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Journal of Optics B Quantum and Semiclassical Optics

Polarization-dependent laser-induced orientation in antiprotonic helium

Leonid A. Melnikov, Maria V. Ryabinina

Published Aug 1, 2001• 0 citations
Publisher

Abstract

Laser-induced orientation and population dynamics in antiprotonic helium under the action of a short laser pulse are studied numerically to investigate polarization effects. Stimulated transition from the metastable long-lived state to the Auger-decayed state is considered, taking into account the Zeeman structure of the levels. The regimes when nearly all atoms can be transferred to the lower level (similar to a π-pulse in a two-level system) are demonstrated.

DOI
10.1088/1464-4266/3/4/312

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Mar 4, 1999•0 citations
Laser-induced orientation and population dynamics in antiprotonic helium under the action of a short laser pulse are studied numerically to investigate polarization effects. Stimulated transition from the metastable long-lived state to the Auger-decayed state is considered, taking into account the Zeeman structure of the levels. The regimes when nearly all atoms can be transferred to the lower level (similar to a π-pulse in a two-level system) are demonstrated.
We report the results of numerical modeling of the coherent action of a single short laser pulse on a multilevel quantum system of metastable and short-lived states of antiprotonic helium. Increasing the pulse intensity we observed more new states getting involved into the coherent interaction, their populations being affected by the pulse. Examples of intensity-dependent dynamics of the metastable states populations are considered. Extraction of spectroscopic information about the states of antiprotonic helium from the experiments with fixed-frequency variable-intensity laser pulses is discussed.
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).
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.
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 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.