho}=6\ifmmode\times\else\texttimes\fi{}{10}^{20}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, a short-lived component with a time constant $\ensuremath{\tau}\ensuremath{\sim}0.2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ appeared at early times in the delayed annihilation time spectrum of $\overline{p}{\mathrm{He}}^{+}$, while the total fraction of long-lived antiprotons decreased from 3.0 to $2.5%$. These effects were caused by the lifetime-shortening of specific $\overline{p}{\mathrm{He}}^{+}$ states such as $(37,34)$.

DOI
10.1103/physreva.70.012504

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The time evolution of the state populations in the v\ensuremath≡n\ensuremath−\ensuremathℓ\ensuremath−1=2v\ensuremath{\equiv}n\ensuremath{-}\ensuremath{\ell}\ensuremath{-}1=2v\ensuremath≡n\ensuremath−\ensuremathℓ\ensuremath−1=2 and 3 metastable cascades of antiprotonic helium (p‾4He+{\overline{p}}^{4}{\mathrm{He}}^{+}p​4He+) atoms were studied using laser spectroscopy. The antiprotonic states (n,\ensuremathℓ)=(39,35)(n,\ensuremath{\ell})=(39,35)(n,\ensuremathℓ)=(39,35), (40,36), and (41,37) in the v=3v=3v=3 cascade were estimated to initially contain, respectively, (0.28\ifmmode±\else\textpm\fi0.04)(0.28\ifmmode\pm\else\textpm\fi{}0.04)%(0.28\ifmmode±\else\textpm\fi0.04), (0.06\ifmmode±\else\textpm\fi0.02)(0.06\ifmmode\pm\else\textpm\fi{}0.02)%(0.06\ifmmode±\else\textpm\fi0.02), and (0.02\ifmmode±\else\textpm\fi0.01)(0.02\ifmmode\pm\else\textpm\fi{}0.01)%(0.02\ifmmode±\else\textpm\fi0.01) of the antiprotons stopped in the helium target, while the corresponding values for states (37,34)(37,34)(37,34), (38,35)(38,35)(38,35), and (39,36)(39,36)(39,36) in the v=2v=2v=2 cascade were (0.21\ifmmode±\else\textpm\fi0.04)(0.21\ifmmode\pm\else\textpm\fi{}0.04)%(0.21\ifmmode±\else\textpm\fi0.04), (0.49\ifmmode±\else\textpm\fi0.07)(0.49\ifmmode\pm\else\textpm\fi{}0.07)%(0.49\ifmmode±\else\textpm\fi0.07), and (0.19\ifmmode±\else\textpm\fi0.07)(0.19\ifmmode\pm\else\textpm\fi{}0.07)%(0.19\ifmmode±\else\textpm\fi0.07). The shortening of the state lifetimes due to collisions between p‾He+\overline{p}{\mathrm{He}}^{+}p​He+ and helium atoms was studied. As the atomic density of the target was increased from \ensuremathρ=2\ifmmode×\else\texttimes\fi1020\ensuremath{\rho}=2\ifmmode\times\else\texttimes\fi{}{10}^{20}\ensuremathρ=2\ifmmode×\else\texttimes\fi1020 to 2\ifmmode×\else\texttimes\fi1021cm\ensuremath−32\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}2\ifmmode×\else\texttimes\fi1021cm\ensuremath−3, the lifetime of state (37,34)(37,34)(37,34) decreased by an order of magnitude, whereas the higher states (38,35)(38,35)(38,35) and (39,35)(39,35)(39,35) were relatively unaffected. Between densities \ensuremathρ=6\ifmmode×\else\texttimes\fi1020\ensuremath{\rho}=6\ifmmode\times\else\texttimes\fi{}{10}^{20}\ensuremathρ=6\ifmmode×\else\texttimes\fi1020 and 2\ifmmode×\else\texttimes\fi1022cm\ensuremath−32\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}2\ifmmode×\else\texttimes\fi1022cm\ensuremath−3, a short-lived component with a time constant \ensuremathτ\ensuremath∼0.2\ensuremathμs\ensuremath{\tau}\ensuremath{\sim}0.2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}\ensuremathτ\ensuremath∼0.2\ensuremathμs appeared at early times in the delayed annihilation time spectrum of p‾He+\overline{p}{\mathrm{He}}^{+}p​He+, while the total fraction of long-lived antiprotons decreased from 3.0 to 2.52.5%2.5. These effects were caused by the lifetime-shortening of specific p‾He+\overline{p}{\mathrm{He}}^{+}p​He+ states such as (37,34)(37,34)(37,34).
