Table of contents

We report a new method to enhance the x-ray emission from nano-cluster plasmas formed upon irradiation by intense femtosecond-duration laser pulses. Our experiments demonstrate that when Ar clusters are doped with H₂O the time-integrated yield of Ar K x-ray emission is enhanced by approximately 12-fold in comparison to that obtained from pure Ar clusters under otherwise identical experimental conditions. A significant alteration in the time-dependent electron density is achieved by the presence of an H₂O dopant, and this could be the possible reason for the enhancement that is observed.

We consider the interaction of antiatoms with a solid surface and present calculations of the reflection probability using the exact retarded potential for the antihydrogen impinging on the ideally conducting surface. We present the reflection probability as a function of collision energy and show that the sufficiently cold atoms are reflected (rather than annihilated) from the surface, which opens prospects for storing and guiding antiatoms between material walls. The reflection probability in the ultracold regime is determined entirely by the van der Waals–Casimir tail of the atom–wall interaction and contains information on the retardation effects. We argue on the possibility of exploiting the ultracold antihydrogen–surface interactions as a probe of the electrodynamical and gravitational properties of an antiatom. We found that the lifetime of cold bouncing on the surface in the gravitational field of Earth is τ = ℏ/(2Mg|Im a|) ≃ 0.11 s. The measurement of that lifetime in conjunction with our calculation of the scattering length a allows determination of the gravitational force Mg on the antihydrogen atom.

We describe a method to create a circular magnetic waveguide for deBroglie waves on a microchip. The guide is a two-dimensional magnetic minimum for trapping weak-field seeking states of atoms or molecules with a magnetic dipole moment. It is created completely by electric currents in wires that are lithographically patterned on a multi-level chip. We describe the geometry and time-dependent currents of the wires and show that it is possible to wrap the waveguide in a complete circle with minimal perturbations due to the leads or wire crossings. This maximal area geometry is suited for rotation sensing with atom interferometry via the Sagnac effect using either cold thermal atoms and molecules or Bose-condensed systems.

We have made a first calculation for S-wave resonances in positron–hydrogen scattering under the influence of Debye plasma environments. A screened Coulomb potential obtained from the Debye model is used to represent the interaction between the nucleus, the electron and the positron. Hylleraas-type wavefunctions are used to represent the correlation effect between the charge particles. The stabilization method is used to calculate the density of resonance states, from which resonance energies and widths are deduced. We have found two resonances of which one is associated with the hydrogen n = 2 threshold, and the other one is found lying below the Ps(2S) threshold. The resonance parameters (energies and widths) calculated for various Debye lengths are reported.

It is recalled that when using an adiabatic basis χ_i set to describe inelastic and rearrangement processes in ion–atom collisions, it is important to specify the coordinate system in which the adiabatic separation of the variables is performed. In this work, a numerical test, requiring a direct calculation of nonadiabatic matrix elements of the form , is proposed to asses the completeness of any specific finite adiabatic basis. Results are presented using a minimal basis set needed to describe the three main electron-capture channels in collisions of C⁴⁺ ions with neutral H atoms. Completeness is not obtained with any of the Jacobi coordinate systems. On the other hand, completeness of the minimal basis set is satisfactory when appropriate reaction coordinates are used.

Simple correlated wavefunctions considering two K-shell active electrons of neutral atoms from He to Xe are presented in this paper, describing a variational method subject to a local Hartree potential representing the presence of outer shell electrons. Three kinds of electron–electron correlation functions have been studied with rigorous observation of the exact behaviour of the wavefunctions at the electron–electron and electron–nucleus coalescence points (Kato cusp conditions). Global properties, such as the energies and virial coefficients, as well as local properties, such as spatial mean values, together with scaling laws with the nuclear charge for the variational parameters, are also analysed. We calculated the expansion of the functions in terms of bipolar spherical harmonics. Finally, comparisons are made with a more rigorous, fully quantal close-coupling method, which also includes the same Hartree potential for the outer electrons.

Emission cross sections (Q_em) of CF⁺₄(, ; 200–500 nm), CF⁺₄(, 350–440 nm), F(3p ⁴D^o_7/2–3s ⁴P_5/2; 685.6 nm), He(3d ³D–2p ³P^o; 587.6 nm), and Ne(3p ²[5/2]–3s ²[3/2]; 640.2 nm) produced in charge-exchange collisions between He⁺ and Ne⁺ and CF₄ molecule were measured in the energy range below 5000 eV down to 1 eV (E_Lab). Emission cross sections of F(3s ²P–2p⁵²P^o; 95.5 nm) and F(3s ⁴P–2p⁵²P^o; 97.5 nm) were also measured in He⁺+CF₄. No emission peaks were observed in the spectrum of Ar⁺+CF₄ (E_Lab = 2000 eV) in the wavelength range of 200–800 nm. The emission cross section of CF⁺₄(, ; 200–500 nm) observed in Ne⁺+CF₄ decreased with increasing the energy with a relation of Q_em ∝ E^−1/2_CM up to 60 eV, whereas it increased with increasing the energy above 100 eV. This result agreed well with one of Sasaki et al [1]. It was concluded that such dependence below 60 eV originated from an orbiting resonance into Ne+CF⁺₄ channel. Emission cross sections of F, He and Ne rapidly increased with increasing the energy from the near thresholds.

