Physical Review Letters

First observation of laser-induced resonant annihilation in metastable antiprotonic helium atoms

N. Morita, M. Kumakura, Toshitsugu Yamazaki, E. Widmann, H. Masuda, I. Sugai, R. Hayano, Frank E. Maas, H. A. Torii, F. J. Hartmann, H. Daniel, Till von Egidy, B. Ketzer, Werner Müller, W. Schmid, D. Horváth, J. A. Eades

Published Feb 21, 1994• 140 citations

Publisher

Abstract

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{}${\mathrm{He}}^{+}$. 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).

DOI: 10.1103/physrevlett.72.1180

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Physical Review Letters

First observation of laser-induced resonant annihilation in metastable antiprotonic helium atoms

Published Feb 21, 1994• 140 citations

Publisher

Abstract

{\mathrm{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).

DOI: 10.1103/physrevlett.72.1180

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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.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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{\mathrm{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

(\overline{p}{}^{4}{\mathrm{He}}^{+}).

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,\mathcal{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)

was found to shorten from 1.3 to 0.1 $\ensuremath{\mu}$s with increasing density. The lifetimes of the states

(38,34)

and

(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)

becomes short lived at high density, the resonance $(38,35)\ensuremath{\rightarrow}(37,34)$ at 529.62 nm was detected.

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

Another laser-induced resonant transition in the v=n-l-1=2 metastable cascade of antiprotonic

^{4}\mathrm{He}

atoms has been found by using a double-resonance technique. This was done by setting the first laser to the already known 470.724-nm resonance [(n,l)=(37,34)\ensuremath{\rightarrow}(36,33)], while the (38,35)\ensuremath{\rightarrow}(37,34) transition was searched for with the second laser. The resonant transition was found at a wavelength of 529.622\ifmmode\pm\else\textpm\fi{}0.003 nm, showing excellent agreement with a recent prediction of Korobov.