Quasi-molecular resonances in muonic charge-exchange reactions

We present the first thorough theoretical discussion of non-monotonic structure in the temporal behavior of X-ray fluorescence intensity spectra of muon charge-exchange reactions of the type pμ + Z→p + μZ. Our discussion is based on semiclassical methods; our results are almost entirely analytical. We find that the reported experimental maxima of muon charge-exchange rates are very close to the theoretical limits. We identify a new quantum mechanism, quantum impedance matching, which explains observed inelastic transitions close to the Unitarity Limit. We investigate the specific example of the reaction pμO ^{8 +} in detail using two-center Coulomb adiabatic potential terms. We find that quantum impedance matched higher-order partial-wave resonances yielding muon-transfer rates close to the Unitarity Limit are responsible for the remarkable non-monotonic structure in the X-ray fluorescence spectra. Received 7 January 2002 / Received in final form 15 April 2002 Published online 24 September 2002     

Collisional excitation of Na-Rydberg atoms

The scattering of heavy ion with a multilevel Rydberg atom in the presence of an electromagnetic field is studied. The interaction of Rydberg atom and the e.m field is explored using non-perturbative quasi-energy technique. Although the results are presented for selected excitations but in actual calculations we have included many levels of the atom. The effect of various parameters are shown on collisional excitation process. As an illustration detailed calculations are performed for the inelastic proton-Na Rydberg atom collision accompanied by the transfer of photons and the effects of dressing due to the field are considered. The emphasis of the present work is on collision induced transitions especially the case that involves change of orbital as well as principal quantum number. Received 26 December 2001 / Received in final form 8 April 2002 Published online 19 July 2002     

Scattering of light exotic atoms in excited states

The differential and total cross-sections for the scattering of muonic, pionic, kaonic and antiprotonic hydrogen in excited states from atomic hydrogen have been calculated for the purpose of atomic cascade calculations. The scattering problem is treated in a fully quantum mechanical framework which takes the energy shifts and, in the case of the hadronic atoms, the widths of the ns states into account. The validity of semiclassical approximations is critically examined. Received 4 December 2001 and Received in final form 4 February 2002     

Rydberg transitions for positron-hydrogen collisions

e⁺ + H(ns) ↦e⁺ + H(n^′s) transitions for arbitrary n and n^′ have been studied using the distorted-wave formalism in the momentum space [Ghoshal and Mandal, Phys. Rev. A 72, 032714 (2005)]. The distorted-wave scattering amplitudes have been written in a simple closed analytical form. A detailed study has been made on differential and total cross sections in the energy range 20–300 eV. Resonance-like behaviour of the differential cross section has been observed in the the region of lower scattering angles for high Rydberg transitions. To the best of our knowledge the distorted-wave results for differential and total cross sections for such arbitrary transitions are reported for the first time in the literature.     

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     

Dynamics of muonic atom cascade in hydrogen-helium mixtures

The deexcitation of excited muonic protium and deuterium in the mixture of hydrogen and helium isotopes is considered. Methods of experimental determination of the probability of direct atomic muon capture by hydrogen and muon transfer rates from excited muonic hydrogen to helium are proposed. Theoretical results for the population of the muonic atoms in the ground state, , are compared with the existing experimental data. Results obtained for mixtures are of interest for investigation of nuclear fusion in muonic molecules. Received: 6 August 1998 / Revised: 1st October 1998 / Accepted: 2 October 1998     

Kinetics of muon catalyzed fusion processes in solid H/D mixture

This paper presents the final results of an experimental study of the kinetics of the muon catalysed fusion (MCF) in solid hydrogen-deuterium mixtures (H/D) at a temperature of 3 K. The experiment E742 was carried out on a TRIUMF meson facility in Canada. Four exposures were performed at different deuterium concentrations in the H/D mixture: c_d = 0.0005, 0.02, 0.15 and 0.75. A simultaneous analysis of the measured time distributions of the 5.5 MeV γ-quanta and the 5.3 MeV conversion muons obtained from nuclear fusion in the pdμ molecule allowed to extract the values of MCF chain parameters in the H/D mixture: the fusion partial rates for different nuclear spin states of the pdμ molecule, and the pdμ molecule formation rate. The experimental data fitting procedure was conducted in two ways: using solely the analytical formulae describing the kinetics of the MCF processes in the H/D mixture, and the detailed Monte Carlo simulation of the entire experiment. The results obtained by these two methods are consistent with each other and confirm the existence of the Wolfenstein-Gerstein effect.     

