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Physical Review Letters

Hydrogen-Assisted Laser-Induced Resonant Transitions between Metastable States of Antiprotonic Helium Atoms

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

Published Mar 3, 1997• 35 citations
Publisher

Abstract

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 ${\mathrm{H}}_{2}$ molecules. By selectively shortening the lifetimes of states with higher principal quantum number $n$ as compared to those of lower $n$, this method for the first time provides access to all initially populated metastable states of $\overline{p}{\mathrm{He}}^{+}$ atoms. This was demonstrated by observing the transitions $(n,l)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(38,l)\ensuremath{\rightarrow}(39,l+1)$, $l\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}35,36,37$ and $(n,l)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(37,l)\ensuremath{\rightarrow}(38,l+1)$, $l\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}34,35,36$.

DOI
10.1103/physrevlett.78.1671

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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)=(38,l)\ensuremath→(39,l+1)(n,l)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(38,l)\ensuremath{\rightarrow}(39,l+1)(n,l)=(38,l)\ensuremath→(39,l+1), l=35,36,37l\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}35,36,37l=35,36,37 and (n,l)=(37,l)\ensuremath→(38,l+1)(n,l)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(37,l)\ensuremath{\rightarrow}(38,l+1)(n,l)=(37,l)\ensuremath→(38,l+1), l=34,35,36l\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}34,35,36l=34,35,36.
Laser resonance transitions between normally metastable states of antiprotonic helium atoms were induced making use of state dependent quenching effects caused by trace admixtures of H2 to the target helium gas. With this method of “H2-assisted inverse resonances” the decay rates of the states (n,l)=(39,l),l=36,37,38, and (38,l),l=35,36,37 of p̄He+ were determined as a function of the H2 admixture. The quenching cross sections at 30 K deduced therefrom for the states with n=39 were found to be of the order of the geometrical cross section for p̄He+–H2 collisons (2⋅10−15 cm2), with a moderate decrease with increasing l. Within a given cascade with constant v=n−l−1, the quenching cross sections for states with n=38 are smaller by a factor of 4–6 than those for states with n=39.
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.
We have observed the first laser-induced resonant transitions in antiprotonic helium atoms. These occur between metastable states and Auger dominated short lived states, and show that the anomalous longevity of antiprotons previously observed in helium media results from the formation of high-n high-l atomic states of p\ifmmode\bar\else\textasciimacron\fi{}He+{\mathrm{He}}^{+}He+. The observed transition with vacuum wavelength 597.259\ifmmode\pm\else\textpm\fi{}0.002 nm is tentatively assigned to (n,l)=(39,35)\ensuremath{\rightarrow}(38,34).
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.
Initial distributions of metastable antiprotonic 4He^{\mathrm{4}}\mathrm{H}\mathrm{e}4He and 3He^{\mathrm{3}}\mathrm{H}\mathrm{e}3He atoms over principal (nnn) and angular momentum ($\ensuremath{\ell}$) quantum numbers have been deduced using laser spectroscopy experiments. The regions n=37−−40n=37--40n=37−−40 and n=35−−38n=35--38n=35−−38 in the two atoms account for almost all of the observed fractions [$(3.0\ifmmode\pm\else\textpm\fi{}0.1)%$ and $(2.4\ifmmode\pm\else\textpm\fi{}0.1)%$] of antiprotons captured into metastable states.