Please check the document version of this publication:• A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement: This paper investigates the possibility of suppressing the ionization rate in a magnetostatic trap of metastable neon atoms by spin-polarizing the atoms. Suppression of the ionization is critical for the possibility of reaching Bose-Einstein condensation with such atoms. We estimate the relevant long-range interactions for the system, consisting of electric quadrupole-quadrupole and dipole-induced dipole terms, and develop short-range potentials based on the Na 2 singlet and triplet potentials. The autoionization widths of the system are also calculated. With these ingredients we calculate the ionization rate for spin-polarized and for spin-isotropic samples, caused by anisotropy of the long-range interactions. We find that spin polarization may allow for four orders of magnitude suppression of the ionization rate for Ne. The results depend sensitively on a precise knowledge of the interaction potentials, however, pointing out the need for experimental input. The same model gives a suppression ratio close to unity for metastable xenon in accordance with experimental results, due to a much increased anisotropy in this case.

Quantum Monte Carlo calculations of the dissociation energies of three-electron hemibonded radical cationic dimersOptical potentials are used in a quantum mechanical treatment of loss processes, e.g., ionization, where the loss of flux is described by the imaginary part. We present a numerical method for calculating two-center two-electron integrals necessary to construct the imaginary part of the optical potential. By introducing Slater-type orbitals with complex-valued exponents ͑CSTOs͒, we are able to represent the free electron wave with a limited number of CSTOs. For the representation of free electron wave functions with many oscillations, i.e., in a large r range or for a high kinetic energy, these new CSTOs form a more natural set of basis functions. The introduction of CSTOs is inevitable for the calculation of integrals concerning collisions in the mK energy range, where the interaction acts over large internuclear distances. Extensive numerical checks show that the final imaginary part of the optical potentials can be calculated with an accuracy better than 2%.

The possibility of probing the collisions of aligned Rydberg atoms by stimulated emission is assessed with studies of a polarized state and a new measurement of a collisional alignment effect in atomic Ca. The stimulated emission method uses a laser to dump the desired state to a lower level which subsequently fluoresces. The technique can be used to obtain populations and polarization dependent information. First, the method is tested by applying it to an aligned Ca(4s17d 1 D 2 ) state. Alignment curves are measured when the initial state is prepared with both parallel and perpendicular relative polarizations. The experimentally observed alignment compares well with that derived from theoretical considerations of a saturated stimulated transition. Second, a two-vector collisional alignment experiment ͑initial state and relative velocity vector͒ is performed to study the energy transfer process Ca(4s7d, and alignment effects are measured by both stimulated emission and conventional direct fluorescence detection. A preference for the ͉m͉ϭ1 and 2 initial states is observed in the relative cross sections. Essentially identical data are obtained with the two detection methods when elliptically polarized light is used for the stimulated emission detection method. The stimulated emission technique can provide alignment and population information of the final states, making it an excellent new tool for both three-vector correlation experiments and state-to-state Rydberg transitions.

Ab initio autoionization widths for the Ne(3s)-Ar and Ne(3p)-Ar systems are calculated in a two-electron approximation. Using correctly normalized wave functions, the calculations result in absolute values for the autoionization widths. We use a modified coupled-channel code with ab initio potentials and ionization widths as input to calculate cross sections for ionization and intramultiplet mixing. The influence of ionization on the intramultiplet mixing process can thus be studied. Experimental results for intramultiplet mixing cross sections for the Ne(3p)-Ar system at a collision energy ͗E͘ϭ1.7 eV are presented. These data and ionization cross sections obtained by Bussert et al. ͓Z. Phys. A 320, 105 ͑1985͔͒ are in good agreement with coupled-channel calculations. For the Ne(3s)-Ar system the calculated ionization widths are in good agreement with empirical ionization widths derived in a multiproperty analysis of a wide range of scattering data. For this system we have to conclude that the ab initio results cannot explain the experimental cross section ratio Q( 3 P 0 )/Q( 3 P 2 )ϭ1.3. Therefore, ab initio calculations of the real part of the potential are essential for a correct quantum-mechanical coupled-channel analysis.

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