Plasma screening within Rydberg atoms in circular states

A Rydberg atom embedded in a plasma can experience penetration by slowly moving electrons within its volume. The original pure Coulomb potential must now be replaced by a screened Coulomb potential which contains either a screening length R_s or a screening factor A = R_s ^-1 . For any given discrete energy level, there is a Critical Screening Factor (CSF) A_c beyond which the energy level disappears (by merging into the continuum). Analytical results are obtained for the classical dependence of the energy on the screening factor, for the CSF, and for the critical radius of the electron orbit for Circular Rydberg States (CRS) in this screened Rydberg atom. The results are derived for any general form of the screened Coulomb potential and are applied to the particular case of the Debye potential. We also show that CRS can temporarily exist above the ionization threshold and are therefore the classical counterparts of quantal discrete states embedded into continuum. The results are significant not only to Rydberg plasmas, but also to fusion plasmas, where Rydberg states of multi-charged hydrogen-like ions result from charge exchange with hydrogen or deuterium atoms, as well as to dusty/complex plasmas.     

X-ray transitions from antiprotonic noble gases

The onset of antiprotonic X-ray transitions at high principal quantum numbers and the occurrence of electronic X-rays in antiprotonic argon, krypton, and xenon has been analyzed with the help of Multiconfiguration Dirac-Fock calculations. The shell-by-shell ionisation by Auger electron emission, characterised by appearance and disappearance of X-ray lines, is followed through the antiprotonic cascade by considering transition and binding energies of both the antiproton and the remaining electrons. Electronic lines could be attributed partly to specific states of the antiprotonic atom de-excitation.     

Simultaneous position and state measurement of Rydberg atoms

We present a technique for state-selective position detection of cold Rydberg atoms. Ground state Rb atoms in a magneto-optical trap are excited to a Rydberg state and are subsequently ionized with a tailored electric field pulse. This pulse selectively ionizes only atoms in e.g. the 54d state and not in the 53d state. The released electrons are detected after a slow flight towards a micro channel plate. From the time of flight of the electrons the position of the atoms is deduced. The state selectivity is about 20:1 when comparing 54d with 53d and the one-dimensional position resolution ranges from 6 to 40 μm over a range of 300 μm. This state selectivity and position resolution are sufficient to allow for the observation of coherent quantum excitation transport.     

Excited states of Ne isoelectronic ions: SAC-CI study

Excited states of the s, p, and d symmetries up to principal quantum number n = 4 are studied for the first eight members of Ne isoelectronic sequence (Ne to Cl⁷⁺) by the SAC-CI (symmetry-adapted-cluster configuration-interaction) method. The valence STO basis sets of Clementi et al. and the optimized excited STO are used by the STO-6G expansion method. The calculated transition energies agree well with the experimental values wherever available. Received 26 July 2000 and Received in final form 14 September 2000     

The positronium negative ion: Classical properties aaand semiclassical quantization

Properties of collinear and planar periodic orbits for the positronium negative ion are examined with respect to the possibilities for semiclassical quantization. In contrast to other two-electron atomic systems as helium and H^- the relevant orbits for quantization are fully stable and permit a full torus quantization. However, for lower excitations the area of stability in phase-space is too small for a reliable torus quantization. Instead, a quasi-separability of the three-body system is used to apply effective one-dimensional (WKB) quantization. Received 19 January 2001     

Ionization and plasma formation in high n cold Rydberg samples

The experiments reported here show that the dipole-dipole interaction, the fundamental interaction between the cold Rydberg atoms, is the dominant initial ionization mechanism for evolution from a frozen Rydberg gas into a plasma. The study also indicates that plasma formation follows a path of initial ionization, redistribution of Rydberg population to higher angular momentum states, and rapid avalanche ionization due to electron-Rydberg collisions.     

