We discuss the accuracy of various published ab initio calculations of highly excited Born-Oppenheimer bound and doubly excited states in diatomic molecules. We show that some of these results appear questionable and in a few cases even clearly unreliable, a fact which may also affect the prediction of collision cross sections based on such data. Our analysis uses Quantum Defect Theory scaling laws and is supported by variational R-matrix calculations. Some simple recipes are discussed, which may help users of ab initio quantum chemistry packages to assess the quality of their results.

Coherence among rotational ion channels during photoionization is exploited to control the anisotropy of the resulting photoelectron angular distributions at specific photoelectron energies. The strategy refers to a robust and single parameter control using two ultrashort light pulses delayed in time. The first pulse prepares a superposition of a few ion rotational states, whereas the second pulse serves as a probe that gives access to a control of the molecular asymmetry parameter beta for individual rotational channels. This is achieved by tuning the time delay between the pulses, leading to channel interferences that can be turned from constructive to destructive. The illustrative example is the ionization of the E ((1)Sigma(+)(g)) state of Li-2. Quantum wave-packet evolutions are conducted including both electronic and nuclear degrees of freedom to reach angle-resolved photoelectron spectra. A simple interference model based on coherent phase accumulation during the field-free dynamics between the two pulses is precisely exploited to control the photoelectron angular distributions from almost isotropic to marked anisotropic.

Using a quantum wave packet simulation including the nuclear and electronic degrees of freedom, we investigate the femtosecond and picosecond energy- and angle-resolved photoelectron spectra of the E(Σg+1) electronic state of Li2. We find that the angular distributions of the emitted photoelectrons depend strongly on the pulse duration in the regime of ultrashort laser pulses. This effect is illustrated by the extraction of a time-dependent asymmetry parameter whose variation with pulse duration can be explained by an incoherent average over different ion rotational quantum numbers. We then derive for the variation of the asymmetry parameter a simple analytical formula, which can be used to extract the asymptotic CW asymmetry parameters of individual transitions from measurements performed with ultra-short pulses.

Variational ab initio R-matrix theory combined with generalized multichannel quantum defect theory is used to calculate singly excited Rydberg states of the hydrohelium molecular ion, HeH+, for (1.3)Sigma(+), (1.3)Pi, (1.3)Delta, (1.3)Phi, and (1.3)Gamma symmetry. Bound levels are calculated for n values up to n approximate to 10, and continuum states up to approximate to 3 eV above the HeH2+ threshold. The calculations span the range of internuclear distances R from 1 to 5 bohrs. The present work follows a preliminary study on the (1.3)Delta states of HeH+ [Bouhali, Bezzaouia, Telmini, and Jungen, EPJ Web Conf. 84, 04004 (2015)] which was also based on R-matrix theory. Further-although limited to rather small R values-the present work extends the recent ab initio computations of Jungen and Jungen [Mol. Phys. 113, 2333 (2015)] to higher excitation energies which are not accessible to standard quantum-chemical methods. Where a comparison with the calculations of Jungen and Jungen and other older results can be made, namely for n <= 5, very good agreement with previous ab initio results is obtained.

The halfium model combines the eigenchannel R-matrix method and the generalized multichannel quantum defect theory in prolate spheroidal coordinates to characterize Rydberg and doubly-excited states of two-electron molecular systems from first pricnciples. The background picture for the model is a collisional process between the outermost (Rydberg) electron on the ionic core. Various applications to molecular hydrogen and hydrohelium ion will be shown.

In Jaouadi et al. [Phys. Rev. A 83, 023616 (2011)] the authors derive an analytical finite-size expansion for the Bose-Einstein condensation critical temperature of an ideal Bose gas in a generic power-law trap. In the case of a harmonic trap, this expansion adds higher order terms to the well- known first order correction. We point out a delicate point in connection to these results, showing that the claims of Jaouadi et al. should be treated with caution. In particular, for a harmonic trap, the given expansion yields results that, depending on what is considered to be the critical temperature of the finite system, do not generally improve on the established first order correction. For some non-harmonic traps, the results differ at first order from other results in the literature.

Long-baseline precision tests based on atom interferometry require drastic control over the initial external degrees of freedom of atomic ensembles to reduce systematic effects. The use of optical lattices (OLs) is a highly accurate method to manipulate atomic states in position and momentum allowing excellent control of the launch in atomic fountains. The simultaneous lattice launch of two atomic species, as required in a quantum test of the equivalence principle, is however problematic due to crosstalk effects. In this article, we propose to selectively address two species of alkalines by applying two OLs at or close to magic-zero wavelengths of the atoms. The proposed scheme applies in general for a pair of species with a vastly different ac Stark shift to a laser wavelength. We illustrate the principle by studying a fountain launch of condensed ensembles of 87Rb and 41K initially co-located. Numerical simulations confirm the fidelity of our scheme up to few nm and nm s−1 in inter-species differential position and velocity, respectively. This result is a pre-requisite for the next performance level in precision tests.

We report R-matrix calculations of low-lying Rydberg states of the hydrohelium molecular ion HeH+ corresponding to 1Δ and 3Δ symmetry. The calculations include states with principal quantum numbers 3 ≤ n ≤ 10. For n = 3 and n = 4 the present results are compared with those of Green et al. [1,2] and Loreau et al. [3].

Recent advances in the stepwise multichannel quantum defect theory approach of electron/molecular cation reactive collisions have been applied to perform computations of cross sections and rate coefficients for dissociative recombination and electron-impact ro-vibrational transitions of H2+, BeH+ and their deuterated isotopomers. At very low energy, rovibronic interactions play a significant role in the dynamics, whereas at high energy, the dissociative excitation strongly competes with all other reactive processes.

