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Material symmetry: a key to specification of interatomic potentials

Nalepka K.

Bulletin of the Polish Academy of Sciences. Technical Sciences

2013

Vol. 61, nr 2

441--450

The paper shows that symmetry forms a basis for relations between different properties of material. In this way, the key quantities for specification of an atomistic model are identified. Material symmetry distinguishes representative processes of small strains. It is proved that the errors in the densities of the energies stored in these processes determine the range of inaccuracies with which an atomistic model recreates processes of small deformations. The errors are equal to the inaccuracies in the eigenvalues of the elasticity tensor, that is in the Kelvin moduli. For cubic crystals, the elementary processes indicated by the symmetry initiate the key paths of large deformations: Bain and trigonal ones. Therefore, the substantial errors in the Kelvin moduli lead to incorrect reconstructing the metastable phases: bcc, sc and bct. The elastic constants commonly used in the literature do not provide such information as the Kelvin moduli. Using the eigenvalues of the elasticity tensor as well as other key properties indicated by the symmetry, the EAM model proposed by A.F. Voter for copper is specified. The obtained potential more accurately reproduces small and large deformations and additionally, correctly describes defect formation as well as Cu dimer properties.

From molecular spectroscopy to entanglement of atoms - a trek with supersonic velocity

Koperski J.

Optica Applicata

2012

Vol. 42, nr 2

417-431

The supersonic free-jet expansion technique is being used in different fields of research in physics, and physical chemistry to study vibrational and rotational molecular structures in ground and excited electronic energy states. The supersonic beam technique exploits a source of monokinetic, rotationally and vibrationally cold van der Waals (vdW) molecules that are very weakly bound in their ground electronic states. In this article we review experiments at the Jagiellonian University in Kraków (Poland) in which the supersonic free-jet beam serves as a source of ground-state vdW molecules in studies of neutral-neutral interactions between 12-group metal (Me = Zn, Cd, Hg) and 18-group noble gas (Ng = He, Ne, Ar, Kr, Xe) atoms. The experiments lead to determination of spectroscopical characteristics and interatomic potentials of MeNg and Me2 molecules, allowing determination of distinct trends in the Me-Ng and Me-Me interactions in different regions of internuclear separation. The determined interatomic potentials are also used in designing mechanisms of internal vibrational cooling of molecules photoassociated in magneto-optical traps. Recently, versatility of supersonic beams is confirmed in quantum information where the technique is planned to be used to create pairs of entangled atoms in experiments dedicated for testing of Bell's inequality for atoms. A purpose of the experiment - which is in a preparational stage in our laboratory - is to create pairs of entangled cadmium atoms with regard to their nuclear spin orientations. It is planned to be achieved in supersonic molecular beams of cadmium dimers using two dye-laser pulses and stimulated Raman process leading to a controlled photodissociation of the molecule.

Excited 1u(51P1) state of Cd2 and the dipole moment of the 1u(51P1)–X0g+ electronic transition

Grycuk T, Kloda T, Kubkowska M K, Kutner T

Optica Applicata

2006

Vol. 36, nr 4

s. 505--510

The satellite band of the 228.8 nm Cd line associated with the 1u(51P1)–X0g+ electronic transition in Cd2 is measured in absorption and used for probing and correcting the excited state potential by means of quantum simulations of the spectrum. Best theoretical potential curves available are employed as the initial input data and the spectrum calculated for a single molecule is compared with the experimental spectrum of the absolute absorption coefficient per atom pair. The method yields considerable correction of the upper state potential which, finally, reproduces the experimental spectrum quite well. The dipole transition moment function is roughly determined.