The two-photon transition of the fully-ionized muonic boron migrating in helium

In recent experiments performed at the Paul Scherrer Institute (PSI) measurements were made of the yield of the two-photon transition of the fully-ionized muonic boron formed and quenched in a gaseous mixture of diborane B₂H₆ and helium. In the present work this yield is calculated for an idealized case of a very low diborane density when the main 2 s state decay modes competing with the two-photon transition are due to the electron transfer from helium to the μ-ion. As the rate of this transfer depends strongly on the relative velocity, a treatment is needed of the whole kinetics of processes occurring with the μ-ion in helium. Accordingly, within the optical model with a complex potential constructed before we calculate cross-sections of the elastic scattering and electron transfer. Then the time evolution of the μ-ion energy is considered and, finally, the yield is calculated. It proves that at helium pressures Torr this yield may be written in the form: ,where the factor C is greater than unity and increases rapidly with the initial energy E₀ which the μ-ion has after its formation in the diborane molecule. Thus measurement of the pressure dependence of allows E₀ to be estimated. The results obtained make it possible to suggest a similar parametrization of the PSI data. Received: 21 October 1997 / Revised: 16 January 1998 / Accepted: 2 March 1998     

Quenching of metastable antiprotonic helium atoms in collisions with deuterium molecules

Quenching of metastable antiprotonic helium atoms in collisions with deuterium molecules has been studied using laser spectroscopy at CERN's new Antiproton Decelerator facility. The quenching cross-sections of the states (n, l )= (39, 36), (39, 37), and (39, 38) were determined from the decay rates of the states which were observed using the “deuterium-assisted inverse resonance” (DAIR) method. The results revealed a similar (n, l )-dependence of the quenching cross-sections as in the case of hydrogen but the values were smaller by a factor of ∼1.5. Received 6 July 2001     

From solar proton burning to pionic deuterium through the Nambu-Jona-Lasinio model of light nuclei

Within the Nuclear Nambu-Jona-Lasinio model of light nuclei (the NNJL model), describing strong low-energy nuclear interactions, we compute the width of the energy level of the ground state of pionic deuterium. The theoretical value fits well the experimental data. Using the cross-sections for the reactions _e + d p + p + e^- and _e + d p + n + _e, computed in the NNJL model, and the experimental values of the events of these reactions, detected by the SNO Collaboration, we compute the boron neutrino fluxes. The theoretical values agree well with the experimental data and the theoretical predictions within the Standard Solar Model by Bahcall. We argue the applicability of the constraints on the astrophysical factor for the solar proton burning, imposed by helioseismology, to the width of the energy level of the ground state of pionic deuterium. We show that the experimental data on the width satisfy these constraints. This testifies an indirect measurement of the recommended value of the astrophysical factor for the solar proton burning in terrestrial laboratories in terms of the width of the energy level of the ground state of pionic deuterium.     

Extracting generalized multiphoton ionization cross-sections from nonperturbative time-dependent calculations: An application in positronium

Ab initio time-dependent (TD) calculations of the behavior of positronium (Ps) under strong subpicosecond laser pulses are presented. The results are compared with results in H through scaling. It is found that a substantial amount of the population can be found in excited states after the pulse. In the perturbative regime, generalized multiphoton ionization cross-sections are extracted from the results of the time-dependent calculations. The generalized cross-sections are used to predict the response of Ps to nanosecond laser pulses at wavelengths of current experimental interest. Beyond the application to Ps, the generality of the method for extracting generalized cross-sections from TD nonperturbative calculations is discussed. Received 8 June 1999     

On kaonic hydrogen. Phenomenological quantum field theoretic model revisited

We argue that due to isospin and U-spin invariance of strong low-energy interactions the S-wave scattering lengths a ⁰ ₀ and a ¹ ₀ of ˉN scattering with isospin I = 0 and I = 1 satisfy the low-energy theorem a ⁰ ₀ +3a ¹ ₀ = 0 valid to leading order in chiral expansion. In the model of strong low-energy ˉN interactions at threshold (Eur. Phys. J. A 21, 11 (2004)) we revisit the contribution of the Σ(1750) resonance, which does not saturate the low-energy theorem a ⁰ ₀ +3a ¹ ₀ = 0, and replace it by the baryon background with properties of an SU(3) octet. We calculate the S-wave scattering amplitudes of K^-N and K^-d scattering at threshold. We calculate the energy level displacements of the ground states of kaonic hydrogen and deuterium. The result obtained for kaonic hydrogen agrees well with recent experimental data by the DEAR Collaboration. We analyse the cross-sections for elastic and inelastic K^-p scattering for laboratory momenta 70MeV/c < p _K < 150MeV/c of the incident K^--meson. The theoretical results agree with the available experimental data within two standard deviations.