About 3% of antiprotons (p) stopped in helium are long-lived with microsecond lifetimes, against picoseconds in all other materials. This unusuallongevity has been ascribed to the trapping of p on metastable bound states in pHe+ helium atom-molecules thus named atomcules. Apart from their unique dual structure investigated by laser spectroscopy - a near-circular quasi-classical Rydberg atom with l ~ n - l ~ 37 or a special diatomic molecule with a negatively charged p nucleus in high rotational state with J = l - the chemical physics aspects of their interaction with other atoms or molecules constitute an interesting topic for molecular physics. While atomcules may resist to million collisions in helium, molecular contaminants such as H2 are likely to destroy them in a single one, down to very low temperatures. In the Born-Oppenheimer framework, we interpret the molecular interaction obtained by ab initio quantum chemical calculations in terms of classical reactive channels, with activation barriers accounting for the experiments carried out in He and H2 . From classical trajectory Monte Carlo simulations, we show that the thermalization stage strongly quenches initial populations, thus reduced to a recovered 3 % trapping fraction. Alternatively, we investigate the reactive association of pHe+ with e+e- in a classical trajectory approach for nuclei: we predict antihydrogen synthesis with 10 % branching ratio and a new class of metastable atomcules (alpha,p, e+, 2e-), which could be confirmed by spectroscopy. This work illustrates the pertinence of chemical physics concepts to the study of exotic processes involving antimatter. New insights into the physico-chemistry of cold interstellar radicals are anticipated.
About 3% of antiprotons (p) stopped in helium are long-lived with microsecond lifetimes, against picoseconds in all other materials. This unusuallongevity has been ascribed to the trapping of p on metastable bound states in pHe+ helium atom-molecules thus named atomcules. Apart from their unique dual structure investigated by laser spectroscopy - a near-circular quasi-classical Rydberg atom with l ~ n - l ~ 37 or a special diatomic molecule with a negatively charged p nucleus in high rotational state with J = l - the chemical physics aspects of their interaction with other atoms or molecules constitute an interesting topic for molecular physics. While atomcules may resist to million collisions in helium, molecular contaminants such as H2 are likely to destroy them in a single one, down to very low temperatures. In the Born-Oppenheimer framework, we interpret the molecular interaction obtained by ab initio quantum chemical calculations in terms of classical reactive channels, with activation barriers accounting for the experiments carried out in He and H2 . From classical trajectory Monte Carlo simulations, we show that the thermalization stage strongly quenches initial populations, thus reduced to a recovered 3 % trapping fraction. Alternatively, we investigate the reactive association of pHe+ with e+e- in a classical trajectory approach for nuclei: we predict antihydrogen synthesis with 10 % branching ratio and a new class of metastable atomcules (alpha,p, e+, 2e-), which could be confirmed by spectroscopy. This work illustrates the pertinence of chemical physics concepts to the study of exotic processes involving antimatter. New insights into the physico-chemistry of cold interstellar radicals are anticipated.
About 3% of antiprotons () stopped in helium are long-lived with microsecond lifetimes, against picoseconds in all other materials. This unusual longevity has been ascribed to the trapping of on metastable bound states in He+ helium atom-molecules thus named atomcules. Apart from their unique dual structure investigated by laser spectroscopy – a near-circular quasi-classical Rydberg atom with l ~ n – 1 ~ 37 or a special diatomic molecule with a negatively charged nucleus in high rotational state with J = l – the chemical physics aspects of their interaction with other atoms or molecules constitute an interesting topic for molecular physics. While atomcules may resist to million collisions in helium, molecular contaminants such as H2 are likely to destroy them in a single one, down to very low temperatures. In the Born-Oppenheimer framework, we interpret the molecular interaction obtained by ab initio quantum chemical calculations in terms of classical reactive channels, with activation barriers accounting for the experiments carried out in He and H2. From classical trajectory Monte Carlo simulations, we show that the thermalization stage strongly quenches initial populations, thus reduced to a recovered 3 % trapping fraction. This work illustrates the pertinence of chemical physics concepts to the study of exotic processes involving antimatter. New insights into the physico-chemistry of cold interstellar radicals are anticipated.
The current status of the physics of exotic atoms is reviewed. The problem of the anomalous width of the kaonic helium 2p level is analyzed, experimental CPT invariance tests for the antihydrogen are discussed, and results of investigations on the metastability of the antiproton (as well as of ??, ??, K?, ??, ...) in helium are considered. A comparison is made of the well-known Condo model and the ionic model. In the latter, the ion (?ee) ? a so-called 'ioncule' ? forms after is stopped in its motion in helium. One of the ioncule's electrons is weakly bound, resides at one of the Efimov levels, and orbits at ~ 20 a.u. in the dipole field of the neutral 'atomcule' (?e). The lifetime of the ioncule is long (> 10? 5 s) and the antiproton's vibrational ? rotational transition frequencies ? are practically equal to the atomcule value (?? /? ~ 10? 5 ?10? 6 ) ? in agreement with laser-spectroscopy data. While the ionic model results on the quenching cross sections of metastable states by impurity atoms and molecules in helium agree well with experiment, the atomcule-based Condo model underrates the cross sections by 3 ? 4 orders of magnitude. The ionic model is also consistent with other experiments. Direct experimental tests of this model are discussed. A new interpretation of E Zavattini's group's laser-spectroscopy results on the 2s ? 2p-splitting in muonic helium is presented.
