Recent experiments involving highly excited atoms

Very large and fragile atoms may be produced by exciting normal atoms with light or by collisions with other atomic particles. Atoms as large as 10^?6 m are now routinely produced in the laboratory and their properties studied. In this review some of the simpler experimental methods available for the production and detection of such atoms are described including tunable dye laser-excitation and field ionization. A few recent experiments which illustrate the collision properties and the effects of electric and magnetic fields are also described. The relevance of highly excited atoms in other areas of research including radioastronomy and isotope separation are discussed.     

Resonant charge exchange in slow collisions involving halogens and oxygen

The coupling of electron momenta is considered for the resonant charge exchange process in slow collisions. Because the electron transfer in this process occurs at large distances between the colliding atomic particles, where ion-atom interactions are relatively weak, we can separate different types of interaction and find the character of coupling of the electron momenta in the quasi-molecule, consisting of the colliding ion and its atom, for real collision pairs. Since the real number of interaction types for colliding particles exceeds that used in the classical Hund coupling scheme, there are intermediate cases of momentum coupling outside the standard Hund scheme. This occurs for the resonant charge exchange involving halogens and oxygen where the quantum numbers of the quasi-molecule in the course of the electron transfer are the total momenta J and j of the colliding ion and atom and the projection M or M_J of the atom orbital or total momentum on the quasi-molecule axis. The ion-atom exchange interaction potential is independent of the ion fine state, and under these conditions, the resonant charge exchange process is not entangled with the rotation of electron momenta, as in case “a” of the Hund coupling. The partial cross section of the resonant charge exchange process depends on quantum numbers of the colliding particles. The average cross sections depend weakly on the coupling scheme.     

Electron terms and resonant charge exchange involving oxygen atoms and ions

The electron terms are constructed for oxygen dimer ions at large ion-atom distances taking into account a certain scheme of summation of electron momenta on the basis of a hierarchy of various ion-atom interactions. Because the number of interaction types exceeds that in the Hund scheme, a realistic hierarchy of interactions and corresponding quantum numbers of the diatomic ion are outside the Hund coupling scheme. Electron terms are evaluated for the oxygen dimer ion in the case where the ground and first excited states of an atom and an ion belong to the respective valence electron shells p⁴ and p³ and correspond to the range of separations that determine the cross sections of resonant charge exchange in plasma. These electron terms allow us to calculate the partial and average cross sections for resonant charge exchange involving an oxygen ion and atom in the ground and first excited states in the range of collision energies of interest for oxygen plasmas. The specific features of electron terms of the oxygen ion dimer and the cross section of electron transfer are analyzed.     

Resonant charge exchange with the p-electron transition

The asymptotic resonant charge exchange theory is developed for slow collisions of atoms and ions with valent p-electrons. Because of a small rotation angle of the molecular axis in the course of the p-electron transition, the resonant charge exchange cross section is not sensitive to the rotational energy of colliding particles, and the cross sections are nearly equal for cases “a”, “b”, and “d” of the Hund coupling, and also for cases “c” and “e” of the Hund coupling. The cross sections of the resonant charge exchange process are evaluated under various conditions and for various elements of the periodical table with p-electron shells of atoms and ions.     

Resonant energy exchange between atoms in dispersing and absorbing surroundings

Within the framework of quantization of a macroscopic electromagnetic field, a master equation describing both the resonant dipole-dipole interaction (RDDI) and the resonant atom-field interaction in the presence of dispersing and absorbing macroscopic bodies is derived, with the relevant couplings being expressed in terms of the surroundings-assisted Green’s tensor. It is shown that, under certain conditions, the RDDI can be regarded as being governed by an effective Hamiltonian. The theory, which applies to both weak and strong atom-field coupling, is used to study the resonant energy exchange between two (two-level) atoms initially sharing a single excitation. In particular, it is shown that, in the regime of weak atom-field coupling, there is a time window where the energy transfer follows a transfer-rate law of the type obtained by ordinary second-order perturbation theory. Finally, the spectrum of the light emitted during the energy transfer is studied and the line splittings are discussed.     

