Bound states and scattering resonances induced by spatially modulated interactions

We study the two-body problem with a spatially modulated interaction potential using a two-channel model, in which the interchannel coupling is provided by an optical standing wave and its strength modulates periodically in space. As the modulation amplitudes increase, there will appear a sequence of bound states. Part of them will cause a divergence of the effective scattering length, defined through the phase shift in the asymptotic behavior of scattering states. We also discuss how the local scattering length, defined through short-range behavior of scattering states, modulates spatially in different regimes. These results provide a theoretical guideline for a new control technique in the cold atom toolbox, in particular, for alkaline-earth(-like) atoms where the inelastic loss is small.     

Effective potential theory of atom-solid surface elastic scattering

A new quantum mechanical theory of atom-solid surface elastic scattering is formulated, based on the idea of effective potentials. Two such quantities are defined in the present approach, one, the “optical potential”, regulating specular scattering, and the second, the “transition potential”, responsible for diffractive scattering. Rigorous equations are obtained for the above mentioned quantities, by using field-theoretic techniques, and an approximation scheme is worked out explicitly. The main computational aspect of the present theory requires the solution of an uncoupled integro-differential equation and the formulation can be easily extended to consider more complicated scattering processes.     

Bound state resonances and related interaction potential for atomic hydrogen on NaCl(001)

Bound state resonances in the coherent elastic scattering of a monoenergetic H atom beam (48.5 meV) on a NaCl(001) surface are measured and analysed, consistently yielding three bound state energies. These energy levels fit into a Morse potential (well depth D = 20.8 meV) and range parameter k = 9.3 nm^?1) which compares well to those of similar interaction systems (as H/NaF and H/KCl) but differs from results reported recently in literature.     

Raman scattering resonances in separated fields

The effects in separated standing-wave fields that are resonant to adjacent Doppler-broadened transitions of many-level atoms are studied. It is shown that due to coherent transfer, at large distances, of polarization on a forbidden transition between the initial and final metastable levels of atoms in a gas, light emission arises at combination frequencies. It is also shown that a resonance with the width reciprocal to the time of flight between separated fields is available in the Raman scattering and absorption line shapes.     

Theoretical studies of atom-solid elastic scattering by iterative integration using the Green function

The elastic scattering of low-energy light atoms from a perfect crystalline surface is studied by an iterative integration scheme using the Green function. The atom-solid interaction is represented by the often used Morse type surface potential. A varying number of closed and open channels is included in the calculation, according to necessicity. For a beam incident along the cyrstal symmetry directions, a scheme to utilize the symmetry condition for efficient computation is proposed. The diffraction intensities at a bound state resonance (selective adsorption) are calculated by properly selecting a reference potential for the calculation of the Green function. The calculations yield the resonant diffraction intensity patterns in agreement with previous calculations using a different numerical technique and with the experimental observations for the HeLiF and HeNaF systems. A calculation including 69 allowed diffracted beams (open channels) for the HeLiF system at normal incidence is also presented and comparison with experimental results is made to estimate the periodic potential parameter.     

Cascades of Fano resonances in Mie scattering

The interference nature of resonant Mie scattering, which is described within the Fano model, has been demonstrated. The interference is caused by interaction of an incident electromagnetic wave with reemitted waves that correspond to eigenmodes of a scattering particle. Mie scattering due to the interference can be represented in the form of cascades of resonance lines of different shapes, each of which is described by the classical Fano formula. The effect is observed in resonant light scattering by an arbitrary body of revolution and discussed in detail using the example of scattering by an infinite homogeneous dielectric cylinder.     

Feshbach resonances in positron scattering from sodium

S-, P- and D-wave Feshbach resonances in positron-sodium scattering have been investigated by using the momentum space coupled-channels optical method. The target continuum and positronium (Ps) formation channels are included via an optical potential. Feshbach resonances below the target excitation and Ps (n = 2) formation thresholds are predicted and the effects of channel-coupling scheme, target continuum and Ps formation channels on the resonance energy and resonance width are discussed. We have also found the Wigner cusp structures at the inelastic channel-opening thresholds in positron-sodium scattering cross sections.     

Focussed inelastic resonances in particle scattering from surfaces

Particles, such as atoms or electrons, inelastically scattered from a crystal surface in resonance with a bound state are predicted to focus around a discrete set of final angles with a defined energy for each angle. The final angle and energy of such focussed inelastic resonances (FIR) are shown to be independent of the initial state. A calculation of the FIR amplitude indicates favorable conditions for the observation of the so far elusive bound states of He on low-index metal surfaces and of image states on stepped metal surfaces.     

Isobaric analogue resonances in proton scattering on Ba

Extensive experimental data for the resonance scatttering of protons to the ground state and first excited state in ¹³⁸Ba are presented. A Breit Wigner analysis with a background term of the form Σ_n(a_n/Eⁿ) is performed and the extracted background function is compared with optical-model calculations using the parameters given in a detailed investigation by Perey. Neutron spectroscopic factors calculated in different ways from this data are compared with the corresponding factors from (d, p). An accurate determination of resonance energies for the first two resonances reveals a deviation from a constant Coulomb shift of the order of 10 keV.     

A link between scattering resonances and dilation analytic resonances in few body quantum mechanics

We study the (2 cluster)(2 cluster) scattering amplitudes for classes of two, three, and four particle dilation analytic Schrödinger operators whose two-body potentials fall off exponentially. As functions of the energy, these amplitudes are shown to have meromorphic continuations on certain Riemann surfaces. We prove that all poles of these continuations are necessarily bound states or dilation analytic resonances [i.e., eigenvalues ofH() for some ].Supported in part by the National Science Foundation under Grant PHY 78-08066     

Doppler-free resonances of the second-order raman scattering

The possibility of the observation of Raman scattering resonances completely free from the influence of the Doppler effect has been examined for the first time. The phenomenon is based on the excitation of a Raman oscillation standing wave in a gas by two standing light waves, whose frequency difference is equal to half the Raman frequency. The complete compensation of Doppler shifts results from the simultaneous interactions between atomic particles and two pairs of counter-propagating waves. Doppler-free resonances of the second-order Raman light scattering appear in the number of particles excited to the upper Raman level and in the radiation at the Stokes and anti-Stokes frequencies. The amplitude estimate for the resonance in the number of particles is given for the example of neon.     

Have positron-helium scattering resonances been observed?

We comment on the recent claim by Karwasz [Eur. Phys. J. D 35, 267 (2005)] that he observed resonant structures in the total cross-section for positron scattering from helium. We frame our observations in terms of both the general capabilities of the Trento spectrometer, and new checks we have made on the measurement procedure employed by Karwasz. We conclude that the observed structures are most likely an experimental artefact, rather than being due to the positron-helium interaction.