We report R-matrix calculations of doubly-excited 3Σg− states of molecular hydrogen corresponding to 3d̃πnℓ̃π configurations. These states form Rydberg series converging to the 3d̃π series limit. They lie in the continuum of the doubly-excited states of 3Σg− symmetry built on the 2p̃π ion core, and therefore they are autoionized. Calculations of resonance positions and widths are presented.

A variational R-matrix approach combined with multichannel quantum defect theory is used for a computational study of triplet gerade states of H-2. Electron-ion reaction (quantum defect) matrices are calculated as functions of internuclear distance and energy for the bound and continuum ranges including singly and doubly excited configurations built on the 1 sigma(g) (X+2 Sigma(+)(g)) and 1 sigma(u) (A(+2)Sigma(+)(u)) core states of the H-2(+) ion. It is shown how these matrices can be reduced to effective quantum defect functions adapted to the analysis of high-resolution spectra in the bound range. These R-matrix effective quantum defects are finally adjusted to the available experimental data [Sprecher et al., J. Phys. Chem. A 117, 9462 (2013)], producing agreement with experiment to within 0.5 cm(-1), nearly as good as obtained by Sprecher et al. In addition, the R-matrix calculations predict the evolution of the quantum defects for higher energies, in a range extending far into the electronic continuum.

A series of computations based on multichannel quantum defect theory have been performed in order to produce the cross sections of rotational transitions (excitations N-i(+) - 2 -> N-i(+), deexcitations N-i(+) -> N-i(+) - 2, with N-i(+) = 2 to 10) and of their competitive process, the dissociative recombination, induced by collisions of HD+ ions with electrons in the energy range 10(-5) to 0.3 eV. Maxwell anisotropic rate coefficients, obtained from these cross sections in the conditions of the Heidelberg Test Storage Ring (TSR) experiments (k(B)T(t) = 2.8 meV and k(B)T(l) = 45 mu eV), have been reported for those processes in the same electronic energy range. Maxwell isotropic rate coefficients have been presented as well for electronic temperatures up to a few hundred Kelvins. Very good overall agreement is found between our results for rotational transitions and the former theoretical computations as well as with experiment. Furthermore, due to the full rotational computations performed, the accuracy of the resulting dissociative recombination cross sections is improved considerably.

The sigma(v) reflection symmetry for diatomic molecules is discussed in the context of variational R-matrix theory applied to the H-2 molecule ("halfium model"). It is shown that explicit use of the reflection symmetry in the R-matrix calculations improves the results of singly and doubly excited (1)Sigma(+)(g) states and resonances. R-matrix calculations of non-autoionizing (1)Sigma(-)(9) doubly-excited states of H-2 situated 27 eV above the ground state are also presented and compared with existing ab initio data obtained by other methods.

We report calculations of H-2 Sigma(-) states using a variational R-matrix approach combined with multichannel quantum defect theory. Several Rydberg series converging to the 2p pi state of the H-2(+) ion core are established and their mutual channel interactions characterized. The influence of the external electron on the chemical bond is found to be particularly strong in these electronically and chemically weakly bound states.

We investigate theoretically an original route to achieve Bose-Einstein condensation using dark power-law laser traps. We propose to create such traps with two crossing blue-detuned Laguerre-Gaussian optical beams. Controlling their azimuthal order $\ensuremath{\ell}$ allows for the exploration of a multitude of power-law trapping situations in one, two, and three dimensions, ranging from the usual harmonic trap to an almost square-well potential, in which a quasihomogeneous Bose gas can be formed. The usual cigar-shaped and disk-shaped Bose-Einstein condensates obtained in a 1D or 2D harmonic trap take the generic form of a ``finger'' or of a ``hockey puck'' in such Laguerre-Gaussian traps. In addition, for a fixed atom number, higher transition temperatures are obtained in such configurations when compared with a harmonic trap of the same volume. This effect, which results in a substantial acceleration of the condensation dynamics, requires a better but still reasonable focusing of the Laguerre-Gaussian beams.

We present a detailed, realistic proposal and analysis of the implementation of a cold atom deflector using time-dependent far off-resonance optical guides. An analytical model and numerical simulations are used to illustrate its characteristics when applied to both non-degenerate atomic ensembles and to Bose-Einstein condensates. Using for all relevant parameters values that are achieved with present technology, we show that it is possible to deflect almost entirely an ensemble of 87Rb atoms falling in the gravity field. We discuss the limits of this proposal, and illustrate its robustness against non-adiabatic transitions.

A class of Coulomb radial functions is defined for use in atomic or molecular Rydberg multichannel quantum-defect calculations. The associated accumulated radial phase parameters beta(epsilon) have a simple functional dependence on the energy, analogous to the familiar Rydberg relation where beta(epsilon)=pi(n(*)-l), with n(*)=(-epsilon)(-1/2), enabling easy use in the empirical fitting of complex multichannel spectra. However, "false roots" at low-energy and strong energy dependences of the quantum defects are largely avoided in the approach, which also may be implemented in the framework of ab initio R-matrix calculations in a straightforward manner. The method is illustrated with one-channel and multichannel examples relating to atomic potassium, nitric oxide, and molecular hydrogen.

The manipulation of cold atoms with optical fields is a very promising technique for a variety of applications ranging from laser cooling and trapping to coherent atom transport and matter wave interferometry. Optical fields have also been proposed as interesting tools for quantum information processing with cold atoms. In this paper, we present a theoretical study of the dynamics of a cold Rb-87 atomic cloud falling in the gravity field in the presence of two crossing dipole guides. The cloud is either deflected or split between the two branches of this guide. We explore the possibilities of optimization of this device and present preliminary results obtained in the case of zero-temperature dilute Bose-Einstein condensates.