The exotic helium atom, consisting of a helium nucleus He2+, an electron ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$, and a massive, negatively charged particle ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$, such as an antiproton (p\ifmmode\bar\else\textasciimacron\fi{}) or a kaon (${\mathit{K}}^{\mathrm{\ensuremath{-}}}$), is studied theoretically. This exotic atom is more like a polar molecule with two nuclei He2+ and ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$, if ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ is in a low-lying orbital and ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ is in a highly excited orbital. Such states are generally formed in the capture of ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ by helium. The Born-Oppenheimer (BO) separation of the ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ motion and the He2+-${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ motion is a good approximation for these states, and affords a transparent, unified perspective of exotic helium with different ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ and different isotopes of He2+. A propensity rule that favors small transition energies (except for the smallest one) is found for radiative emission by low vibrational and high rotational levels. This rule resembles the selection rule for infrared emission by the usual diatomic polar molecules, and contradicts the conventional expectation that radiative transitions in exotic atoms favor larger transition energies. The radiative lifetimes \ensuremath{\tau} of highly excited states formed immediately after the capture of ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ are of the order of \ensuremath{\mu}sec for 4He2+-p\ifmmode\bar\else\textasciimacron\fi{}-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ and 3He2+-p\ifmmode\bar\else\textasciimacron\fi{}-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$, and those for 4He2+-${\mathit{K}}^{\mathrm{\ensuremath{-}}}$-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ are about half the values for the antiprotonic helium.The effect of the electronic charge polarization by ${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ on \ensuremath{\tau} is explained quantitatively in a simple picture in terms of the molecular dipole moment. Low vibrational, high rotational states are described as circular or near-circular states in the atomic picture. They are stable against Auger decay, which the nonadiabatic perturbation on the BO states induces. An infinite series of bound \ensuremath{\Sigma} electronic states of exotic helium negative ions (He2+-${\mathit{X}}^{\mathrm{\ensuremath{-}}}$-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$)-${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ exists for any He2+-${\mathit{X}}^{\mathrm{\ensuremath{-}}}$ distances R beyond 0.819 a.u. in the BO approximation. An infinite \ensuremath{\Pi} series exists for R>3.811 a.u., and an infinite \ensuremath{\Delta} series for R>9.532 a.u.
A theoretical study is conducted on the stability of exotic antiproton-hydrogen molecular anions (pH), which in the separated atom limit correlate with p+H(n=2) (n being the principal quantum number). The pH molecule in the electronic ground state is highly unstable due to pair annihilation and autodetachment ($\ensuremath{\rightarrow}\overline{p}p+e$). However, if the pH molecule is electronically excited, the molecular stability can be drastically improved. Since the excited states of the H atom have accidental degeneracy, the asymptotic form of the Born-Oppenheimer potential curve of p+H(n=2) behaves as 1/R2 (R being the relative distance), which can play a critical role in the bonding and stability of the molecule. It is found that the pH molecule in the first electronic excited state and in a high rotational state is sufficiently stable and has a lifetime dominated by spontaneous radiative emission ($\ensuremath{\rightarrow}\overline{p}+\mathrm{H}+h\ensuremath{\nu}$). The pH system is dynamically similar to ${\mathrm{H}}_{2}{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{-}}$, which had been commonly considered to be unstable due to autodetachment. However, recent measurements of ion-beam sputtering unambiguously verified the existence of long-lived ${\mathrm{H}}_{2}{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{-}}$ molecules in high rotational states. This suggests that the pH molecules may actually be created if one employs experimental means such as the chemical sputtering of hydrogen-rich targets with p beams.
We review recent experiments carried out with dense (10(12) cm(-3)) ultracold (T = 40 μK) samples of Cs atoms which have the goal to characterize, by high-resolution spectroscopy, the interactions between Cs atoms, Cs(+) ions and electrons that lead to the formation of metastable long-range molecules. The types of molecules observed in these experiments and the mechanisms leading to the aggregation of atoms in weakly bound molecules are very different from those encountered in warmer samples. In particular, we present results on molecules with binding energies of less than 0.05 J/mol and discuss their properties in the context of a new category of molecular states arising from slow-electron-atom scattering and their relation to atomic and molecular Rydberg states. One of the astonishing aspects of these types of molecules is that they can still be treated in good approximation in the realm of the Born-Oppenheimer approximation despite a huge electronic-state density. Non-Born-Oppenheimer effects are revealed by the decay of the molecules into neutral and charged fragments.
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).