Scattering processes involving bound excitons in highly excited ZnO

ZnO shows, under high excitations at low temperatures, two different extrinsic emission bands on the low energy side of bound exciton emission lines. The temperature dependence of one of these bands shows good agreement with theoretical predictions for a bound exciton-free electron scattering process. A comparison is made with results of other authors, where similar emission structures have been interpreted in terms of biexciton recombination.     

Formation of highly excited hydrogen atoms in a beam-foil interaction

The production of highly excited hydrogen atoms by passing a proton beam though a thin carbon foil has been investigated within the energy range 5–30 keV. The population of atoms in states with principal quantum number, n, between 14 and 25 have been determined using the field ionization technique and found to scale as n^?4.35±0.2.     

Collisional deexcitation of exotic hydrogen atoms in highly excited states

The deexcitation of exotic hydrogen atoms in highly excited states in collisions with hydrogen molecules has been studied using the classical-trajectory Monte Carlo method. The Coulomb transitions with large change of principal quantum number n have been found to be the dominant collisional deexcitation mechanism at high n. The molecular structure of the hydrogen target is shown to be essential for the dominance of transitions with large Δn. The external Auger effect has been studied in the eikonal approximation. The resulting partial wave cross-sections are consistent with unitarity and provide a more reliable input for cascade calculations than the previously used Born approximation. Received 28 May 2002 Published online 15 October 2002 RID="a" ID="a"e-mail: thomas@physik.unizh.ch     

Collisional deexcitation of exotic hydrogen atoms in highly excited states

The atomic cascades in μ^- p and p atoms have been studied in detail using new results for the cross-sections of the scattering of highly excited exotic atoms from molecular hydrogen. The cascade calculations have been done with an updated version of the extended standard cascade model that computes the evolution in the kinetic energy from the beginning of the cascade. The resulting X-ray yields, kinetic energy distributions, and cascade times are compared with the experimental data.     

Emission of highly excited atoms and molecules in the upper atmosphere

The emission ability of Rydberg atoms and molecules in the orbitally degenerate states is considered. The mechanisms of their formation in the F, E, and D upper atmosphere layers are analyzed. The characteristic lifetimes of these states in the microwave range are estimated. It is shown that radiation in this range can be accompanied by a cascade of transitions. The possibility of studying the influence of intense atmospheric microwave radiation on living organisms is discussed.     

Cross sections for charge exchange of atoms on multiple-charged ions

Processes of charge exchange of Ar^{+ +}(3p ⁵) and Kr^{+ +}(4p ⁵) on He(1s ²) at low collision energies are investigated theoretically. Semiempirical asymptotic calculations of the radial coupling between initial and final states (both neglecting and allowing for the spin-orbit interaction) are presented. The result of semiempirical calculation of the coupling is in reasonable agreement with an earlier ab initio calculation for the system ArHe^{+ +}. The integral cross sections were estimated using the Landau-Zener model. Comparisons are made with available experimental data and numerical quantum calculation.     

Cooling by spontaneous decay of highly excited antihydrogen atoms in magnetic traps

An efficient cooling mechanism of magnetically trapped, highly excited antihydrogen (H) atoms is presented. This cooling, in addition to the expected evaporative cooling, results in trapping of a large number of H atoms in the ground state. It is found that the final fraction of trapped atoms is insensitive to the initial distribution of H magnetic quantum numbers. Expressions are derived for the cooling efficiency, demonstrating that magnetic quadrupole (cusp) traps provide stronger cooling than higher order magnetic multipoles. The final temperature of H confined in a cusp trap is shown to depend as approximately 2.2T(n0)n(0)(-2/3) on the initial Rydberg level n0 and temperature T(n0).