Electron-impact excitation of the 5s → 5p resonance transition in rubidium is investigated using a semi-relativistic Breit–Pauli R-matrix with pseudo-states (close-coupling) approach. For the example of 20 eV incident energy, we illustrate the importance of channel coupling and relativistic effects in the computational model. Similar to the case of e–Cs collisions discussed by Andersen and Bartschat (2002 J. Phys. B: At. Mol. Opt. Phys.35 4507), relativistic effects exhibit themselves only on selected parameters, such as the spin-asymmetry function for the scattering of a spin-polarized electron beam from an unpolarized target beam, while they are almost invisible in the (pseudo-) Stokes parameters that can be measured either in an electron–photon coincidence setup or via 'time-reversed' superelastic collisions. The overall agreement with the available experimental data is satisfactory.

Experimental and theoretical results are presented for the A₂ spin asymmetry for elastic and inelastic scattering of spin-polarized electrons from rubidium, at incident electron energies of 15, 20, 30, 50 and 80 eV. Theoretical calculations are performed within a semi-relativistic Breit–Pauli R-matrix (close-coupling) model. The experimental data reveal significant asymmetries at all energies. The experimental results are reasonably well described by the present theoretical calculations and by other calculations using a relativistic distorted-wave approach, and provide a clear relativistic signature.

In this paper, a study is made of the interaction of atomic helium with an antiproton within the Born–Oppenheimer approximation. The electronic energy curve is calculated using the variational method with basis sets containing Hylleraas-type basis functions. The energies and nuclear wave functions for 50 s states are then obtained using this potential by solving the nuclear wave equation numerically using the Cooley–Numerov algorithm. It is possible that the state with the lowest energy is a bound state. The remaining states are all above the lowest continuum threshold for and are thus quasi-bound states.

The leading relativistic and QED corrections to the ground-state energy of the three-body system e⁻e⁺e⁻ are calculated numerically using a Hylleraas correlated basis set. The accuracy of the nonrelativistic variational ground state is discussed with respect to the convergence of the energy with increasing size of the basis set, and also with respect to the variance of the Hamiltonian. The corrections to this energy include the lowest order Breit interaction, the vacuum polarization potential, one and two photon exchange contributions, the annihilation interaction and spin–spin contact terms. The relativistic effects and the residual interactions considered here decrease the one-electron binding energy from the nonrelativistic value of 0.012 005 070 232 980 107 69(28) au to 0.011 981 051 246(2) au (78 831 530 ± 5 MHz).

Highly correlated ab initio methods were used in order to generate the potential energy functions (PEFs) of the bound electronic states of the SN⁻ anion and the long-range parts of the PEFs of its excited states and their mutual spin–orbit couplings. The SN (X²Π and a⁴Π) potential energy curves are also computed. In addition to the two bound electronic states of SN⁻ (i.e. X³Σ⁻ and ¹Δ) already known, our calculations show that the ³Δ state is lying energetically below its quartet parent neutral state (a⁴Π). The depletion of the J = 3 component of SN⁻(³Δ) will mainly occur via weak interactions with the electron continuum wave. At large internuclear distances, SN⁻(⁵Π) state is predicted to possess a shallow polarization minimum supporting long-lived SN⁻ ions. Finally, the reaction between S⁻(²P_u) and N(⁴S_u) involves the electronic states of SN⁻ and their mutual couplings, in competition with the autodetachment processes.

We study the applicability of various screening approaches within the context of laser–cluster interactions. Comparisons between well-known screening models, such as the static screened Coulomb, Debye–Hückel, Thomas–Fermi and ion–sphere ones, and a more elaborated approach based on the Lindhard dielectric function within the linear response theory (Gupta and Rajagopal 1982 Phys. Rep.87 261) show that none of the simpler classical models can be used through the complete range of plasma situations encountered in laser–cluster interactions. Therefore, we have clearly specified the validity domains of the various models as well as the regions of electronic temperature and density where Friedel oscillations appear.

We extend a recently introduced mapping model, which explains the bunching phenomenon in an ion beam resonator for two ions (Geyer and Tannor 2004 J. Phys. B: At. Mol. Opt. Phys.37 73), to describe the dynamics of the whole ion bunch. We calculate the time delay of the ions from a model of the bunch geometry and find that the bunch takes on a spherical form at the turning points in the electrostatic mirrors. From this condition we derive how the observed bunch length depends on the experimental parameters. We give an interpretation of the criteria for the existence of the bunch, which were derived from the experimental observations by Pedersen et al (2002 Phys. Rev. A 65 042704).

In collisions of antiprotons with hydrogen atoms or molecules, antiprotonic hydrogen (protonium) is produced mostly in very high principal (n) and angular momentum (ℓ) quantum number states, which are practically stable against particle–antiparticle annihilation. However, since annihilation is very fast for the ns state, ℓ mixing induced by surrounding electric fields or by collisions has a serious influence on the stability of the produced atoms, as was first pointed out by Day et al (1959 Phys. Rev. Lett.3 61). This paper calculates the cross sections for annihilation which stem from ℓ-changing transition by ion collisions. Because the collision time can be comparable to the lifetime against ns annihilation in the present case, collisional transition and annihilation must be treated in a coherent manner. We carry out an impact-parameter close-coupling calculation, simultaneously including the annihilation channel by means of an optical potential model. At low collision energies, the ℓ mixing due to the polarization effect makes the annihilation cross section enormously large.