We report the results of numerical modeling of the coherent laser multi-pulse technique, proposed in our earlier papers to measure the population of the metastable states of antiprotonic helium and to control the decay kinetics in the adjacent transition chains. It is demonstrated that a sequence of properly chosen (pi) -pulses may result in a sequence of annihilation spikes whose amplitudes are nearly proportional to the populations of individual metastable states. The limitations related to the pulse duration and delay are studied as well as the role of the lifetime of the short-lived final state.
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
(p4He+). 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) 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.
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 (
p4He+) 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
pHe+ 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
pHe+, 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
pHe+ states such as
(37,34).