A laser spectroscopic method was developed to directly observe the lifetimes of individual states in metastable antiprotonic helium atoms (p‾4He+).(\overline{p}{}^{4}{\mathrm{He}}^{+}).(p​4He+). Collisional effects with surrounding helium atoms were studied in cryogenic gas targets at pressures between 0.1 and 9.0 bars and at temperatures between 5.5 and 7.0 K. The metastable state (n,l)=(39,35)(n,\mathcal{l})=(39,35)(n,l)=(39,35) showed a lifetime of 1.5 $\ensuremath{\mu}\mathrm{s}$, regardless of density, whereas the lifetime of the energetically lower-lying state (37,34)(37,34)(37,34) was found to shorten from 1.3 to 0.1 $\ensuremath{\mu}$s with increasing density. The lifetimes of the states (38,34)(38,34)(38,34) and (36,33)(36,33)(36,33) were determined to be 10 ns and 5 ns, respectively, showing the effects of the internal Auger process. When extrapolated to zero density, the measured lifetimes agreed with the results of calculations for the radiative and Auger widths. Using the fact that the state (37,34)(37,34)(37,34) becomes short lived at high density, the resonance $(38,35)\ensuremath{\rightarrow}(37,34)$ at 529.62 nm was detected.
Laser studies of metastable antiprotonic helium atoms, which we recently initiated by observing a sharp increase of the antiproton annihilation rate induced by laser-stimulated resonant transitions, have been extended. With a single laser tuned to the resonance already found at 597.26 nm, we have now established the time dependence of the upper state population. With two lasers ignited at variable time separation, we also studied the feeding of the upper state from higher atomic levels. The initial populations and level lifetimes of excited exotic atoms were determined.
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Calculations of low-energy collisions of antiprotons p‾\overline{p}p​ with helium atoms in the ground (11S)(1\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})(11S) and metastable (23S(2\phantom{\rule{4pt}{0ex}}^{3}\mathit{S}(23S and 21S)2\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})21S) states are carried out by using a semiclassical method and by further assuming that one of the two electrons is passive. Antiproton capture to form antiprotonic helium $(\ensuremath{\rightarrow}\overline{p}{\mathrm{He}}^{+}+e)$ and breakup ionization $(\ensuremath{\rightarrow}\overline{p}+{\mathrm{He}}^{+}+e)$ are investigated. The energy dependence of the capture and ionization cross sections and the distributions of the product states are presented and compared with previous theoretical calculations. For the metastable helium targets, most of the p‾He+\overline{p}{\mathrm{He}}^{+}p​He+ atoms produced in the capture have long lifetimes against subsequent Auger decays and can be observed experimentally. For the ground-state targets, however, the formation of such long-lived states is rather a rare event at low collision energies.
Laser resonance transitions between normally metastable states of antiprotonic helium atoms were observed making use of state dependent quenching effects caused by small admixtures of H2{\mathrm{H}}_{2}H2​ molecules. By selectively shortening the lifetimes of states with higher principal quantum number nnn as compared to those of lower nnn, this method for the first time provides access to all initially populated metastable states of p‾He+\overline{p}{\mathrm{He}}^{+}p​He+ atoms. This was demonstrated by observing the transitions $(n,l)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(38,l)\ensuremath{\rightarrow}(39,l+1)$, l=35,36,37l\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}35,36,37l=35,36,37 and $(n,l)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(37,l)\ensuremath{\rightarrow}(38,l+1)$, l=34,35,36l\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}34,35,36l=34,35,36.
A systematic study of the decay rates of antiprotonic helium (p‾4He+{\overline{p}}^{4}{\mathrm{He}}^{+}p​4He+ and p‾3He+{\overline{p}}^{3}{\mathrm{He}}^{+}p​3He+) at CERN Antiproton Decelerator has been made by a laser spectroscopic method. The decay rates of some of its short-lived states, namely those for which the Auger rates ${\ensuremath{\gamma}}_{A}$ are much larger than their radiative decay rates $({\ensuremath{\gamma}}_{\mathrm{rad}}\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}{s}^{\ensuremath{-}1})$, were determined from the time distributions of the antiproton annihilation signals induced by laser beams, and the widths of the atomic resonance lines. The magnitude of the decay rates, especially their relation with the transition multipolarity, is discussed and compared with theoretical calculations.